🦻 Bike Fit Tool

Crank Arm Length Calculator for US Cyclists

Built and reviewed by (MBA Finance) · Updated October 2026

All five professional fitting methods in one place: Lennard Zinn upper and lower, Machine/Bontrager, Graeme Obree 9.5%, Phil Burt femur method, and the 2024 to 2026 WorldTour Short trend. Every result rounded to the nearest US stock size. Saddle height adjustment calculated automatically when you enter your current setup.

🤯 Five Methods Side-by-Side 📈 WorldTour 2026 Short Trend 📈 Phil Burt Femur Method 🔎 Nearest US Stock Size 📈 Saddle Height Adjustment 📄 PDF Fit Report

Five Fitting Methods: Zinn, Obree, Machine, Phil Burt, and WorldTour Short

Body Measurements

inches
Bare-feet inseam from floor to crotch. Stand against a wall, press a book firmly up into your crotch, mark the top edge, measure from floor to mark. This is longer than your trouser inseam.
inches
Used for Obree 9.5% method only.
mm
Enter for saddle adjustment output.
mm
Measure from the greater trochanter (hip bone top) to the lateral femoral epicondyle (outside of knee). Typical US male: 440 to 510 mm. A tape measure works; have someone help for accuracy.

Discipline

Applies a discipline-specific adjustment to the recommended result only (not to individual method outputs). MTB disciplines subtract 2.5 to 7.5 mm for ground clearance. Triathlon subtracts 5 mm for hip angle. Track sprint adds 2.5 mm for standing-start leverage.
🔎 Recommended Crank
–
mm
Nearest US Stock Size
Zinn Lower (REC)
–
WorldTour Short
–
Zinn Upper
–
Machine / Bontrager
–
Obree 9.5%
–
Phil Burt Femur
–
All Five Methods: Raw Results vs Available Stock Sizes
Horizontal bar chart comparing all method outputs for your inseam and height. Red bar = recommended (Zinn Lower). Enter measurements and click Calculate to populate.
US Market Stock Sizes (Shimano and SRAM, 2026)
160 mm 162.5 mm 165 mm 167.5 mm 170 mm 172.5 mm 175 mm 177.5 mm 180 mm

How Do Inseam and Femur Measurements Determine Optimal Pedal Stroke Size?

The crank arm connects the pedal to the bottom bracket axle. When you push the pedal through its circular path, your leg traces an arc whose diameter is exactly twice the crank length. A 170 mm crank produces a 340 mm diameter circle. A 165 mm crank produces a 330 mm circle. These 10 mm differences in diameter are small enough that most recreational cyclists would never feel them in a blind test, but over thousands of pedal strokes per hour they accumulate into measurable differences in hip flexion angle, knee load, and sustainable cadence.

The inseam is the primary input in every fitting formula because it is the most direct measurement of the effective leg length that drives the pedal. Your cycling inseam (measured barefoot, from floor to crotch with a book pulled firmly upward) correlates strongly with the optimal pedal-circle diameter. Riders with longer inseams can tolerate larger circles because their legs have the range of motion to complete each stroke without excessive hip compression at the top. Riders with shorter inseams need smaller circles to avoid hip impingement at twelve o’clock of the pedal stroke.

The femur, the bone running from the hip socket to the knee, contributes a second layer of precision. Two riders with identical inseam measurements can have very different femur lengths if one has proportionally longer tibias (shin bones) and shorter femurs, and the other has the reverse. The femur length determines how much the hip closes at the top of the pedal stroke for a given crank radius. Phil Burt, the former lead physiotherapist for British Cycling and Team Sky, developed a formula based specifically on femur length: crank mm = femur mm divided by 5, plus 65. This formula directly addresses the hip-closure problem that drives most fitting pain issues at the top of the pedal stroke, and is often the most useful formula for riders dealing with hip impingement or anterior knee pain.

CPSC and Crank Arm Safety (16 CFR Part 1512): The United States Consumer Product Safety Commission has identified crank arm fractures and detachments as documented bicycle injury hazards. CPSC’s 2024 Advance Notice of Proposed Rulemaking for electric bicycles specifically cites crank arm and pedal detachments among the most common mechanical hazard incidents. CPSC 16 CFR Part 1512 sets structural integrity requirements for bicycle components. These federal standards cover material failure under typical rider loads, which is why crank length selection (determining the mechanical leverage and bending stress applied to the crank) is relevant to safe component operation. Source: Federal Register 2024-05472 (CPSC) and 16 CFR Part 1512.

