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Tap Drill Size Calculator: UNC, UNF, UNEF, and Metric Thread Drill Sizes for US Machine Shops

Find the correct drill size before tapping any thread. Full UNC, UNF, UNEF, and metric series with adjustable thread engagement percentage. Returns decimal inch, fractional, letter, and number drill sizes plus metric equivalent. Free, no login required.

🪕 Full UNC Thread Series 🔢 UNF and UNEF Series 🌐 ISO Metric Threads 📈 Adjustable Engagement % 📄 PDF Drill Sheet 📨 WhatsApp Share
Thread Selection
Sizes populate automatically when you switch thread standards above.
75%
75% is US standard per Machinery’s Handbook. Use 65-70% for hard materials to reduce tap breakage.
Drill Size Results
🪕 Select a thread standard, choose a thread size, and click Find Tap Drill.
Tap Drill Size vs Thread Size for Current Series (Calculate to populate)

What Is a Tap Drill and Why Every Machinist Must Choose It Correctly

A tap drill is the drill bit you use to prepare a hole in a workpiece before running a tap through it to cut the threads. The tap drill creates a hole that is slightly smaller than the major diameter of the thread you are cutting, leaving enough material for the tap’s cutting edges to form the thread flanks. Get the drill size right and you have a strong, complete thread and a tap that turns freely through the hole. Get it wrong and you either snap a tap in an almost-finished part or cut threads so shallow they strip under the first load.

This is one of the most looked-up reference values in any US machine shop. The standard reference is Machinery’s Handbook, which includes complete tap drill charts for all Unified Inch and metric thread series. This calculator implements the same formulas: Tap Drill = Major Diameter minus (1.299038 times Thread Engagement percent divided by 100) divided by Threads per Inch for inch threads, and the equivalent formula using pitch for metric threads.

Thread Engagement: The Variable Every Tap Drill Chart Assumes

Thread engagement is the percentage of the theoretical full thread height that is actually formed in the tapped hole. The most widely used standard in US shops is 75 percent thread engagement, which is what Machinery’s Handbook tap drill tables and most published tap drill charts assume. At 75 percent engagement, you get:

  • Approximately the same holding strength as 100 percent engagement in most materials, because threaded joint failure occurs in the fastener shank or the first engaged thread, not across the full thread flank
  • Significantly lower tapping torque than 100 percent engagement, reducing tap breakage risk in production tapping operations
  • Greater tolerance for hole diameter variation without losing joint integrity

This calculator lets you adjust thread engagement from 50 to 100 percent. Use 65 to 70 percent for hard or tough materials like 17-4 PH stainless or titanium where tap breakage is a higher concern. Use 80 to 85 percent for softer materials like aluminum or plastic where thread stripping is the failure mode you are designing against.

Why Broken Taps Are the Most Expensive Mistake in a Machine Shop

A broken tap in a nearly finished part ranks among the most costly single events in machine shop production. When a tap snaps below the surface of a workpiece, you face three choices, none of them good: scrapping the part, attempting EDM (electrical discharge machining) tap removal, or drilling out and installing a thread insert like a Helicoil. EDM tap extraction requires specialized equipment, adds hours to the job, and is not available in every shop. A Helicoil insert changes the fastener specifications and may not be acceptable for the application. Scrapping the part means all machining time and material cost is lost. For high-value workpieces in titanium, stainless, or a complex multi-operation setup, a single broken tap can cost a hundred to several thousand dollars in rework or loss. Correct tap drill selection is the first and most effective prevention measure because the vast majority of tap breakage is caused by excessive tapping torque from an undersized drill hole.

UNC, UNF, and UNEF: The Three Unified Thread Series Used in US Shops

UNC (Unified National Coarse) is the most widely used thread series in the United States for general fastening. Fewer threads per inch means faster assembly, better resistance to cross-threading, and better thread engagement in ductile materials. A 1/4-20 UNC bolt (1/4 inch diameter, 20 threads per inch) is found in virtually every piece of US mechanical equipment.

UNF (Unified National Fine) has more threads per inch for the same nominal diameter. A 1/4-28 UNF bolt has 28 threads per inch, giving finer adjustment, better resistance to loosening under vibration, and better thread engagement in thin-walled sections. UNF is common in automotive, precision instrument, and aerospace fastening where vibration resistance matters.

