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Sheet Metal Bend Deduction Calculator: Flat Blank Length, Bend Allowance, and K-Factor for Press Brake Work

Calculate bend deduction (BD), bend allowance (BA), and outside set back (OSSB) for any bend angle, material thickness, and inside radius. Covers 8 common sheet metal materials with K-factor presets. Multi-bend flat pattern accumulator included. Free, no login required.

🗡️ BD + BA + OSSB 📈 8 Material K-Factors 🌐 Inch and Metric 🔢 Multi-Bend Accumulator 📄 PDF Shop Traveler 📨 WhatsApp Share
Units:
Bend Parameters
in
Decimal inches (e.g. 0.060 for 18ga CRS, 0.125 for 1/8″)
in
Measure inside the bend, not centerline. Typical: equal to material thickness.
degrees
Bend angle (not included angle). 90 degrees = square corner.
Range 0.25 (coining) to 0.50 (full thickness). 0.44 is standard air-bend mild steel.
in
in
Flat blank = Flange 1 + Flange 2 minus Bend Deduction
Calculation Results
🗡️ Enter thickness, inside radius, and bend angle, then click Calculate.
Bend Deduction vs Angle for Current Material and Radius (Calculate to populate)

What Is Bend Deduction and Why Every Press Brake Operator Must Know It

When a flat sheet of metal is bent on a press brake, the outer surface of the bend stretches longer than the inner surface compresses. The material along the outer radius elongates; the material along the inner radius compresses. Somewhere in between lies the neutral axis, a theoretical surface that neither stretches nor compresses. The flat length of the bend arc along this neutral axis is the bend allowance (BA). But because the neutral axis sits inside the material, the total length of material consumed by the bend is always slightly more than what a purely geometric calculation of flat flanges would suggest.

Bend deduction (BD) is the practical quantity a fabricator subtracts from the sum of the flat flange dimensions to find the correct flat blank length. If you lay out a flat pattern using only the finished part dimensions without subtracting bend deduction, your bent part will always come out oversize. The amount of the oversize exactly equals the bend deduction for each bend in the part.

This is the most important calculation in sheet metal layout, and it is the one most often done incorrectly or inconsistently between shops. Different shops use different K-factors, different formulas, and sometimes seat-of-the-pants estimates that only work for the specific gauge and radius their press brake operator has been running for twenty years. This calculator implements the standard formulas from Machinery’s Handbook and the SME Sheet Metal Forming Handbook, with material-specific K-factor presets so your calculations are consistent and documented.

How Bend Deduction Affects Production Economics in US Metal Fab Shops

In a production environment, consistently accurate bend deduction directly affects material cost and shop efficiency. If your blank is cut 0.040 inch too long because the wrong BD was applied, every part in the batch has flanges that are too long after bending. Depending on the design, you either scrap the parts, re-machine the flanges to length, or weld/fill the oversize. Each of these outcomes adds cost. In a shop running 500 brackets per week from 14 gauge CRS, an error of even 0.020 inch in the flat blank means each part is cut slightly long, consuming approximately 1.2 percent more material than necessary. On a material cost of $2.80 per square foot, that is a recurring waste that accumulates to hundreds of dollars per month. Conversely, if blanks are cut slightly short, flanges come out undersized and parts fail dimensional inspection. The standard practice in lean US fabrication shops is to verify the BD formula, calibrate it quarterly from test bends, and document the calibrated K-factor on the job traveler. This takes thirty minutes per material type per year and eliminates a source of chronic waste and rework.

The Three Values You Need: BA, OSSB, and BD

Bend Allowance (BA) is the arc length along the neutral axis through the bend zone. Formula: BA = (pi / 180) x A x (R + K x T), where A is the bend angle in degrees, R is the inside bend radius, K is the K-factor, and T is the material thickness. BA tells you how much flat material length is consumed by the arc.

Outside Set Back (OSSB) is the distance from the tangent point of the bend arc to the theoretical outside corner of the finished bend. Formula: OSSB = tan(A/2) x (R + T). OSSB is used to locate punch centers relative to the part print dimensions, and it appears explicitly in the bend deduction formula.