Why Inseam Is More Reliable Than Total Height

Two riders of identical total height can have very different optimal crank sizes if their body proportions differ. A six-foot rider with long legs and a short torso has a very different inseam than a six-foot rider with average legs and a long torso. Using total height for crank length (the Obree 9.5% method) is a useful cross-check but a less precise primary method than inseam-based formulas. The BikeDynamics research by Paul Swift showed that combining inseam with GT height (the greater trochanter height) gives the best statistical correlation (R² of 0.953) for crank length prediction, but since most riders do not have access to a GT height measurement, inseam alone provides a reliable and practical foundation for all the formulas in this calculator.

The WorldTour Shorter Paradigm: Why Pogacar and Pidcock Dropped to 165mm

Until about 2019, the conventional wisdom in professional and recreational road cycling was that taller riders should run longer cranks for more leverage and power. A 180cm rider would routinely be fitted with 175mm cranks. A 185cm rider might go to 177.5mm. The industry shipping default for riders over 180cm was 175mm as recently as 2022 on most major brands.

The WorldTour peloton in 2024 and 2025 tells a dramatically different story. Tadej Pogacar, who is 176cm tall, runs 165mm Carbon-Ti cranks on his road bike and has been on that length since the 2023 Tour de France. Tom Pidcock, at 170cm, also runs 165mm. Wout van Aert and Mathieu van der Poel both transitioned from 172.5mm to 170mm in the 2024 to 2025 off-season. Filippo Ganna, one of the most powerful time trialists in professional cycling at 193cm tall, runs 167.5mm on his road bike and 165mm on his TT rig. These are not marginal adjustments. They represent moves of 7.5mm to 10mm below what the Lennard Zinn upper formula would have recommended for the same riders a decade ago.

The Aerodynamic Rationale: The mechanism driving this shift is not power but aerodynamics. A shorter crank arm reduces hip flexion angle at the top of the pedal stroke (12 o’clock position). Less hip flexion at TDC allows the rider to lower their front end more aggressively and maintain a flatter back without diaphragm compression. Bradley Wiggins’s Hour Record team documented that moving from 177.5mm to 170mm cranks let him drop the front end 30mm, producing a 3.5 percent drag reduction. For a GC contender, that is worth 90 seconds in a 30 km time trial. For a recreational rider, the same principle applies even at lower speeds: a shorter crank that allows a more comfortable, lower position can be faster and more sustainable than a longer crank that forces a compromised posture.

This calculator implements the WorldTour Short method as Lennard Zinn Lower minus 5mm. For a rider with an 83cm inseam: Zinn Lower = 83 times 2.10 = 174.3mm. WorldTour Short = 174.3 minus 5 = 169.3mm, rounding to stock 170mm. That is the same result a 176cm WorldTour GC contender would have reached through professional fitting. In 2026, 160mm and 167.5mm cranks are available off the shelf from Shimano Dura-Ace R9200 and Ultegra R8100, making the full WorldTour short spectrum accessible to US recreational riders for the first time without custom ordering.

Is Shorter Always Better for US Riders?

Not universally. Riders who generate most of their power through steady torque at low cadence (typically bigger riders, climbers, and those doing long ultra-distance events) may lose meaningful feel when switching more than 5mm below their inseam-based recommendation. The research by Martin and Spirduso (2001) found no significant power difference for crank lengths within a 10mm window around the rider’s optimum, but the key word is “within.” Going 12.5mm or more shorter than the Zinn upper recommendation without a professional fit to optimize saddle height, cleat position, and position simultaneously can produce a genuine cadence and power penalty for some riders, particularly those with limited hip mobility or strong neuromuscular patterns developed over years on longer cranks.

Three Real American Riders and the Methods That Fit Each of Them

Boulder, Colorado
Road Climber: Following the WorldTour Short Trend

Alex is 5 feet 11 inches (180cm) with a cycling inseam of 33 inches (83.8cm). He has been on 175mm cranks for eight years following the traditional formula, but his fitter noticed his hip angles at the top of the stroke were causing slight saddle rocking and anterior knee discomfort on long climbs. Running all five methods: Zinn Upper = 181mm (stock 180mm), Zinn Lower = 176mm (172.5mm), Machine = 169.7mm (170mm), Obree 9.5% = 171mm (170mm), WorldTour Short = 171mm (170mm). The convergence of Machine, Obree, and WorldTour Short at 170mm is the clear signal. Alex switches to 170mm cranks, raises his saddle 5mm, and reports that his cadence on climbs improved from 74 rpm to 81 rpm within three weeks as the hip restriction resolved.