UNEF (Unified National Extra Fine) is used in very thin-walled sections and for maximum thread engagement per unit length where wall thickness is severely limited. It is less common in general machining but important in aerospace and instrument applications.

All three series follow the same tap drill formula. The difference is TPI: the same nominal diameter drilled for UNF needs a slightly larger tap drill than for UNC because UNF has more threads per inch, meaning each individual thread is shallower relative to the pitch.

How to Read a Thread Callout on a US Engineering Drawing

US engineering drawings follow ASME Y14.5 and ANSI thread callout conventions. A thread callout on a tapped hole typically reads: 1/4-20 UNC-2B THRU, where 1/4 is the nominal diameter, 20 is the threads per inch, UNC is the thread series, 2B is the tolerance class (A for external/male threads, B for internal/female), and THRU means through the full thickness of the part. The tolerance class 2B is the most common for general-purpose tapped holes in US manufacturing. A 3B callout indicates a tighter tolerance for precision applications. Metric callouts on US drawings following ISO conventions read: M10 x 1.5 6H, where M10 is the nominal metric diameter, 1.5 is the pitch in millimeters, and 6H is the ISO tolerance class for internal threads. When you see a thread callout on a drawing, the first step before selecting a tap is to identify the nominal diameter and pitch (or TPI), which are all the formula needs to calculate the correct tap drill.

Metric ISO Thread Series for US Shops Working with International Drawings

Automotive and aerospace OEM customers frequently supply drawings with metric thread callouts per ISO 68-1. An M10 x 1.5 callout means 10mm nominal diameter with a 1.5mm pitch (distance between thread crests). The tap drill formula for metric threads uses pitch instead of TPI: Drill = Nominal Diameter minus (1.299038 times Engagement percent divided by 100) times Pitch. All values are in millimeters.

The widely cited shortcut in metric tapping is simply “drill diameter = nominal minus pitch,” which gives approximately 57 to 60 percent thread engagement for coarse metric threads. For stronger metric threads, the calculator lets you specify 75 percent engagement and returns the correct drill size for that target. The difference matters: for an M10 x 1.5, the D-P shortcut gives 8.5mm, while 75 percent engagement requires 8.54mm, which rounds to 8.5mm anyway in this case but diverges for coarser pitch ratios.

How the Tap Drill Size Calculator Works: Formula and Output Explained

Select your thread standard (UNC, UNF, UNEF, or Metric), pick the thread size from the dropdown, set your thread engagement percentage, and click Find Tap Drill. Here is what each output means.

Calculated Drill Size in Decimal Inches and Millimeters

The primary output is the theoretically correct drill size calculated from the formula. This is not a rounded value from a table: it is the exact result of the formula for the thread and engagement percentage you selected. You use this number to find the closest standard drill.

Recommended Drill vs Alternate Drill

The recommended drill is the nearest standard drill size at or below the calculated value. Using a drill that is equal to or slightly smaller than the calculated size ensures your tapped hole meets at least the specified thread engagement percentage. The alternate drill is the nearest standard drill above the calculated value, which gives slightly less engagement but may produce a cleaner tapped hole in some materials with thinner chips and less torque.

In practice, for most steel and aluminum tapping at 75 percent engagement, either the nearest below or nearest above will give a strong, functional thread. The choice depends on material, tap type, and how critical the joint strength is. For through-hole tapping in standard structural applications, the nearest above is often used to reduce tap breakage. For blind-hole tapping where engagement length is limited, the nearest below ensures you hit the 75 percent target.

The Drill Size Hierarchy: Fractional, Letter, Number, and Metric

US shops carry drills in four naming systems: Number drills (#80 smallest to #1 largest, covering 0.0135 to 0.228 inch) are the primary drill series for small tap sizes in #0 through #10 screws. Letter drills (A through Z, from 0.234 to 0.413 inch) fill the range between number drills and fractional drills. Fractional drills (1/64 through 4 inch in standard increments) cover the full range for larger tap sizes. Metric drills in millimeter increments are increasingly common in shops that work with both metric and inch threads.