Bend Deduction (BD) is the quantity subtracted from the sum of the outside face dimensions to get the flat blank length. Formula: BD = (2 x OSSB) minus BA. BD is always positive for bends less than 180 degrees. For a 90-degree bend in 18 gauge mild steel at R = 0.060 inch with K = 0.44: BA = 0.1357 inch, OSSB = 0.1200 inch, BD = 0.1043 inch. So for a 2-inch flange and a 1.5-inch flange at 90 degrees, the flat blank = 2.000 + 1.500 minus 0.104 = 3.396 inches, not 3.500 inches as the outside dimensions would suggest.

K-Factor: What It Is and Why It Varies by Material

The K-factor is the ratio of the neutral axis location to the material thickness: K = t / T, where t is the distance from the inner surface of the bend to the neutral axis, and T is the total material thickness. At K = 0.50, the neutral axis is exactly at the midpoint of the material. In real bending, the neutral axis shifts inward from the midpoint due to the compressive stress on the inner radius side being greater than the tensile stress on the outer side.

K-factor varies by material ductility, bend method, and the ratio of inside radius to material thickness. Soft, ductile materials like 3003-H14 aluminum shift the neutral axis closer to the inner surface (K closer to 0.33). Harder materials like 6061-T6 and stainless steel have a neutral axis closer to the midpoint (K closer to 0.44 to 0.46). Air bending produces a different K-factor than bottom bending (coining), because coining forces the material against the die and changes the stress distribution. The K-factor presets in this calculator are industry-consensus values for air bending, which is the dominant bending method in US sheet metal shops today.

Flat Pattern Length: Using Bend Deduction for Full Part Layout

For a part with multiple bends, the flat blank length is the sum of all flat flange lengths minus the sum of all bend deductions. For a three-flange U-channel with two 90-degree bends in 0.060-inch CRS with 0.060-inch radius: each bend has BD = 0.1043 inch. If the flanges are 2.0, 4.0, and 2.0 inches (outside dimensions): flat blank = 2.0 + 4.0 + 2.0 minus 2 x 0.1043 = 8.0 minus 0.2086 = 7.791 inches. The multi-bend accumulator in this calculator lets you add each bend sequentially to build up the full flat pattern for complex parts with many bends.

How This Bend Deduction Calculator Works: Inputs, Formulas, and Outputs

Select your material from the dropdown, enter thickness (T) and inside bend radius (R) in decimal inches or millimeters, set the bend angle, and click Calculate. Here is what each input and output means.

Material Selection Auto-Fills K-Factor

When you pick a material from the dropdown, the calculator fills the K-factor field with the industry-standard value for that material in air bending. You can override the K-factor manually if your specific tooling, bend method, or process yields a calibrated K-factor from test bends. Many production shops run test bends in each material and measure the actual flat length consumed, then back-calculate their true K-factor for that material on their specific press brake. That calibrated value is more accurate than any table, and this calculator accepts it directly.

Unit Toggle: Inch and Metric

Press the Decimal Inch or Millimeter button at the top to switch units. When you switch, the output values convert automatically. Many US shops that run automotive programs work in millimeters for part dimensions while still thinking in gauge numbers for material thickness. The unit toggle handles both workflows without requiring manual conversion.

Optional Flat Blank Calculation

Entering Flange 1 and Flange 2 lengths (outside face dimensions from the part print) unlocks the flat blank length output. This is the direct layout answer: blank your flat sheet to this length, and the finished bent part will come out to your print dimensions. The multi-bend accumulator in the results panel lets you add the BD from this bend to a running total for multi-bend parts.

Three Real US Fabrication Shop Examples: Bend Deduction in Practice

Example 1: Custom Sheet Metal Shop in Denver, Colorado: 90-Degree Bracket in 16 Gauge CRS

A Denver custom fabrication shop is bending a simple L-bracket from 16 gauge cold-rolled steel (0.0598 inch thick) on a 90-ton press brake. The print calls for a 2.000-inch flange and a 1.500-inch flange, both measured to the outside face. Inside radius is 0.062 inch (1/16 inch punch nose radius).