Inputs: 83.8cm inseam, 180cm height. Methods point to 170mm. Saddle raised 5mm after switch from 175mm.
Portland, Oregon
MTB Trail Rider: Ground Clearance Discipline Adjustment

Sarah is 5 feet 7 inches (170cm) with an inseam of 30.5 inches (77.5cm). She is building a new Specialized Stumpjumper trail bike and wants to know the right crank length before ordering components. Running the formulas: Zinn Lower = 162.7mm (stock 162.5mm or 165mm). With the MTB Trail discipline adjustment of minus 5mm: 162.7 minus 5 = 157.7mm (rounds to 160mm). She opts for 165mm as a compromise between the unadjusted Zinn Lower and the full MTB trail adjustment, which lands comfortably in the mid-range. When she test rides a friend’s 170mm trail bike on the same section of the Sandy Ridge trail system, she immediately notices two pedal strikes on rooted corners that she cleared cleanly on 165mm. The ground clearance argument is confirmed in one ride.

Inputs: 77.5cm inseam, MTB Trail discipline. Zinn Lower 162.5mm; MTB-adjusted 160mm; final choice 165mm.
Austin, Texas
Triathlete: Phil Burt Femur Method for TT Position

Marcus is training for Ironman Texas and working with a fitter on his TT position. He is 5 feet 9 inches (175cm) with a 32-inch (81.3cm) inseam. His fitter measured his femur at 455mm. Phil Burt method: 455 divided by 5 plus 65 = 156mm, rounding to stock 160mm. The Zinn Lower gives 170.7mm (170mm). The 10mm difference between the two methods is explained by the fact that Marcus has proportionally shorter femurs relative to his total inseam (longer tibia). Running 160mm cranks in his TT position gives him a hip angle at TDC that his fitter verifies is above 45 degrees, the threshold for minimizing diaphragm compression in an aggressive aero position. With Triathlon discipline adjustment applied to Zinn Lower (minus 5mm), that method also converges at 165mm, within one stock size of the Burt result. Marcus chooses 165mm as the consensus of both methods.

Inputs: 81.3cm inseam, 455mm femur, TT discipline. Phil Burt gives 160mm; Zinn Lower TT-adjusted gives 165mm. Final: 165mm.

Should You Round Up or Down When the Formula Falls Between Stock Sizes?

Almost every formula result falls between standard stock sizes, because inseam and height vary continuously while cranks come in fixed increments of 2.5mm. When you get a result of 173.2mm, the nearest stock sizes are 172.5mm and 175mm. Which should you choose?

The research-based answer, supported by multiple studies and the current professional fitting consensus, is to round down when the result is within 1.5mm of the lower stock size, and round up when it is closer to the upper. But in practice, the broader principle from fitters in 2025 and 2026 is to default to rounding down, especially if you are interested in any of the following: riding in an aggressive or aerodynamic position, training at higher cadences (above 90 rpm), dealing with hip tightness or anterior knee pain, or transitioning to a gravel or road bike from a mountain bike background. In all of these scenarios, the slightly shorter crank produces measurably better outcomes with no power penalty at recreational to intermediate riding speeds.

The only common case where rounding up is clearly preferred is track sprint racing. Standing start efforts require maximum torque at very low pedal velocity (sub-60 rpm), and the additional mechanical leverage of the longer crank translates directly to more speed off the line. This is why the track sprint discipline in this calculator applies a plus 2.5mm adjustment to push the recommendation toward the upper stock size. For every other discipline, the modern consensus among American bike fitters is to take the lower size if you are within 2mm of it, and to take the lower size even when you are closer to the upper if you have any hip mobility restrictions or aero position goals.

How This Calculator Rounds

This calculator uses nearest-neighbor rounding to the actual available stock sizes. The raw formula result is shown in the output so you can see exactly where you fall between sizes and make an informed choice. For example, a raw result of 173.8mm is 1.3mm from 172.5mm and 1.2mm from 175mm, making 175mm the technically nearest. But if you are a road racer targeting a sub-90-gram weight build and have access to a 175mm Shimano Dura-Ace R9200, the rounding is a footnote. If you are buying off Amazon and the choice is between a 172.5mm and a 175mm of the same groupset, this is the number that helps you make that call.