This calculator returns the standard drill designation (e.g., “#7” for a 1/4-20 UNC tap at 75 percent) alongside the decimal inch and metric equivalents, giving you what you need whether you are pulling from a number drill index or a fractional drill set.

Three Real US Shop Examples: Tap Drill Selection in Practice

Example 1: Automotive Shop in Detroit, Michigan: 1/4-20 UNC in Mild Steel Bracket

A Detroit parts supplier is tapping 1/4-20 UNC threaded holes in a 1018 mild steel bracket for an exhaust heat shield assembly. Standard application, standard materials.

Formula: 0.250 minus (1.299038 x 0.75) / 20 = 0.250 minus 0.04871 = 0.2013 inch.

Nearest drill below 0.2013″: #7 drill (0.2010″). This is the standard recommendation found in every US tap drill chart for 1/4-20 UNC. The #7 is 0.0003″ below the calculated value, giving 75 percent engagement. The shop buys cobalt HSS #7 drills in packs of ten and taps the holes on a drill press with cutting oil, producing clean threads that pass go/no-go gauge inspection.

Example 2: Aerospace Supplier in Wichita, Kansas: M8 x 1.25 in Titanium Ti-6Al-4V

A Wichita aerospace machining house needs M8 x 1.25 threaded holes in a Ti-6Al-4V structural fitting for a fixed-wing aircraft. Titanium requires reduced engagement to protect the tap.

At 70% engagement (reduced to protect the tap in titanium): 8.0 minus (1.299038 x 0.70) x 1.25 = 8.0 minus 1.1367 = 6.863mm.

Nearest drill below 6.863mm: 6.8mm drill. The shop uses a solid carbide drill with through-spindle coolant, running at 50 SFM to minimize work-hardening. The reduced engagement at 70 percent still produces threads that meet the design strength requirement per the stress analysis, while the slightly larger hole reduces tapping torque by approximately 15 percent, dramatically improving tap life in this difficult alloy.

Example 3: General Machine Shop in Columbus, Ohio: 1/2-20 UNF in Hydraulic Manifold

A Columbus hydraulic component shop is tapping 1/2-20 UNF port threads in a 6061-T6 aluminum manifold block. Fine thread, soft material, full engagement required for hydraulic sealing.

At 80% engagement (increased for hydraulic sealing integrity): 0.500 minus (1.299038 x 0.80) / 20 = 0.500 minus 0.05196 = 0.4480″.

Nearest drill below 0.4480″: 29/64″ drill (0.4531″) is the nearest above. At 75%, standard charts show 29/64″ anyway. The shop uses a spiral flute tap with aluminum-specific TiN coating at 200 SFM, producing a clean thread that seals reliably with an O-ring face seal fitting at 3,000 PSI working pressure.

Five Expert Tapping Tips for US Machine Shop Applications

🦋
Never go larger than the calculated drill to “make tapping easier”

A common mistake is upsizing the tap drill when a tap feels stiff, thinking the larger hole will reduce torque. A drill that is too large reduces thread engagement to the point where the threads can strip under load. The correct response to a stiff tap is to use cutting fluid, reduce speed, use a more appropriate tap geometry for the material, or reduce engagement percent by 5 to 10 points and recalculate. Never arbitrarily upsize the drill without recalculating engagement.

🔢
Check your drill diameter before tapping, not after the tap breaks

Drill bits wear and sometimes break in use, producing a hole that is undersized. Measure the drill point diameter with a micrometer or drill gauge before tapping, particularly for production runs. A drill that is 0.003 to 0.005 inches undersized from wear can increase tapping torque enough to snap a smaller tap. For critical tapping operations in hard materials, replace drill bits on a scheduled interval rather than running them to failure.

🔥
Reduce engagement to 65% for stainless steel, titanium, and Inconel

Work-hardening alloys like 304/316 stainless, 17-4 PH, Ti-6Al-4V, and Inconel 718 generate significantly higher tapping torques than plain steel at the same thread engagement. Using the calculator at 65 to 70 percent engagement rather than 75 percent produces a tap drill that is 0.003 to 0.010 inches larger for most sizes, reducing tapping torque by 10 to 20 percent. This trade-off is well established in the machining literature: the slight reduction in thread engagement is compensated by the dramatically higher tool life and lower tap breakage rate. For Inconel, some shops go as low as 60 percent engagement with carbide spiral-point taps.

