The tap drill is the hole you drill before cutting an internal thread. Drill it too large and the thread has nothing to bite; too small and you snap the tap. The sizes below give roughly 75% thread engagement, which is the standard compromise between strength and tapping torque.
For metric threads the rule is simple arithmetic:
Tap drill (mm) = major diameter − pitch
M6 with a 1.0 mm pitch needs a 5.0 mm drill. Imperial sizes do not follow a clean formula, which is why the charts exist.
Metric coarse
| Thread | Pitch (mm) | Tap drill (mm) | Clearance drill (mm) |
|---|---|---|---|
| M1.6 | 0.35 | 1.25 | 1.8 |
| M2 | 0.4 | 1.6 | 2.4 |
| M2.5 | 0.45 | 2.05 | 2.9 |
| M3 | 0.5 | 2.5 | 3.4 |
| M3.5 | 0.6 | 2.9 | 3.9 |
| M4 | 0.7 | 3.3 | 4.5 |
| M5 | 0.8 | 4.2 | 5.5 |
| M6 | 1.0 | 5.0 | 6.6 |
| M8 | 1.25 | 6.8 | 9.0 |
| M10 | 1.5 | 8.5 | 11.0 |
| M12 | 1.75 | 10.2 | 13.5 |
| M14 | 2.0 | 12.0 | 15.5 |
| M16 | 2.0 | 14.0 | 17.5 |
| M20 | 2.5 | 17.5 | 22.0 |
Metric fine
| Thread | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M8 × 1.0 | 1.0 | 7.0 |
| M10 × 1.25 | 1.25 | 8.8 |
| M10 × 1.0 | 1.0 | 9.0 |
| M12 × 1.5 | 1.5 | 10.5 |
| M12 × 1.25 | 1.25 | 10.8 |
| M16 × 1.5 | 1.5 | 14.5 |
UNC (Unified National Coarse)
| Thread | TPI | Tap drill | Decimal (in) |
|---|---|---|---|
| #0-80 | 80 | 3/64 | 0.0469 |
| #1-64 | 64 | #53 | 0.0595 |
| #2-56 | 56 | #50 | 0.0700 |
| #3-48 | 48 | #47 | 0.0785 |
| #4-40 | 40 | #43 | 0.0890 |
| #5-40 | 40 | #38 | 0.1015 |
| #6-32 | 32 | #36 | 0.1065 |
| #8-32 | 32 | #29 | 0.1360 |
| #10-24 | 24 | #25 | 0.1495 |
| #12-24 | 24 | #16 | 0.1770 |
| 1/4-20 | 20 | #7 | 0.2010 |
| 5/16-18 | 18 | F | 0.2570 |
| 3/8-16 | 16 | 5/16 | 0.3125 |
| 7/16-14 | 14 | U | 0.3680 |
| 1/2-13 | 13 | 27/64 | 0.4219 |
| 9/16-12 | 12 | 31/64 | 0.4844 |
| 5/8-11 | 11 | 17/32 | 0.5312 |
| 3/4-10 | 10 | 21/32 | 0.6562 |
UNF (Unified National Fine)
| Thread | TPI | Tap drill | Decimal (in) |
|---|---|---|---|
| #2-64 | 64 | #50 | 0.0700 |
| #4-48 | 48 | #42 | 0.0935 |
| #6-40 | 40 | #33 | 0.1130 |
| #8-36 | 36 | #29 | 0.1360 |
| #10-32 | 32 | #21 | 0.1590 |
| 1/4-28 | 28 | #3 | 0.2130 |
| 5/16-24 | 24 | I | 0.2720 |
| 3/8-24 | 24 | Q | 0.3320 |
| 7/16-20 | 20 | 25/64 | 0.3906 |
| 1/2-20 | 20 | 29/64 | 0.4531 |
| 9/16-18 | 18 | 33/64 | 0.5156 |
| 5/8-18 | 18 | 37/64 | 0.5781 |
| 3/4-16 | 16 | 11/16 | 0.6875 |
Notes
- These are 75% engagement figures. Going to 100% adds very little holding strength but sharply increases the chance of breaking the tap.
- In hard or gummy material, drill one size larger and accept 60–65% engagement.
- Aluminium and brass tap easily; stainless work-hardens, so use a sharp tap, cutting fluid and steady pressure.
- The clearance column is the through-hole for a bolt to pass without threading.
Not sure what a thread is called in the other system? See our standards cross-reference.