Adjusting Saddle Height After Switching to a Shorter or Longer Option

Changing crank length is not a standalone adjustment. The crank arm sets the height of the pedal at its lowest point in the stroke, which directly determines the effective leg extension at the six o’clock position. If you shorten your cranks without raising your saddle, your leg is never reaching full extension at the bottom of the stroke, which creates a knee-bent, inefficient power position and can cause medial knee pain over time. If you lengthen your cranks without lowering your saddle, your leg over-extends at the bottom and can cause lateral knee pain or hip rocking.

The rule is elegant in its simplicity: adjust your saddle height by exactly the same amount you changed your crank length, in the opposite direction. Shorten the crank by 5mm, raise the saddle 5mm. Lengthen the crank by 5mm, lower the saddle 5mm. This maintains the same leg extension angle at six o’clock as your previous fit, preserving the biomechanics that were optimized by your previous saddle height measurement or bike fit. This adjustment is the mandatory first step, and it should be made before any other positional changes.

This calculator shows the saddle adjustment automatically. When you enter your current crank length and your recommended new length differs, the result box shows the exact mm to raise or lower your saddle. Print the PDF fitting report and keep it with your bike tools as a reference during installation.

Secondary Adjustments After Changing Crank Length

After making the saddle height correction and riding for two to three weeks to adapt, you may find that other positional elements need re-evaluation. Cleat position may shift slightly when switching to significantly shorter cranks, because the arc traced by the foot changes. Riders who switch 7.5mm or more shorter sometimes find they need to bring the cleat back (toward the heel) by 1 to 2mm to maintain the same knee-over-pedal position. Handlebar height may also feel different, particularly if a shorter crank is allowing you to drop into a flatter back position for the first time. Give the body three to six weeks of regular riding to fully adapt before making secondary adjustments.

Why Do Mountain Bikes Favor Shorter Pedal Circles Than Road Setups?

Mountain bike ground clearance is the most practical argument for shorter pedaling geometry, and it is the primary reason this calculator applies a discipline adjustment for MTB categories. The bottom bracket height on a trail or enduro mountain bike is intentionally set low for stability and cornering confidence. On most US trail bikes (Santa Cruz, Yeti, Trek Slash, Specialized Stumpjumper), the bottom bracket sits between 320mm and 360mm above the ground. The pedal at its lowest point (six o’clock position) adds the crank length to the BB drop. A 175mm crank on a bike with 340mm BB height has its lowest pedal point 165mm from the ground. Hit a 180mm root or ledge with that pedal and you stop, get thrown, or break a pedal axle. Shorten to 165mm and the lowest pedal point is now 175mm from the ground, giving 10mm more clearance on the same obstacle.

On technical American trail networks (Pisgah in North Carolina, Downieville in California, Marquette in Michigan, Moab in Utah), pedal strikes on rooted, rocky terrain are one of the most common causes of rider incidents. A 5mm shorter crank on a trail bike is a meaningful safety improvement, not just a performance optimization. The US cycling community has been slower to adopt short cranks for MTB than the European scene, partly because the American tradition of big-ring climbing at low cadence favors longer cranks for torque, and partly because 160mm cranks were until recently only available from specialty brands at premium prices. With Shimano’s 2024 expansion to 160mm across Dura-Ace and XT, and SRAM’s full 160mm to 175mm range across Eagle, the supply barrier has been removed.

The calculator’s discipline adjustments are: MTB XC minus 2.5mm (hardtail racing, less ground clearance concern), MTB Trail minus 5mm (enduro and trail full-suspension, moderate to aggressive terrain), and MTB Enduro or DH minus 7.5mm (full enduro competition where maximum clearance is essential and cadence is secondary). These adjustments are applied to the Lennard Zinn Lower base recommendation before rounding to the nearest stock size.

Six Pro Tips from American Fitters on Getting Your Pedal Stroke Right

1

Measure Your Inseam Three Times and Average the Result

The cycling inseam measurement is the most important input in every crank length formula and also the most commonly measured incorrectly. Minor variations in book pressure, stance width, and posture can produce inseam readings that vary by 1 to 2cm between measurements taken five minutes apart. The calculation-level impact of a 1cm inseam error is 2.1mm in the Zinn Lower formula, which can be the difference between rounding to 172.5mm and rounding to 175mm. Take three measurements: morning, afternoon, and after a shower (when muscles are most relaxed). Average them for the most reliable input. Wear cycling shorts during measurement, since the fit between clothing and book affects the crotch contact point.