📄
Save the PDF drill sheet with your job traveler

Use the PDF Drill Sheet button to generate a printed record showing the thread specification, calculated drill size, recommended drill, and engagement percentage. Attach it to the job traveler for each tapping operation. When a job repeats in six months, you have the exact drill specification documented without recalculating. For shops running multiple thread sizes on the same part, the PDF gives the operator a complete reference at the machine.

🌐
For metric threads on US drawings, always verify pitch in the callout

A metric thread callout without a pitch (e.g., just “M10”) implies coarse pitch (M10 x 1.5 in this case), but fine pitch variants exist (M10 x 1.25, M10 x 1.0). The tap drill size changes significantly: M10 x 1.5 at 75 percent requires 8.54mm, while M10 x 1.0 at 75 percent requires 9.03mm, a difference of nearly 0.5mm. Always confirm the pitch from the drawing or material specification before selecting the tap drill, and verify that the tap you are using matches both diameter and pitch before tapping. Pitch mismatch between tap and thread callout is a costly and common error when shops encounter metric threads infrequently.

Quick Reference Tap Drill Chart for the Most Common US Thread Sizes

This table shows the standard tap drill recommendations for the most frequently used thread sizes in US production and job shop environments at 75 percent thread engagement. Values match Machinery’s Handbook 31st Edition.

ThreadSeriesMajor Dia (in)TPICalc Drill (in)Recommended DrillMetric Equiv
#4-40UNC0.1120400.0876#43 (0.0890″)2.26mm
#6-32UNC0.1380320.1076#36 (0.1065″)2.74mm
#8-32UNC0.1640320.1336#29 (0.1360″)3.40mm
#10-24UNC0.1900240.1559#23 (0.1540″)3.96mm
#10-32UNF0.1900320.1596#21 (0.1590″)4.06mm
1/4-20UNC0.2500200.2013#7 (0.2010″)5.11mm
1/4-28UNF0.2500280.2152#3 (0.2130″)5.47mm
5/16-18UNC0.3125180.2603F (0.2570″)6.61mm
3/8-16UNC0.3750160.31885/16″ (0.3125″)8.10mm
1/2-13UNC0.5000130.425127/64″ (0.4219″)10.80mm
1/2-20UNF0.5000200.451329/64″ (0.4531″)11.46mm
3/4-10UNC0.7500100.652621/32″ (0.6563″)16.58mm
M6 x 1.0Metric6.0mm1.0 pitch5.026mm5.0mm drill0.197″
M8 x 1.25Metric8.0mm1.25 pitch6.782mm6.8mm drill0.268″
M10 x 1.5Metric10.0mm1.5 pitch8.539mm8.5mm drill0.335″
M12 x 1.75Metric12.0mm1.75 pitch10.295mm10.3mm drill0.406″

Source: Machinery’s Handbook 31st Edition (Industrial Press) | ANSI/ASME B1.1 | ISO 68-1 Metric Threads

