Drill & Tap Size Chart
Tap drill sizes for UNC, UNF, and metric coarse threads at 75% thread engagement — the standard for general-purpose tapping. Clearance hole references included.
UNC (Unified Coarse) — Tap Drill Sizes
| Thread | Tap drill (75% thread) | Decimal | Close fit clearance | Free fit clearance |
|---|---|---|---|---|
| #4-40 | #43 | 0.0890 | #32 (0.116) | #30 (0.1285) |
| #6-32 | #36 | 0.1065 | #27 (0.144) | #25 (0.1495) |
| #8-32 | #29 | 0.1360 | #18 (0.1695) | #16 (0.177) |
| #10-24 | #25 | 0.1495 | #9 (0.196) | #7 (0.201) |
| 1/4-20 | #7 | 0.2010 | F (0.257) | H (0.266) |
| 5/16-18 | F | 0.2570 | P (0.323) | Q (0.332) |
| 3/8-16 | 5/16 | 0.3125 | W (0.386) | X (0.397) |
| 7/16-14 | U | 0.3680 | 29/64 (0.453) | 15/32 (0.469) |
| 1/2-13 | 27/64 | 0.4219 | 33/64 (0.516) | 17/32 (0.531) |
| 9/16-12 | 31/64 | 0.4844 | 37/64 (0.578) | 19/32 (0.594) |
| 5/8-11 | 17/32 | 0.5313 | 41/64 (0.641) | 21/32 (0.656) |
| 3/4-10 | 21/32 | 0.6563 | 49/64 (0.766) | 25/32 (0.781) |
| 7/8-9 | 49/64 | 0.7656 | 57/64 (0.891) | 29/32 (0.906) |
| 1-8 | 7/8 | 0.8750 | 1-1/64 (1.016) | 1-1/32 (1.031) |
| 1-1/8-7 | 63/64 | 0.9844 | 1-9/64 (1.141) | 1-5/32 (1.156) |
| 1-1/4-7 | 1-7/64 | 1.1094 | 1-17/64 (1.266) | 1-9/32 (1.281) |
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UNF (Unified Fine) — Tap Drill Sizes
| Thread | Tap drill | Decimal |
|---|---|---|
| #6-40 | #33 | 0.1130 |
| #8-36 | #29 | 0.1360 |
| #10-32 | #21 | 0.1590 |
| 1/4-28 | #3 | 0.2130 |
| 5/16-24 | I | 0.2720 |
| 3/8-24 | Q | 0.3320 |
| 7/16-20 | 25/64 | 0.3906 |
| 1/2-20 | 29/64 | 0.4531 |
| 9/16-18 | 33/64 | 0.5156 |
| 5/8-18 | 37/64 | 0.5781 |
| 3/4-16 | 11/16 | 0.6875 |
| 7/8-14 | 13/16 | 0.8125 |
| 1-12 | 59/64 | 0.9219 |
Metric Coarse — Tap Drill Sizes (mm)
| Thread | Pitch (mm) | Tap drill (mm) | Close clearance (mm) |
|---|---|---|---|
| M3 | 0.50 | 2.5 | 3.2 |
| M4 | 0.70 | 3.3 | 4.3 |
| M5 | 0.80 | 4.2 | 5.3 |
| M6 | 1.00 | 5.0 | 6.4 |
| M8 | 1.25 | 6.8 | 8.4 |
| M10 | 1.50 | 8.5 | 10.5 |
| M12 | 1.75 | 10.2 | 13.0 |
| M14 | 2.00 | 12.0 | 15.0 |
| M16 | 2.00 | 14.0 | 17.0 |
| M18 | 2.50 | 15.5 | 19.0 |
| M20 | 2.50 | 17.5 | 21.0 |
| M22 | 2.50 | 19.5 | 23.0 |
| M24 | 3.00 | 21.0 | 25.0 |
| M27 | 3.00 | 24.0 | 28.0 |
| M30 | 3.50 | 26.5 | 31.0 |
| M36 | 4.00 | 32.0 | 37.0 |
About thread engagement percentage
Tap drill diameter sets the percentage of theoretical thread depth you'll get:
- 75% engagement (default): standard for general purpose. Strong enough for most applications.
- 60–65%: common for tougher materials (stainless, hardened steel) where the smaller drill takes less torque to tap.
- 50%: only used on very hard materials and where strength isn't critical. Above this rate, you risk stripping.
Beyond 75% engagement you get diminishing returns — going to 100% theoretical only adds about 5% holding strength but dramatically increases tapping torque and risk of breaking a tap. 75% is the sweet spot.
Clearance hole fits
- Close fit: for bolts that are pinned or located precisely. The bolt body just fits the hole.
- Free fit: for general assembly — gives room for misalignment without binding.