2

Use Multiple Methods as a Confidence Band, Not a Single Answer

The most reliable indication of your correct crank length is not any single formula result but the overlap zone when all five methods are compared side by side. If four of the five methods cluster within 2.5mm of each other and only one outlier, that outlier is likely responding to an unusual body proportion (either proportionally long or short femur relative to total inseam) and should be down-weighted. When all five methods cluster within 2.5mm, you can have high confidence in the recommended size. When they spread across 10mm or more, it is a signal that your body proportions deviate significantly from average and a professional fitter with a dynamic assessment will give more reliable results than any static formula.

3

Consider Going 2.5 to 5mm Shorter Than the Formula If You Have Hip Tightness

Hip flexor tightness and limited hip mobility are extremely common in American office workers who spend 6 to 10 hours per day seated in a chair before getting on a bike. A formula derived from a population of fitter, more mobile cyclists may recommend a crank length that compresses an already-tight hip at the top of the stroke. If you regularly experience anterior hip or groin discomfort when climbing out of the saddle, your hip angle at TDC may be below 45 degrees. Going 2.5mm shorter than the formula recommendation is a low-risk first intervention. If anterior hip discomfort resolves within two weeks of the switch, that is strong evidence that a shorter length is correct for your hip mobility profile regardless of what the formula says.

4

Borrow Before You Buy When Possible

Cranks are one of the more expensive bicycle components to test and return, particularly carbon road cranks from top-tier groupsets. Before committing to a length change, try to borrow or rent a set of cranks at your target length. Many bike shops, particularly those with fitting studios, maintain a demo fleet of cranks for this purpose. Some manufacturers (Rotor, Praxis Works) offer adjustable cranks that let you test multiple lengths on the same hardware. If your local shop does not have a demo program, a used crankset at your target length purchased on the US cycling classifieds forums (like PinkBike Marketplace or MTBR for mountain, or the VeloPages Classifieds for road) is a lower-risk way to validate the formula result before buying a full price retail set.

5

Record Your Result and Saddle Height in This Calculator’s PDF

Crank length is one of those fit parameters that riders forget between bike purchases, forcing a repeat fit process that could be avoided. Download this calculator’s PDF fitting report, which includes your inseam, all five method results, your recommended size, and the exact saddle height adjustment needed when switching from your current length. Store it in your cycling files alongside your bike fit certificate. When you buy a new bike, frame, or crankset, the data is immediately available. If you ever take your bike to a new fitter, the report gives them a precise starting point for your fit parameters that eliminates one complete measurement session and lets them focus on dynamic adjustments instead.

6

Allow Six to Eight Weeks of Adaptation After Any Crank Change

Neuromuscular patterns adapt to crank length over years of riding, and changing that length forces the nervous system to relearn the pedal stroke. During the first two to three weeks after a switch, most riders report feeling less powerful and potentially uncomfortable, particularly when switching to shorter cranks that require a higher cadence to maintain the same speed. This is normal. Power and comfort typically return to baseline by week four to six and often improve beyond the previous baseline by week eight as the body adapts to the more efficient motion pattern. Do not abandon a fitting change before eight weeks of regular riding unless you develop acute pain (which warrants immediate reversal and a fitter consultation), not merely discomfort or the subjective sense of feeling less powerful.

Quick Reference: Stock Sizes, Saddle Adjustments, and Discipline Recommendations

Inseam (in)Inseam (cm)Zinn LowerMachineObree (70″/178cm)WorldTour ShortRoad Rec (stock)MTB Trail (stock)
28″71.1149.3153.9169.1144.3150145
29″73.7154.8157.1169.1149.8155150
30″76.2160.0160.3169.1155.0160155
31″78.7165.3163.4169.1160.3165160
32″81.3170.7166.6169.1165.7170 or 172.5165
33″83.8176.0169.8169.1171.0175 or 177.5170
34″86.4181.4173.0169.1176.4180172.5
35″88.9186.7176.1169.1181.7180+175
Discipline Adjustments (mm from Zinn Lower)
  • Road race / endurance: 0 mm
  • Gravel / adventure: minus 2.5 mm
  • MTB XC hardtail: minus 2.5 mm
  • MTB trail / all-mountain: minus 5 mm
  • MTB enduro / DH: minus 7.5 mm
  • Triathlon / TT: minus 5 mm
  • Track sprint (standing start): plus 2.5 mm
Saddle Height Adjustment Rule
  • Shorter by 2.5mm: raise saddle 2.5mm
  • Shorter by 5.0mm: raise saddle 5.0mm
  • Shorter by 7.5mm: raise saddle 7.5mm
  • Longer by 2.5mm: lower saddle 2.5mm
  • Longer by 5.0mm: lower saddle 5.0mm
  • Secondary: recheck cleat position after 4 weeks
  • Adaptation time: 6 to 8 weeks for full adjustment