Tap Drill Selection: 16 Questions from the Shop Floor

What is the standard tap drill formula used in US machine shops?
For Unified Inch threads (UNC, UNF, UNEF): Tap Drill = Major Diameter minus (0.9743 / TPI) at 75 percent thread engagement. The constant 0.9743 equals 1.299038 x 0.75, where 1.299038 is twice the thread height constant for a 60-degree thread profile (2 x 0.649519). For metric ISO threads: Tap Drill (mm) = Nominal Diameter minus 0.9743 x Pitch, where pitch is in millimeters. This formula is from ANSI/ASME B1.1 and Machinery’s Handbook and produces the drill size that leaves 75 percent of the theoretical full thread height in the hole, which is the US industry standard for general tapping applications.
Why is 75 percent thread engagement the US standard rather than 100 percent?
Research on threaded joint strength shows that increasing thread engagement beyond 75 percent provides negligible additional holding strength while significantly increasing tapping torque and tap breakage risk. At 75 percent engagement, the threaded joint fails in the fastener body under tension, not in the thread flanks. Since the fastener is the weaker element, adding more thread depth in the tapped hole does not change the joint’s load capacity. Reducing from 100 percent to 75 percent engagement reduces tapping torque by approximately 25 percent in steel, which meaningfully extends tap life and reduces breakage in production environments. The 75 percent standard is documented in Machinery’s Handbook (Industrial Press), ANSI/ASME B1.1, and is universally assumed in commercial tap drill charts throughout the United States.
How do I choose between the nearest drill above or below the calculated size?
The nearest drill below the calculated size gives at least the specified engagement (conservative choice). The nearest drill above gives slightly less engagement but more chip clearance and lower tapping torque (productive choice). For most general-purpose tapping in steel, cast iron, and brass, either drill produces a functional thread: the difference in drill size is typically 0.001 to 0.003 inch, which translates to only 1 to 3 percentage points of engagement. For safety-critical fastener applications, tapped holes with minimum engagement requirements, or any situation where you need to certify thread engagement, use the nearest below. For production tapping where tool life matters, the nearest above (or even 65 to 70 percent engagement) reduces tap breakage while still producing threads that pass standard go/no-go gauge inspection.
What is the tap drill for a 1/4-20 UNC thread and why is it a #7?
The calculated tap drill for a 1/4-20 UNC thread at 75 percent engagement is 0.2500 minus (0.9743 / 20) = 0.2500 minus 0.04872 = 0.2013 inches. A #7 number drill is 0.2010 inches, which is 0.0003 inches below the calculated value, making it the nearest standard drill at or below the target. This 0.0003-inch difference amounts to less than half a percent of change in thread engagement, which is not measurable in practice. The #7 has been the standard recommendation for 1/4-20 UNC tapping in US shop practice for over a century. It is the most commonly purchased number drill in American job shops precisely because 1/4-20 is the most widely used US fastener thread.
What happens if I use a drill that is too small for a tap?
A hole that is too small generates excessive tapping torque because the tap must displace or cut more material to form the thread. In most materials, the tap will either break before completing the hole or the threads will tear out of the workpiece wall as the tap is advanced. A broken tap in a blind hole or a part with tight tolerances is often unrecoverable without EDM (electrical discharge machining) tap extraction, which is expensive and sometimes impossible depending on part geometry. Even if the tap completes the hole, the excessive torque may fatigue the tap and cause early failure on subsequent holes. The correct action when a tap feels unexpectedly stiff is to stop, back out the tap, and verify the drill size with a micrometer or dowel gauge before continuing.
Can I use the same tap drill size for both through holes and blind holes?
The tap drill size is the same for through and blind holes, but the drill depth for a blind hole must account for the tap point that does not cut threads. A standard spiral point plug tap has a chamfer of 3 to 5 thread pitches at the point that sizes the entry before cutting full threads. If your blind hole is only as deep as the thread length you need, the drill hole must be 5 to 7 thread pitches deeper than the required thread engagement length to allow for the tap point and chip clearing. The rule of thumb for blind tapping: drill depth = required thread depth plus 5 times the pitch (metric) or 5 divided by TPI (inch). Failure to drill deep enough for a blind tapped hole results in the tap bottoming out before reaching full depth, with likely tap breakage.
What is the difference between a spiral point tap and a spiral flute tap?