BA = (pi/180) x 90 x (0.062 + 0.44 x 0.0598) = 1.5708 x 0.0883 = 0.1387 inch.

OSSB = tan(45°) x (0.062 + 0.0598) = 1.0 x 0.1218 = 0.1218 inch.

BD = 2 x 0.1218 minus 0.1387 = 0.2436 minus 0.1387 = 0.1049 inch.

Flat blank = 2.000 + 1.500 minus 0.1049 = 3.395 inches. The operator shears blanks to 3-3/8 inch (3.375″) as the nearest standard shear setting, which gives a slightly small flange and will be trimmed to final dimension after bending.

Example 2: HVAC Duct Fabricator in Houston, Texas: 45-Degree Elbow Flange in 22 Gauge Galvanized Steel

A Houston commercial HVAC shop is fabricating transition pieces in 22 gauge galvanized steel (0.0299 inch). The fitting requires a 45-degree return flange. Inside radius is 0.030 inch (matching the material thickness for a sharp bend). K-factor for galvanized steel: 0.43.

BA = (pi/180) x 45 x (0.030 + 0.43 x 0.0299) = 0.7854 x 0.0429 = 0.0337 inch.

OSSB = tan(22.5°) x (0.030 + 0.0299) = 0.4142 x 0.0599 = 0.0248 inch.

BD = 2 x 0.0248 minus 0.0337 = 0.0496 minus 0.0337 = 0.0159 inch.

At 22 gauge, this is about 0.016 inch of deduction per bend, a small number that experienced sheet metal workers often know intuitively for their common gauge and radius combinations. The calculator confirms the value and documents it for new operators who do not yet have years of feel for the material.

Example 3: Aluminum Enclosure Shop in Portland, Oregon: 90-Degree U-Channel in 1/8″ 5052-H32

A Portland electronics enclosure shop needs a U-channel from 0.125-inch 5052-H32 aluminum with two 90-degree bends and inside radii of 0.125 inch (equal to material thickness, typical for 5052). Flanges are 1.500, 3.000, and 1.500 inch (outside face dimensions). K-factor for 5052: 0.38.

For each 90-degree bend: BA = (pi/180) x 90 x (0.125 + 0.38 x 0.125) = 1.5708 x 0.1725 = 0.2710 inch. OSSB = tan(45°) x (0.125 + 0.125) = 0.2500 inch. BD = 2 x 0.250 minus 0.2710 = 0.2290 inch.

Two bends: total BD = 2 x 0.2290 = 0.4580 inch. Flat blank = 1.500 + 3.000 + 1.500 minus 0.458 = 5.542 inches. The shop programs the brake to bend at 5.542/2 from each end (centering the web), bends both flanges at 90 degrees, and the finished part matches the print dimensions for both flanges and the web dimension.

Five Expert Tips for Accurate Flat Pattern Layout on the Press Brake

🦋
Run test bends to calibrate your actual K-factor

The K-factor values in this calculator are consensus industry values for air bending. Your specific press brake, tooling, and material supplier may produce slightly different results. To calibrate: bend a test piece with known flange dimensions, measure the actual outside flange lengths after bending, back-calculate what K-factor produces that exact BD, and enter that calibrated value for future production runs. A shop that calibrates K-factor for each material and gauge combination will hold tighter tolerances than one that uses published table values without verification.

📈
Air bending vs bottom bending: the K-factor difference matters

Air bending (the tool touches only the edges of the die opening and the punch nose) is the standard method in most US shops. Bottom bending (the punch drives the material fully into the die until it contacts the die face) produces a sharper, more consistent bend but requires significantly higher tonnage. Coining (full contact forming) produces the sharpest radii. Air bending K-factors range from 0.33 to 0.50. Bottom bending and coining produce effective K-factors closer to 0.33 regardless of material because the die forces the neutral axis inward. If your shop bottom-bends or coins, reduce the K-factor by 0.04 to 0.08 from the air-bend preset values in this calculator, or run calibration test bends to measure the actual shift.

