The tap drill is the hole you drill before cutting a thread with a tap. Too small and the tap binds or breaks; too large and the thread is shallow and strips. This chart gives, for every common inch and metric thread, the drill that produces about 75% of full thread depth and the percentage you actually get with the nearest number, letter, fractional and metric drills, so you can choose from the drills you own. For a size not listed, or a different percentage, use the tap drill size calculator.
Tap drill size by thread
Each of these pages gives the drill for one thread with every nearby drill, the clearance hole, blind-hole depth and wrench size:
- 1/2-13 tap drill size
- 3/8-16 tap drill size
- 5/16-18 tap drill size
- 10-24 tap drill size
- 10-32 tap drill size
- 8-32 tap drill size
- 6-32 tap drill size
- 4-40 tap drill size
- M3 x 0.5 tap drill size
- M4 x 0.7 tap drill size
- M5 x 0.8 tap drill size
- M6 x 1 tap drill size
- M8 x 1.25 tap drill size
- M10 x 1.5 tap drill size
- M12 x 1.75 tap drill size
- 3/4-10 tap drill size
- 1/4-28 tap drill size
- 3/8-24 tap drill size
- 1/2-20 tap drill size
- 5/8-11 tap drill size
- 7/16-14 tap drill size
- 1-8 tap drill size
- 1/4-20 tap drill size
Inch threads (UNC and UNF)
| Thread | 75% drill (in) | Closest standard drill | Closest fractional | Closest metric |
|---|---|---|---|---|
| #0-80 UNF | 0.0478 | 3/64" = 81% | 3/64" = 81% | 1.2 mm = 79% |
| #1-64 UNC | 0.0578 | #53 = 67% | 1/16" = 52% | 1.5 mm = 69% |
| #1-72 UNF | 0.0595 | #53 = 75% | 1/16" = 58% | 1.5 mm = 77% |
| #2-56 UNC | 0.0686 | #50 = 69% | 1/16" = 101% | 1.7 mm = 82% |
| #2-64 UNF | 0.0708 | #50 = 79% | 5/64" = 39% | 1.8 mm = 75% |
| #3-48 UNC | 0.0787 | #47 = 76% | 5/64" = 77% | 2 mm = 75% |
| #3-56 UNF | 0.0816 | #45 = 73% | 5/64" = 90% | 2.1 mm = 70% |
| #4-40 UNC | 0.0876 | #43 = 71% | 3/32" = 56% | 2.2 mm = 78% |
| #4-48 UNF | 0.0917 | #42 = 68% | 3/32" = 67% | 2.3 mm = 79% |
| #5-40 UNC | 0.1006 | #38 = 72% | 3/32" = 96% | 2.6 mm = 70% |
| #5-44 UNF | 0.1029 | #37 = 71% | 7/64" = 53% | 2.6 mm = 77% |
| #6-32 UNC | 0.1076 | #36 = 78% | 7/64" = 71% | 2.7 mm = 78% |
| #6-40 UNF | 0.1136 | #33 = 77% | 7/64" = 88% | 2.9 mm = 73% |
| #8-32 UNC | 0.1336 | #29 = 69% | 9/64" = 58% | 3.4 mm = 74% |
| #8-36 UNF | 0.1369 | #29 = 78% | 9/64" = 65% | 3.5 mm = 73% |
| #10-24 UNC | 0.1494 | #25 = 75% | 5/32" = 62% | 3.8 mm = 75% |
| #10-32 UNF | 0.1596 | #21 = 76% | 5/32" = 83% | 4.1 mm = 70% |
| #12-24 UNC | 0.1754 | #16 = 72% | 11/64" = 82% | 4.5 mm = 72% |
| #12-28 UNF | 0.1812 | #14 = 73% | 3/16" = 61% | 4.6 mm = 75% |
| 1/4-20 UNC | 0.2013 | #7 = 75% | 13/64" = 72% | 5.1 mm = 76% |
| 1/4-28 UNF | 0.2152 | #3 = 80% | 7/32" = 67% | 5.5 mm = 72% |
| 5/16-18 UNC | 0.2584 | F = 77% | 17/64" = 65% | 6.6 mm = 73% |
| 5/16-24 UNF | 0.2719 | I = 75% | 17/64" = 87% | 6.9 mm = 75% |
| 3/8-16 UNC | 0.3141 | 5/16" = 77% | 5/16" = 77% | 8 mm = 74% |
| 3/8-24 UNF | 0.3344 | Q = 79% | 21/64" = 87% | 8.5 mm = 75% |
| 7/16-14 UNC | 0.3679 | U = 75% | 3/8" = 67% | 9.3 mm = 77% |
| 7/16-20 UNF | 0.3888 | 25/64" = 72% | 25/64" = 72% | 9.9 mm = 73% |
| 1/2-13 UNC | 0.4251 | 27/64" = 78% | 27/64" = 78% | 10.8 mm = 75% |
| 1/2-20 UNF | 0.4513 | 29/64" = 72% | 29/64" = 72% | 11.5 mm = 73% |
| 9/16-12 UNC | 0.4813 | 31/64" = 72% | 31/64" = 72% | 12.2 mm = 76% |
| 9/16-18 UNF | 0.5084 | 33/64" = 65% | 33/64" = 65% | 12.9 mm = 76% |
| 5/8-11 UNC | 0.5364 | 17/32" = 79% | 17/32" = 79% | 13.6 mm = 76% |