Spiral point taps (also called gun taps) have a straight flute with a spiral-ground point that pushes chips forward ahead of the tap. They are ideal for through-hole tapping where chips can exit through the back of the hole, running at higher speeds than other tap types. Spiral flute taps have helical flutes that carry chips back up out of the hole as the tap advances. They are the correct choice for blind hole tapping in soft and stringy materials like aluminum, stainless steel, and copper, where chips cannot be pushed through. Hand taps (with straight flutes) are general-purpose taps for both through and blind holes but run at lower speeds. Selecting the correct tap type for your hole type and material matters as much as selecting the correct drill size: the best drill selection does not compensate for the wrong tap geometry.
How do I find the tap drill for a thread size not listed in standard charts?
Use the formula directly: Tap Drill (inches) = Major Diameter minus (1.299038 x desired engagement percent divided by 100) divided by TPI. For any thread size, you need the major diameter and threads per inch, both of which are in the thread callout or in the ANSI/ASME B1.1 standard for Unified threads. For metric threads, use pitch instead of TPI: Tap Drill (mm) = Nominal Diameter minus 1.299038 x engagement fraction x pitch. This calculator handles the full range of standard sizes and any engagement percentage from 50 to 100 percent. For non-standard thread forms (ACME, NPT pipe threads, buttress threads), the formula constants are different; ACME tap drills are covered in Machinery’s Handbook Section on Power Screws.
What is the standard tap drill for metric M10 x 1.5 and why is it 8.5mm?
At 75 percent engagement, the calculated tap drill for M10 x 1.5 is: 10.0 minus (1.299038 x 0.75 x 1.5) = 10.0 minus 1.461 = 8.539mm. The nearest standard metric drill below 8.539mm is 8.5mm, which is the universal recommendation for M10 x 1.5 tapping in virtually all published metric tap drill charts. The commonly used shortcut of “drill = nominal minus pitch” (10 minus 1.5 = 8.5mm) happens to produce the same result in this case because the rounding is in the same direction. However, for coarser relative pitches (where pitch is large relative to nominal diameter), the shortcut can diverge from the 75 percent engagement formula. Using the formula-based calculator rather than the D-P shortcut gives the correct drill size for any combination, including fine-pitch metric threads where the shortcut can oversize the hole.
Do I need to adjust tap drill size for tapping in cast iron versus steel?
The tap drill formula does not change by material: the calculation depends only on thread dimensions and desired engagement. What changes by material is the practical target engagement percentage and the tap geometry selection. In gray cast iron, the material is brittle and self-lubricating: 75 percent engagement is fine, and standard taps cut cleanly with minimal cutting fluid. In ductile iron and malleable iron, tapping is similar to mild steel. In stainless steel, reducing engagement to 65 to 70 percent reduces the risk of tap galling and breakage. In aluminum, 75 to 80 percent engagement is appropriate since aluminum threads can strip under load more easily than steel threads due to the lower shear strength of the material. The calculator lets you adjust engagement to match your material, while the formula and drill lookup remain consistent.
What is a go/no-go thread gauge and how does it verify tap drill selection?
A go/no-go plug gauge is the standard inspection tool for verifying tapped hole dimensions. The “go” member is slightly undersize and must screw in freely for a minimum number of turns, confirming that the tap produced threads of sufficient size and pitch. The “no-go” member is slightly oversize and must not screw in more than two turns, confirming that the thread is not oversized. If the go member does not enter, the hole is undertapped (tap drill too small or tap not going in far enough). If the no-go member enters, the hole is oversized, which typically means the drill was too large, the tap is worn oversize, or the material was overcut. In the US, thread gauges for Unified inch threads follow ANSI/ASME B1.2 tolerance classes. For metric threads, ISO 1502 gauge standards apply. Correct tap drill selection using the formula and a sharp, on-size drill should consistently produce threads that pass both members of a class 2B go/no-go gauge, which is the standard tolerance class for most US general-purpose threaded fastener applications.
What cutting fluid should I use for tapping in different materials?