🔢
Inside radius must match the punch nose radius, not be estimated

In air bending, the inside radius of the finished part is not the punch nose radius; it is a function of the die opening width and material thickness. The Machinery’s Handbook relationship for air bending is: R_inside approximately equals 0.156 times the die opening width. A V-die with a 0.500-inch opening produces an inside radius of approximately 0.078 inch in steel, regardless of the punch nose radius (as long as the punch radius is smaller). Measure or calculate your actual inside radius from your die setup, not the punch catalog specification, and enter that measured value into this calculator for accurate results.

🗡️
Minimum inside bend radius is material-dependent: do not guess

Bending below the minimum inside radius for a material causes cracking on the outer surface of the bend. The minimum bend radius is typically expressed as a multiple of material thickness: for 5052-H32 aluminum, minimum R is 0.5T for bends perpendicular to the grain and 1.0T parallel to the grain. For 6061-T6, minimum R is 3.0T to 4.0T. For mild steel, R can be as small as 0.0T (sharp bend, R essentially zero) in thin gauges. The minimum radius values are published by material standards organizations and tooling manufacturers. Always verify your intended radius against the material’s minimum before laying out the flat pattern, because a radius below minimum will crack the part regardless of how accurate your bend deduction calculation is.

📄
Document every setup with a PDF shop traveler

Use the PDF Shop Traveler button to generate a printed record of the bend parameters, K-factor, formulas, and results for each bend in a production run. Attach it to the job router card or work order. When the same job runs again in three months, the operator does not have to recalculate or remember the setup. If a dimension is out of spec, the traveler provides the calculation record needed to troubleshoot whether the problem is in the blank size, the K-factor assumption, the tooling setup, or the measuring method. Documented setups also protect the shop during customer quality audits by showing that blank dimensions were calculated from a defined, repeatable process.

Quick Reference: Bend Deduction for Common US Sheet Metal Gauges and Angles

All values assume mild steel / CRS, K-factor 0.44, air bending, inside radius equal to material thickness. Values in decimal inches.

GaugeThickness (in)Radius (in)BD at 45 degBD at 90 degBD at 120 degSource
11 ga0.11960.1200.06280.20800.4905Machinery’s Hbk.
12 ga0.10460.1050.05500.18200.4293Machinery’s Hbk.
14 ga0.07470.0750.03920.12990.3064Machinery’s Hbk.
16 ga0.05980.0600.03140.10400.2452Machinery’s Hbk.
18 ga0.04780.0480.02510.08310.1960Machinery’s Hbk.
20 ga0.03590.0360.01880.06240.1472Machinery’s Hbk.
22 ga0.02990.0300.01570.05200.1227Machinery’s Hbk.
24 ga0.02390.0240.01250.04150.0980Machinery’s Hbk.
1/8″0.12500.1250.06550.21730.5127Machinery’s Hbk.
3/16″0.18750.1880.09850.32630.7693Machinery’s Hbk.

Source: Machinery’s Handbook 31st Ed. | SME Sheet Metal Forming Handbook | NIST Manufacturing