| 5/8-18 UNF | 0.5709 | 37/64" = 65% | 37/64" = 65% | 14.5 mm = 75% |
| 3/4-10 UNC | 0.6526 | 21/32" = 72% | 21/32" = 72% | 16.6 mm = 74% |
| 3/4-16 UNF | 0.6891 | 11/16" = 77% | 11/16" = 77% | 17.5 mm = 75% |
| 7/8-9 UNC | 0.7668 | 49/64" = 76% | 49/64" = 76% | 19.5 mm = 74% |
| 7/8-14 UNF | 0.8054 | 13/16" = 67% | 13/16" = 67% | 20.5 mm = 73% |
| 1-8 UNC | 0.8782 | 7/8" = 77% | 7/8" = 77% | 22.3 mm = 75% |
| 1-12 UNF | 0.9188 | 59/64" = 72% | 59/64" = 72% | 23.3 mm = 76% |
| 1-1/8-7 UNC | 0.9858 | 63/64" = 76% | 63/64" = 76% | 25 mm = 76% |
| 1-1/8-12 UNF | 1.0438 | 1-3/64" = 72% | 1-3/64" = 72% | 26.5 mm = 75% |
| 1-1/4-7 UNC | 1.1108 | 1-7/64" = 76% | 1-7/64" = 76% | 28.2 mm = 75% |
| 1-1/4-12 UNF | 1.1688 | 1-11/64" = 72% | 1-11/64" = 72% | 29.7 mm = 75% |
| 1-3/8-6 UNC | 1.2126 | 1-7/32" = 72% | 1-7/32" = 72% | 30.8 mm = 75% |
| 1-3/8-12 UNF | 1.2938 | 1-19/64" = 72% | 1-19/64" = 72% | 32.9 mm = 74% |
| 1-1/2-6 UNC | 1.3376 | 1-11/32" = 72% | 1-11/32" = 72% | 34 mm = 75% |
| 1-1/2-12 UNF | 1.4188 | 1-27/64" = 72% | 1-27/64" = 72% | 36 mm = 76% |
Metric threads
| Thread | Tap drill | Thread % | Nearest inch drill |
|---|---|---|---|
| M1 x 0.25 | 0.75 mm | 77% | #69 (0.0292 in) = 80% |
| M1.2 x 0.25 | 0.95 mm | 77% | #63 (0.0370 in) = 80% |
| M1.6 x 0.35 | 1.25 mm | 77% | 3/64" (0.0469 in) = 90% |
| M2 x 0.4 | 1.6 mm | 77% | 1/16" (0.0625 in) = 79% |
| M2.5 x 0.45 | 2.05 mm | 77% | #46 (0.0810 in) = 76% |
| M3 x 0.5 | 2.5 mm | 77% | #40 (0.0980 in) = 79% |
| M3.5 x 0.6 | 2.9 mm | 77% | #33 (0.1130 in) = 81% |
| M4 x 0.7 | 3.3 mm | 77% | #30 (0.1285 in) = 81% |
| M5 x 0.8 | 4.2 mm | 77% | #19 (0.1660 in) = 75% |
| M6 x 1 | 5 mm | 77% | #9 (0.1960 in) = 79% |
| M8 x 1.25 | 6.75 mm | 77% | 17/64" (0.2656 in) = 77% |
| M8 x 1 | 7 mm | 77% | J (0.2770 in) = 74% |
| M10 x 1.5 | 8.5 mm | 77% | Q (0.3320 in) = 80% |
| M10 x 1.25 | 8.75 mm | 77% | 11/32" (0.3438 in) = 78% |
| M10 x 1 | 9 mm | 77% | T (0.3580 in) = 70% |
| M12 x 1.75 | 10.25 mm | 77% | Y (0.4040 in) = 76% |
| M12 x 1.5 | 10.5 mm | 77% | Z (0.4130 in) = 77% |
| M12 x 1.25 | 10.75 mm | 77% | 27/64" (0.4219 in) = 79% |
| M14 x 2 | 12 mm | 77% | 15/32" (0.4688 in) = 81% |
| M14 x 1.5 | 12.5 mm | 77% | 31/64" (0.4844 in) = 87% |
| M16 x 2 | 14 mm | 77% | 35/64" (0.5469 in) = 81% |
| M16 x 1.5 | 14.5 mm | 77% | 37/64" (0.5781 in) = 68% |
| M18 x 2.5 | 15.5 mm | 77% | 39/64" (0.6094 in) = 78% |
| M18 x 1.5 | 16.5 mm | 77% | 21/32" (0.6562 in) = 68% |
| M20 x 2.5 | 17.5 mm | 77% | 11/16" (0.6875 in) = 78% |
| M20 x 1.5 | 18.5 mm | 77% | 47/64" (0.7344 in) = 69% |
| M22 x 2.5 | 19.5 mm | 77% | 49/64" (0.7656 in) = 79% |
| M22 x 1.5 | 20.5 mm | 77% | 13/16" (0.8125 in) = 70% |
| M24 x 3 | 21 mm | 77% | 53/64" (0.8281 in) = 76% |
| M24 x 2 | 22 mm | 77% | 55/64" (0.8594 in) = 84% |
| M27 x 3 | 24 mm | 77% | 15/16" (0.9375 in) = 82% |
| M27 x 2 | 25 mm | 77% | 63/64" (0.9844 in) = 77% |
| M30 x 3.5 | 26.5 mm | 77% | 1-3/64" (1.0469 in) = 75% |
| M30 x 2 | 28 mm | 77% | 1-7/64" (1.1094 in) = 70% |
| M36 x 4 | 32 mm | 77% | 1-17/64" (1.2656 in) = 74% |
| M36 x 3 | 33 mm | 77% | 1-19/64" (1.2969 in) = 79% |
The formula, with a worked example
Tap drill = major diameter − (0.01299 x percent thread) / TPI. For #10-32 at 75%: 0.1900 − (0.01299 x 75) / 32 = 0.1900 − 0.0304 = 0.1596 in. The closest standard drill is #21 (0.1590 in), giving 76% thread. Turn the formula round to check any drill: percent thread = (major − drill) x TPI / 0.01299.