Frequently Asked Questions About Sizing, Fit, and Pedaling Biomechanics

No single formula is universally “most accurate” because different methods optimize for different outcomes. The Lennard Zinn Lower formula (inseam cm times 2.10) is the most widely used by US bike fitters as a starting point because it sits conservatively below the traditional upper end and corresponds to a comfortable, versatile length for most riders. The Phil Burt femur-based method is often more accurate for riders with unusual body proportions (especially those with shorter femurs relative to total leg length) and for riders dealing with hip pain. The most reliable approach is to run all five methods simultaneously, which this calculator does, and look for the zone where multiple methods agree. Consensus within 2.5mm across three or more methods indicates high confidence in that length range.
Stand barefoot with your back against a wall and your feet about 6 inches apart. Take a hardback book (at least 3/4 inch thick) and press it up firmly into your crotch, keeping the spine of the book level and parallel to the floor. Have someone mark the top of the book on the wall, then measure from the mark to the floor. This is your cycling inseam. Do not use your trouser inseam, which is measured at the seam of your pants and is typically 1 to 2 inches shorter than the cycling inseam. Take three measurements at different times of day and average the results. Wear cycling shorts or close-fitting shorts during measurement for consistency. The cycling inseam from this method will match the inputs used in all the Lennard Zinn and related formulas.
A 5’10” (177.8cm) rider with a typical inseam of 32 to 33 inches (81 to 84cm) would land in the following ranges using this calculator’s five methods: Zinn Lower 170 to 176mm, Machine 167 to 170mm, Obree 9.5% approximately 169mm (based on 177.8cm height), and WorldTour Short 165 to 171mm. The recommended stock size for most road disciplines would be 170mm or 172.5mm, with MTB trail riding suggesting 165mm to 167.5mm. This is a range, not a single definitive answer, because body proportions vary significantly at the same total height. Enter your actual inseam into the calculator above for a precise recommendation rather than relying on height alone.
The primary reason is aerodynamics. Shorter cranks reduce hip flexion at the top of the pedal stroke (12 o’clock position), which allows riders to maintain a lower, flatter back without compressing the diaphragm. A lower front end reduces frontal area and wind drag. Bradley Wiggins’s Hour Record team calculated a 3.5 percent drag reduction from going from 177.5mm to 170mm cranks on his track bike, which corresponds to roughly 90 seconds of advantage in a 30 km time trial. The secondary reason is cadence comfort: shorter cranks allow faster, smoother pedaling at high rpm, which is the dominant road racing power delivery method in modern grand tour racing. The research by Martin and Spirduso (2001) confirmed that power is not meaningfully different across crank lengths within a 10mm window, so the aerodynamic benefit of shorter comes at no power cost.
The Phil Burt femur method was developed by the former head physiotherapist of British Cycling and Team Sky, who worked with riders including Bradley Wiggins and Chris Froome. The formula is: crank mm equals femur mm divided by 5, plus 65. This method specifically addresses hip closure angle at the top of the pedal stroke, making it particularly useful for riders with hip impingement, anterior groin pain, or unusual leg proportions. To measure your femur: stand with your weight on one leg. Place your finger on the greater trochanter (the bony prominence on the outer top of your hip, which rises and falls as you lift that leg). Then find the lateral femoral epicondyle (the bony bump on the outside of your knee). Measure straight between these two points with a tape measure. The result in millimeters goes directly into the Phil Burt formula. Typical US adult male femur length: 440 to 510mm. Typical female: 400 to 470mm.
Within the range of most practical crank lengths (160 to 177.5mm for a typical adult rider), the research evidence shows no significant effect on sustainable power output. The most comprehensive study, by Martin and Spirduso (2001), compared maximum power across crank lengths from 120mm to 220mm and found power was only meaningfully reduced at the extremes (120mm and 220mm), far outside the range of normal cycling cranks. Within a 10mm window around any rider’s optimal length, power differences are within measurement noise. What does change is the distribution of the work between hip extension (dominant at longer cranks) and knee extension (more balanced at shorter cranks), and cadence comfort. Many riders who switch to shorter cranks find their sustainable cadence increases, which can improve power output via cadence-related efficiency gains even though the crank length itself is not producing more power per stroke.