Sheet Metal Bending: 16 Questions from the Press Brake Floor

What is the difference between bend allowance and bend deduction?
Bend allowance (BA) is the arc length of material along the neutral axis consumed by the bend. It tells you how much length of flat material becomes the curved section. Bend deduction (BD) is what you subtract from the sum of the outside face flange dimensions to find the correct flat blank length. BD = 2 x OSSB minus BA. Both quantities describe the same bend but from different measurement reference points. BA is referenced to the neutral axis (inside the material). BD is referenced to the outside face of the material (where the part print dimensions are usually measured). Most flat pattern layout in US shops uses BD because engineers and fabricators typically dimension parts to the outside face, making BD the directly usable number for blank calculation.
What is outside set back (OSSB) and when do I need it?
Outside set back (OSSB) is the distance from the point where the flat flange tangentially meets the bend arc to the theoretical sharp outside corner. It is given by OSSB = tan(A/2) x (R + T). OSSB is primarily used for two purposes: first, it appears in the bend deduction formula as BD = 2 x OSSB minus BA. Second, it is used in press brake setup to locate punch centerlines: the distance from the bend line (tangent point) to the punch centerline equals the inside radius R, while the distance from the outside corner reference to the bend line equals OSSB. Understanding OSSB helps you locate the part under the punch when setting up a bend with reference to print dimensions rather than neutral axis positions.
Why is bend deduction always subtracted, never added, from flange dimensions?
Bend deduction is always subtracted because the bend zone consumes material. When a flat sheet is bent, the material in the bend area is redistributed into the arc. If you simply laid out a part by adding up the outside face dimensions with no deduction, the blank would be too long. After bending, the flanges would be too long and the part would be oversize. The deduction corrects for the material that disappears into the bend geometry. Mathematically, the outside face of the bend is longer than what a flat calculation would suggest because the outside arc sweeps through a longer path than the flat projection of those dimensions. BD quantifies exactly how much longer and lets you pre-compensate the blank. All values of BD are positive for standard bends between 1 and 179 degrees.
How do I find the correct K-factor for my specific shop setup?
The most accurate K-factor for your shop is determined empirically from test bends. Bend a piece of the exact material, thickness, and radius you will use in production. Measure the flat blank length before bending. Measure both outside flange lengths after bending. Then solve for K: K = ((BD/2 + BA/2) / (pi x A/360)) / (R+T) minus R/T, where BD = (F1+F2) minus blank length (measured from test), and BA = 2 x OSSB minus BD. A simpler approach: BD_measured = F1 + F2 minus blank_length. Then K = ((2 x tan(A/2) x (R+T)) minus BD) / ((pi/180 x A)) minus R) / T. A single test bend per material type at your standard radius will give you a calibrated K-factor that outperforms any published table. Keep a log of calibrated K-factors in your shop’s setup records.
What is the minimum bend radius for common US sheet metal materials?
Minimum inside bend radius as multiples of material thickness, for bends perpendicular to the rolling direction (easiest direction). Mild steel and low-carbon steel: 0T to 0.5T depending on gauge; typical production minimum is 0.5T. Cold-rolled steel (CRS): 0.5T for gauges 18 and thinner. Aluminum 3003-H14: 0T (very ductile, essentially sharp bends possible). Aluminum 5052-H32: 0.5T perpendicular, 1.0T parallel to grain. Aluminum 6061-T6: 3.0T to 4.0T (hard temper, prone to cracking). Stainless 304/316: 0.5T to 1.0T depending on gauge. Galvanized steel: 1.0T to 1.5T because the zinc coating can crack at sharper radii. Spring steel: 4.0T or more. These are minimum values from material manufacturers and the Precision Sheet Metal Operators Association. Going below minimum radius for your specific material and temper risks cracking on the outer bend surface.
How do I calculate the flat blank length for a part with three or more bends?
For a multi-bend part, flat blank length = sum of all flange lengths (outside face dimensions) minus sum of all bend deductions. Calculate the BD for each individual bend using this calculator, then add all the BDs together. Flat blank = F1 + F2 + F3 + … + Fn minus BD1 minus BD2 minus … minus BDn. For a U-channel with flanges 2.0, 3.0, 2.0 inches and two 90-degree bends at BD = 0.104 inch each: flat blank = 7.000 minus 0.208 = 6.792 inches. The multi-bend accumulator in this calculator’s results panel lets you add each bend sequentially as you calculate them, building up the total deduction without manual addition. For complex parts with many different angles and radii, calculate each bend separately, note the BD for each, and sum them at the end.
What causes springback and how does it affect the bend angle I enter in this calculator?
