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Tap drill chart
Standard cut-tap drills for about 75% thread, for the most common inch and metric sizes, and how to work out any other size.
Inch (UNC and UNF)
| Thread | Tap drill | Decimal (in) |
|---|---|---|
| #10-24 UNC | #25 | 0.1495 |
| #10-32 UNF | #21 | 0.1590 |
| 1/4-20 UNC | #7 | 0.2010 |
| 1/4-28 UNF | #3 | 0.2130 |
| 5/16-18 UNC | F | 0.2570 |
| 3/8-16 UNC | 5/16 | 0.3125 |
| 3/8-24 UNF | Q | 0.3320 |
| 1/2-13 UNC | 27/64 | 0.4219 |
| 1/2-20 UNF | 29/64 | 0.4531 |
Metric (coarse)
| Thread | Tap drill (mm) |
|---|---|
| M3 × 0.5 | 2.5 |
| M4 × 0.7 | 3.3 |
| M5 × 0.8 | 4.2 |
| M6 × 1.0 | 5.0 |
| M8 × 1.25 | 6.8 |
| M10 × 1.5 | 8.5 |
| M12 × 1.75 | 10.2 |
Working out a tap drill
For metric threads, major diameter minus pitch gives roughly 75% thread:
Drill = D − P (M8 × 1.25 → 8 − 1.25 = 6.75, use 6.8)
For inch threads, choose the percent of thread you want:
Drill = Major − (0.01299 × % thread ÷ TPI)
% thread = TPI × (Major − Drill) ÷ 0.01299
Check: 1/4-20 with a #7 drill (0.201") gives 20 × 0.049 ÷ 0.01299 = 75%. In tough materials or deep holes, 60–65% thread is often enough and much easier on the tap. Form (roll) taps need a larger drill than cut taps; use the tap maker's chart.
Every size, any percent. The app's tap drill calculator works out the drill for any thread and percent of thread, and the drill chart lists number, letter, fractional and metric drills.
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