As of 2026, the standard available crank arm lengths in the US market (Shimano and SRAM at most major retailers) are 160, 162.5, 165, 167.5, 170, 172.5, 175, 177.5, and 180mm. Shimano expanded to 160mm and 167.5mm options for Dura-Ace R9200 and Ultegra R8100 in 2024, making the full short-crank spectrum available from a major manufacturer for the first time. SRAM Red AXS ships in 160, 162.5, 165, 167.5, 170, 172.5, and 175mm as of 2026. Specialized has standardized 165mm cranks across most S-Works builds in size 56cm and below. Specialty manufacturers (Rotor, Praxis Works, Carbon-Ti, Zinn Cycles) offer a wider range, including sub-160mm sizes for riders with very short inseams or extreme fitting requirements.
Ground clearance at the pedal equals the bottom bracket height minus the crank arm length. On a typical US trail bike with 340mm BB height, a 175mm crank brings the lowest pedal point to 165mm from the ground. A 165mm crank raises that to 175mm. A 160mm crank raises it to 180mm. In practical terms, that 10mm to 15mm of additional clearance (from going from 175mm to 160mm to 165mm) means clearing obstacles that you would have struck on the longer crank: small rocks, roots, ledge lips, and trail ruts. On aggressive enduro terrain like the Pisgah National Forest in North Carolina, Fruita in Colorado, or the Kingdom Trails in Vermont, pedal strikes are a leading cause of loss-of-control incidents. The MTB Trail discipline adjustment in this calculator (minus 5mm from Zinn Lower) reflects this real-world safety consideration, not merely a performance optimization.
The Machine method (attributed to Keith Bontrager, the engineer behind Bontrager bicycle components) uses the formula: crank mm equals inseam cm times 1.25, plus 65mm. This formula consistently produces shorter recommendations than the Lennard Zinn formulas, which makes it a more conservative, lower-risk starting point, particularly for newer riders who have not yet developed strong neuromuscular patterns on a specific crank length. For a rider with an 82cm inseam: Machine = 82 times 1.25 plus 65 = 167.5mm. Zinn Lower for the same rider = 82 times 2.10 = 172.2mm. The 4.7mm difference is significant: it is the difference between the 167.5mm and 172.5mm stock sizes. The Machine result tends to cluster near the current WorldTour Short recommendations for many riders, which partly explains its continued relevance in the era of shorter professional cranks.
Yes, definitively. Children’s bicycle cranks should be sized proportionally to their inseam just as adult cranks are, but most entry-level and mid-range children’s bikes ship with cranks that are too long relative to the child’s body size. Oversized cranks on children’s bikes cause the same hip impingement and ankle compensation problems as they do on adult bikes, but on growing bodies with developing musculoskeletal patterns the effects can be more lasting. For young US riders in USA Cycling junior development programs, proper crank sizing is considered an important injury prevention measure. USA Cycling’s junior gear restriction rules also apply to cranks in certain age categories, making appropriate sizing relevant for competition eligibility. Use this calculator by entering the child’s cycling inseam in the inseam field, selecting the appropriate discipline, and applying the result to find the nearest available junior crank size.
Shoe stack height (the distance from the pedal spindle center to the bottom of the foot inside the shoe) affects effective leg extension in a similar way to saddle height, but it does not directly affect optimal crank length. The crank length determines the arc the pedal traces. Whether your foot is higher or lower above that pedal spindle (due to shoe stack height or cleat wedging) changes your knee angle throughout the stroke but not the fundamental question of how large a circle your leg needs to complete comfortably. The ONLY exception is when switching from extremely flat-soled mountain bike shoes to very high-stack road shoes (or vice versa): if the stack height change is 10mm or more, it is functionally equivalent to a saddle height change and your fitter may use it to fine-tune position without changing saddle height. This is a secondary fitting consideration and does not factor into the primary crank length formulas in this calculator.
The Graeme Obree 9.5% method (crank mm equals 9.5 percent of total height in mm, or equivalently height cm times 0.95) is reliable as a cross-check but less precise than the inseam-based methods because it uses total height rather than leg length. Two riders of identical height but different body proportions (one long-torsoed and one long-legged) will get the same Obree recommendation but likely have different optimal crank lengths. The Obree method is most useful as a sanity check: if your inseam-based recommendation is more than 7mm away from the Obree result, it suggests unusual body proportions that may warrant a professional fit with dynamic assessment rather than relying on static formula outputs alone. For riders of typical proportions, the Obree result usually falls within 2 to 5mm of the Zinn Lower result, providing confirmation of the recommendation.