Cutting fluid selection for tapping is as important as tap drill selection in difficult materials. For carbon steel (1018, 1045, 4140): sulfurized cutting oil or tapping compound provides best lubrication and thread finish. For stainless steel (304, 316, 17-4 PH): sulfurized or chlorinated tapping compound; never use dry or water-based fluid since it causes galling. For aluminum and aluminum alloys: kerosene-based tapping fluid or WD-40 for light tapping; aluminum-specific tapping compounds for production volumes. For titanium and Inconel: heavy chlorinated tapping compound; proper lubrication is critical to preventing tap seizure and work hardening. For cast iron: dry tapping is common since cast iron is self-lubricating from graphite content; light oil or compressed air for chip clearing. For plastic (Delrin, nylon, HDPE): dry tapping works well; light oil if needed for heat control at higher speeds. The Society of Manufacturing Engineers (SME) Machining Data Handbook provides comprehensive tapping fluid recommendations by material.
How does thread pitch affect the tap drill size when comparing UNC to UNF for the same nominal diameter?
For the same nominal diameter, a finer pitch (UNF) requires a larger tap drill than coarse pitch (UNC), even though you might expect the opposite. The reason is in the formula: Drill = Major – (0.9743 / TPI). Higher TPI (finer pitch) makes the denominator larger, making 0.9743/TPI smaller, so the drill size increases. For 1/4 inch: UNC (20 TPI) drill = 0.250 – 0.9743/20 = 0.250 – 0.04872 = 0.2013″. UNF (28 TPI) drill = 0.250 – 0.9743/28 = 0.250 – 0.03480 = 0.2152″. The UNF tap drill is 0.014″ larger. This makes sense physically: a finer pitch means each thread is shallower (smaller height per thread), so you do not need to drill as far below the major diameter to get the same engagement percentage.
What is a bottoming tap and when do I need a different drill for it?
A bottoming tap has a very short chamfer (1 to 1.5 threads) so it can cut threads nearly to the bottom of a blind hole. It requires a pilot hole that has already been threaded with a plug or taper tap: you use plug or taper tap first to cut threads through most of the hole depth, then finish to the bottom with the bottoming tap. The tap drill size is the same regardless of which tap type you use (taper, plug, or bottoming), because the drill creates the hole before any tapping begins and the drill diameter determines the thread engagement. What changes with a bottoming tap setup is the drill depth: you must drill deeper than the finished thread depth to ensure the tap can reach the required depth with a 1.5-thread chamfer rather than the 3 to 5 threads of a plug tap. The drill depth calculation: required thread depth plus 2 times pitch clearance at the bottom for the bottoming tap to seat and cut full threads.
Can this calculator be used for pipe threads (NPT) or ACME threads?
This calculator covers Unified Inch threads (UNC, UNF, UNEF) and ISO Metric threads (ISO 68-1), which are straight parallel threads. NPT (National Pipe Taper) threads are tapered at 3/4 inch per foot and use completely different tap drill tables, because the thread is self-sealing through taper wedge action rather than engagement depth. NPT tap drill sizes are listed in ASME B1.20.1 and Machinery’s Handbook under pipe threads. ACME threads (used for power screws and lead screws) have a 29-degree thread form with much deeper threads than Unified threads; their tap drill formula uses different constants. Buttress threads and square threads are also outside the scope of this calculator. For NPT and ACME tap drill sizes, consult Machinery’s Handbook Section 8 (Screw Thread Systems) directly from Industrial Press or the relevant ASME standard.
Where can I find the official ANSI and ISO thread standards for US shops?
The authoritative sources for US thread standards are: ASME publishes ANSI/ASME B1.1 (Unified Inch Screw Threads), B1.13M (Metric Screw Threads), and B1.20.1 (Pipe Threads). These standards define all thread dimensions, tolerance classes, and recommended limits for tap drill sizes. Machinery’s Handbook (Industrial Press, 31st Edition) consolidates these standards into practical shop reference tables including the complete UNC/UNF/UNEF tap drill chart and metric tap drill charts. For metric threads, ISO 68-1 is the international standard for metric thread geometry, and ISO 262 specifies the preferred metric thread series. ANSI/ASME B1.1 is available for purchase from ASME; Machinery’s Handbook (available from Industrial Press and most technical bookstores) is the practical shop reference that most US machinists and engineers use day-to-day.

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Legal Disclaimer and Editorial Transparency: Tap drill sizes calculated by this tool implement the ANSI/ASME B1.1 formula for 60-degree Unified Inch threads and the ISO 68-1 equivalent for metric threads, sourced from Machinery’s Handbook 31st Edition (Industrial Press). Results are for planning and setup reference and should be verified against the applicable thread standard and tooling manufacturer recommendations before production tapping. Thread engagement percentage, material properties, tap geometry, and hole surface finish all affect tapped joint strength; consult a qualified manufacturing engineer for safety-critical threaded joint design. USCalculators.com is not affiliated with ASME, ISO, or Industrial Press. Authority references: ASME B1.1 Thread Standard | Machinery’s Handbook | NIST Manufacturing. Last updated August 2026.