For 5/16-18: 0.3125 − (0.01299 x 75) / 18 = 0.2584 in. That falls between letter drills, and the closest is F (0.2570 in) at 77%; the fractional 17/64″ gives 65%, which is why charts usually list F for 5/16-18.
Choosing the percentage of thread
- 75% is the normal target in steel and aluminium. A full 100% thread is only about 5% stronger but needs roughly three times the tapping torque.
- 60 to 70% in stainless steel, hard alloys and for small taps (#6 and below, M3 and below), where breaking the tap is the main risk. With enough length of engagement (typically at least one bolt diameter in steel) the bolt usually breaks before a thread like this strips.
- 80 to 85% in soft materials such as brass, cast aluminium and plastics, and in thin sheet where only a few threads engage.
- Thread-forming (roll) taps need a larger hole than cutting taps, because they push the metal rather than cut it. Use the tap maker’s drill size for those, not this chart.
Common mistakes
- Using the bolt’s nominal diameter minus one thread depth: that is the 100% thread size, too tight for a hand tap.
- Tapping a blind hole to the bottom. Drill deeper than the thread you need (see the tapped hole depth calculator) so the tap’s tapered lead and the chips have room.
- Not chamfering the hole. A small countersink at the top helps the tap start square and stops a raised burr.
Frequently asked questions
What size drill for a 1/4-20 tap?
A #7 drill (0.201 in), which gives about 75% thread. The fractional substitute is 13/64 in. See our 1/4-20 tap drill guide for all the options.
What size drill for a 10-32 tap?
A #21 drill (0.1590 in) for about 76% thread; 5/32 in is the fractional alternative at 83%.
What is the tap drill for M6?
5.0 mm for M6 x 1 (6 − 1 = 5). For fine metric threads see our metric fine thread tap drill chart.
Why do tap drill charts disagree?
Charts are built for different thread percentages (anything from 65% to 83%) and round to different drill series. This chart shows the percentage for each drill so you can see the trade-off.
Sources and method
Every dimension on this page is computed from the formulas below, not copied from another chart, so you can check any row by hand. Values are basic (nominal) dimensions; real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric) and are slightly smaller (bolts) or larger (nuts and tapped holes).
- ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form): basic profile, pitch-diameter and minor-diameter relationships, and the tensile stress area formula. The same relationships are published in the US federal standard FED-STD-H28/2B, Screw-Thread Standards for Federal Services, Section 2 (a US Government work).
- ISO 68-1, ISO general purpose screw threads, basic profile; ISO 261, general plan of diameters and pitches; ISO 898-1, stress area of metric bolts.
- ASME B94.11M, Twist Drills: number, letter and fractional drill diameters (the drill list used by our calculators).
- ISO 273, Fasteners, clearance holes for bolts and screws (fine, medium, coarse series); inch close / normal / loose fits as published in the ASME B18.2.8 clearance classes and reproduced in fastener suppliers’ charts.
- Percentage-of-thread formula for tap drills: Machinery’s Handbook (Industrial Press), as used by our tap drill size calculator.
More thread and fastener tools
- All thread and fastener charts
- UNC and UNF thread chart
- Metric thread pitch chart
- Tap drill chart (inch and metric)
- Clearance hole chart
- Nail size chart
- Wood screw size chart
- Lag screw pilot hole chart
- Bolt torque chart
- Wrench size chart
- Drill bit size chart
- Thread engagement chart
- Nail gun nail sizes by gauge
- Screw and nail size calculator
- Tap drill size calculator
- Clearance hole size calculator
- Tapped hole depth calculator
Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.