No. A standard crankset has matched left and right arms, and using different lengths on each side would create a fundamentally asymmetric pedal stroke that would cause serious biomechanical problems and likely lead to injury. Some adaptive cycling setups for riders with limb differences or prosthetic limbs do use asymmetric cranks, but this is a specialized clinical fitting domain requiring professional guidance. If you have a leg length discrepancy, the standard fitting approach is to use equal crank length on both sides and compensate with saddle height adjustment and cleat shimming rather than using different crank arm lengths. Rotor makes Uno cranks with variable effective length through an offset spindle design, but this system still maintains equal effective lengths on both sides simultaneously.
Crank arm length and Q-factor (the distance between the pedal attachment faces on opposite cranks, measured laterally) are independent dimensions that are sometimes confused. Crank arm length is the distance from the bottom bracket spindle center to the pedal axle center, determining the arc traced during pedaling. Q-factor determines how wide apart your feet are while pedaling and affects knee tracking and hip alignment. Changing crank length does not change Q-factor on the same crankset. However, some cranksets that offer a wider range of crank lengths (such as Rotor or Praxis Works) may have different Q-factors at different arm lengths depending on the manufacturing design. When switching cranksets entirely to access a different arm length, verify the Q-factor of the new crankset matches or is within 5mm of your current setup to avoid introducing a lateral position change alongside the length change.
A static crank length calculator based on inseam and height measurements cannot tell you your actual optimal length the way a dynamic professional fit can. It cannot assess hip mobility limitations that would push you shorter than the formula suggests. It cannot detect existing injury patterns or neuromuscular compensations in your current pedal stroke that might indicate a dramatically different length than any formula would predict. It cannot account for the interaction between crank length, cleat position, shoe flexibility, and saddle height that a trained fitter adjusts simultaneously. What this calculator does well is give you an accurate, research-based starting point that is right for the majority of riders, and save you the time and cost of a fit consultation purely for crank length selection when you are within a normal proportion range. Use it as the first step, not the final word.
Several physical symptoms suggest your current cranks may be the wrong length. Anterior knee pain (front of the knee) during or after rides can indicate cranks that are too long, forcing excessive knee bend at TDC. Hip impingement or groin tightness during climbing or high-cadence riding also often indicates cranks that are too long and are compressing the hip joint excessively. Saddle rocking from side to side at high cadence sometimes indicates cranks that are too long, causing the rider to accommodate hip compression by rocking the pelvis. On the other side, cranks that are too short can cause an inability to generate power at low cadence and a feeling that the pedaling circle is uncomfortably small. If you notice any of these patterns and your current cranks are outside the range suggested by this calculator’s five-method consensus, a crank length change may be the intervention worth trying before more complex and expensive fitting adjustments.

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Legal Disclaimer and Editorial Transparency. Crank arm length recommendations from this calculator are derived from publicly documented professional fitting formulas including those of Lennard Zinn, Graeme Obree, Phil Burt, and the Machine/Bontrager methodology. These are starting-point estimates only. They are not a substitute for dynamic professional bicycle fitting by a certified fitter who can assess hip mobility, injury history, and existing neuromuscular patterns through motion analysis. The WorldTour Short method is based on trend data from publicly available professional cycling reporting current through May 2026. Saddle height adjustment guidance follows the standard professional fitting protocol of matching saddle height change to crank length change in the opposite direction. Martin and Spirduso (2001) research citation is from a peer-reviewed sports science publication; consult the original study for methodology details. CPSC 16 CFR Part 1512 references are provided for informational context; consult the current eCFR for authoritative regulatory text. Discipline adjustments are approximations based on consensus professional fitting practice. USCalculators.com is editorially independent and receives no compensation from bicycle component brands mentioned on this page.