Springback is the elastic recovery of the material after the bend load is released. When you bend sheet metal to 90 degrees under the punch, the material stores elastic energy. When the punch is removed, the elastic portion of the strain recovers and the bend opens slightly, ending at perhaps 87 to 92 degrees instead of 90. The amount of springback depends on the material’s yield strength and modulus, the bend radius relative to material thickness, and the bending method. Higher strength materials spring back more. Aluminum springback is significant; stainless springback is substantial; soft copper springback is minimal. The bend angle you enter in this calculator should be the final desired bend angle on the finished part, not the over-bend angle needed to hit that dimension. The over-bend correction is determined empirically on your press brake and is a separate setup variable. Most modern press brake controls handle springback correction automatically through closed-loop angle measurement and compensation.
Why do different bending software programs give slightly different flat pattern lengths for the same part?
Different CAD/CAM and sheet metal design programs use slightly different methods for flat pattern development. Some use bend allowance (BA) tables, some use bend deduction (BD) tables, some use Y-factor tables instead of K-factor, and some implement proprietary algorithms based on experimental data. Even programs that use the same formula may apply different default K-factor values or different rounding behavior. The differences are usually small (0.001 to 0.010 inch per bend for typical part sizes) but can accumulate to meaningful error on parts with many bends and tight tolerances. The best practice is to use a consistent calculation method, calibrate the K-factor for each material from test bends, document the calibrated values, and apply them uniformly. This calculator uses the standard BD formula from Machinery’s Handbook, which is compatible with most US shop practice and gives results consistent with manually entered calibrated K-factors in major sheet metal CAD programs like SolidWorks Sheet Metal, CATIA, and Autodesk Inventor Sheet Metal.
What is the difference between inside radius and centerline radius in bend calculations?
Inside radius (R) is the radius at the inner surface of the bend, where the material touches the punch nose or die. Centerline radius is the radius at the mid-plane of the material thickness, equal to R plus T/2. Neutral axis radius (R_n) is the radius at the neutral axis, equal to R plus K times T. This calculator uses inside radius as the primary input because that is the value you can physically measure from the punch tooling catalog or by measuring a completed bend with a radius gauge. Some older formulas and some press brake OEM documentation use centerline radius; if your source data uses centerline radius, convert to inside radius by subtracting T/2 before entering in this calculator.
How does bending direction relative to the sheet grain affect the flat pattern?
Sheet metal has a rolling direction (grain direction) from the mill processing. Bending perpendicular to the rolling direction (across the grain) is easier, allows tighter bend radii, and produces more ductile behavior. Bending parallel to the rolling direction (with the grain) requires a larger minimum radius and risks cracking, particularly in harder tempers like 6061-T6 and half-hard stainless. The K-factor itself can shift slightly between the two directions. However, for the purpose of flat pattern development and bend deduction calculation, the K-factor adjustment for bending direction is typically 0.01 to 0.03 in practical production scenarios and is captured in the calibration process. The standard K-factor values in this calculator assume the more common case of bending perpendicular to the grain (which gives the more ductile result). If you regularly bend parallel to grain in a material with known direction sensitivity, adjust K by +0.01 to +0.03 or use a calibrated empirical value.
What gauge is 0.060 inch sheet metal and how do I convert gauge to decimal thickness?
Gauge systems for sheet metal are not universal: the same gauge number means different thicknesses in different systems. In the US, the most common gauge system for steel is the US Standard / Manufacturers Standard Gauge (MSG), where 16 gauge = 0.0598 inch, 18 gauge = 0.0478 inch, and 20 gauge = 0.0359 inch. For aluminum, the Brown and Sharpe (B&S) or American Wire Gauge (AWG) system is used, where 16 gauge = 0.0508 inch and 18 gauge = 0.0403 inch. Galvanized steel and stainless steel follow yet another convention. The safest approach is to work in decimal thickness for all calculations and verify the actual material thickness with a micrometer before setting up the press brake. Never assume gauge without confirming the decimal equivalent. This calculator accepts decimal thickness in either inches or millimeters, bypassing gauge ambiguity entirely.
Can I use this calculator for tube and pipe bending?
No. Tube and pipe bending involves different geometry and different material behavior than sheet metal bending. In tube bending, the cross-section ovality, wall thinning on the outside of the bend, and wall wrinkling on the inside are the critical variables, not bend deduction and K-factor. Tube bending calculations use different formulas that account for the tube’s outer diameter, wall thickness, bend radius, and the interaction between these parameters. The parameters that determine whether a tube bend is feasible (minimum bend radius relative to tube diameter, required mandrel diameter, required wiper die radius) are specific to tube bending technology. For tube bending layout, consult resources from tube bending equipment manufacturers such as Ercolina, BendPak, or Huth, or the SME Tube Bending Guide.
How accurate is the bend deduction formula compared to actual press brake results?
When the correct K-factor is used (ideally calibrated from test bends in your specific material and thickness), the formula predicts flat blank length to within 0.002 to 0.005 inch per bend for most production scenarios. This is accurate enough for parts with tolerances of plus or minus 0.010 inch and better. For tighter tolerances (plus or minus 0.003 to 0.005 inch), empirical calibration is essential because small variations in material yield strength, die clearance, and tool wear can shift the effective K-factor enough to affect final part dimensions. For loose tolerances (plus or minus 0.030 inch or more), the published K-factor table values without calibration are typically sufficient. The formula becomes less reliable for very sharp bends (R less than T), very thick materials (over 0.375 inch), high-strength materials with significant springback, or bending near the minimum radius limit of the material where cracking becomes a concern.
What is an included angle and how is it different from the bend angle used in this calculator?
The bend angle (A) used in this calculator is the supplement of the included angle. If you bend a part so the two flanges form a 90-degree corner (a square bracket), the bend angle is 90 degrees. The included angle (the angle inside the part between the two flanges) is also 90 degrees in this case because 90 degrees is its own supplement. But for an obtuse bend: if you want two flanges that open to 120 degrees (included angle = 120 degrees), the bend angle = 180 minus 120 = 60 degrees. Press brake angle measurement systems vary: some measure the bend angle (how many degrees of bend you introduced), some measure the included angle (the angle between the flanges). Make sure you know which convention your press brake control uses, and enter the correct convention in this calculator. When in doubt: bend angle + included angle = 180 degrees. This calculator uses bend angle (degrees of bend applied to the flat sheet).
Where can I find official US standards for sheet metal fabrication?
The primary US standards and references for sheet metal bending are: Machinery’s Handbook (Industrial Press, 31st Edition) contains the sheet metal bending formulas, K-factor tables, and minimum bend radius guidelines. The SME Sheet Metal Forming Handbook (Society of Manufacturing Engineers) covers process theory and practical setup guidance. The Fabricators and Manufacturers Association International (FMA) publishes technical papers and standards for sheet metal shop practice. For aluminum specifically, The Aluminum Association publishes Aluminum Design Manual, which includes bend radius recommendations by alloy and temper. The ASTM standards for sheet metal material properties (ASTM A36 for carbon steel, ASTM B209 for aluminum sheet) provide material property data used to estimate K-factor and minimum bend radius.
What is a Y-factor and how does it relate to K-factor?
Y-factor (also called the ANSI Y-factor) is an alternative way to express the neutral axis location used in some sheet metal design software. The relationship between Y-factor and K-factor is: Y = K times pi divided by 2, or approximately Y = K times 1.5708. At K = 0.44 (CRS, air bend): Y = 0.44 x 1.5708 = 0.691. At K = 0.33 (soft aluminum): Y = 0.33 x 1.5708 = 0.518. The Y-factor formula for bend allowance is: BA = Y x (pi/180) x A x 2 x T… but this formulation is less intuitive and less directly tied to material properties than the K-factor version. Most modern US sheet metal software and calculation references use K-factor. If your design system asks for Y-factor, convert from K using Y = K x pi/2. Older Machinery’s Handbook editions (pre-1990s) used a different Y-factor convention; always check the edition’s own conversion tables if using older references.

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Legal Disclaimer and Editorial Transparency: Bend deduction and bend allowance values produced by this calculator implement the standard formulas from Machinery’s Handbook 31st Edition (Industrial Press) and the SME Sheet Metal Forming Handbook. K-factor presets are consensus air-bend values from published industry references. Actual flat pattern dimensions may vary from calculated values due to material variation, tooling condition, press brake calibration, springback, and bending method. Calibrate K-factor from test bends for production work requiring close tolerances. This calculator is provided for planning and layout reference only; verify all dimensions in the actual part before production runs. USCalculators.com is not affiliated with Industrial Press, SME, or the Fabricators and Manufacturers Association. Authority references: Industrial Press | SME | The Aluminum Association | NIST Manufacturing. Last updated August 2026.