Metric · UNC · UNF · 75% thread
Tap Drill Size Chart
Standard tap drill sizes at 75% thread engagement for metric coarse/fine, UNC and UNF threads — plus a calculator for any thread percentage.
Tap drill calculator (any %)
TD = D − 1.299 × % × pitchThread % from drilled hole
% = (D − drill) / (1.299·p)Metric tap drill chart (75%)
Metric threads
| Thread | Major Ø (mm) | Pitch (mm) | Tap drill (mm) | Tap drill (in) | Minor Ø ext. (mm) |
|---|---|---|---|---|---|
| M1 × 0.25 | 1 | 0.25 | 0.756 | 0.0298 | 0.729 |
| M1.6 × 0.35 | 1.6 | 0.35 | 1.259 | 0.0496 | 1.221 |
| M2 × 0.4 | 2 | 0.4 | 1.610 | 0.0634 | 1.567 |
| M2.5 × 0.45 | 2.5 | 0.45 | 2.062 | 0.0812 | 2.013 |
| M3 × 0.5 | 3 | 0.5 | 2.513 | 0.0989 | 2.459 |
| M4 × 0.7 | 4 | 0.7 | 3.318 | 0.1306 | 3.242 |
| M5 × 0.8 | 5 | 0.8 | 4.221 | 0.1662 | 4.134 |
| M6 × 1 | 6 | 1 | 5.026 | 0.1979 | 4.917 |
| M8 × 1.25 | 8 | 1.25 | 6.782 | 0.2670 | 6.647 |
| M8 × 1 | 8 | 1 | 7.026 | 0.2766 | 6.917 |
| M10 × 1.5 | 10 | 1.5 | 8.539 | 0.3362 | 8.376 |
| M10 × 1.25 | 10 | 1.25 | 8.782 | 0.3458 | 8.647 |
| M12 × 1.75 | 12 | 1.75 | 10.295 | 0.4053 | 10.106 |
| M12 × 1.5 | 12 | 1.5 | 10.539 | 0.4149 | 10.376 |
| M14 × 2 | 14 | 2 | 12.051 | 0.4745 | 11.835 |
| M16 × 2 | 16 | 2 | 14.051 | 0.5532 | 13.835 |
| M18 × 2.5 | 18 | 2.5 | 15.564 | 0.6128 | 15.294 |
| M20 × 2.5 | 20 | 2.5 | 17.564 | 0.6915 | 17.294 |
| M22 × 2.5 | 22 | 2.5 | 19.564 | 0.7702 | 19.294 |
| M24 × 3 | 24 | 3 | 21.077 | 0.8298 | 20.752 |
| M27 × 3 | 27 | 3 | 24.077 | 0.9479 | 23.752 |
| M30 × 3.5 | 30 | 3.5 | 26.590 | 1.0469 | 26.211 |
| M33 × 3.5 | 33 | 3.5 | 29.590 | 1.1650 | 29.211 |
| M36 × 4 | 36 | 4 | 32.103 | 1.2639 | 31.670 |
| M42 × 4.5 | 42 | 4.5 | 37.616 | 1.4809 | 37.129 |
| M48 × 5 | 48 | 5 | 43.129 | 1.6980 | 42.587 |
UNC — Unified National Coarse
| Thread | Major Ø (in) | TPI | Tap drill (in) | Tap drill (mm) | Pitch (mm) |
|---|---|---|---|---|---|
| 0-80 | 0.0600" | 80 | 0.0478 | 1.215 | 0.3175 |
| 1-64 | 0.0730" | 64 | 0.0578 | 1.468 | 0.3969 |
| 2-56 | 0.0860" | 56 | 0.0686 | 1.742 | 0.4536 |
| 3-48 | 0.0990" | 48 | 0.0787 | 1.999 | 0.5292 |
| 4-40 | 0.1120" | 40 | 0.0876 | 2.226 | 0.6350 |
| 5-40 | 0.1250" | 40 | 0.1006 | 2.556 | 0.6350 |
| 6-32 | 0.1380" | 32 | 0.1076 | 2.732 | 0.7937 |
| 8-32 | 0.1640" | 32 | 0.1336 | 3.392 | 0.7937 |
| 10-24 | 0.1900" | 24 | 0.1494 | 3.795 | 1.0583 |
| 12-24 | 0.2160" | 24 | 0.1754 | 4.455 | 1.0583 |
| 1/4"-20 | 0.2500" | 20 | 0.2013 | 5.113 | 1.2700 |
| 5/16"-18 | 0.3125" | 18 | 0.2584 | 6.563 | 1.4111 |
| 3/8"-16 | 0.3750" | 16 | 0.3141 | 7.978 | 1.5875 |
| 7/16"-14 | 0.4375" | 14 | 0.3679 | 9.345 | 1.8143 |
| 1/2"-13 | 0.5000" | 13 | 0.4251 | 10.796 | 1.9538 |
| 9/16"-12 | 0.5625" | 12 | 0.4813 | 12.225 | 2.1167 |
| 5/8"-11 | 0.6250" | 11 | 0.5364 | 13.625 | 2.3091 |
| 3/4"-10 | 0.7500" | 10 | 0.6526 | 16.575 | 2.5400 |
| 7/8"-9 | 0.8750" | 9 | 0.7667 | 19.475 | 2.8222 |
| 1"-8 | 1.0000" | 8 | 0.8782 | 22.307 | 3.1750 |
| 1-1/8"-7 | 1.1250" | 7 | 0.9858 | 25.040 | 3.6286 |
| 1-1/4"-7 | 1.2500" | 7 | 1.1108 | 28.215 | 3.6286 |
| 1-1/2"-6 | 1.5000" | 6 | 1.3376 | 33.976 | 4.2333 |
UNF — Unified National Fine
| Thread | Major Ø (in) | TPI | Tap drill (in) | Tap drill (mm) | Pitch (mm) |
|---|---|---|---|---|---|
| 0-80 | 0.0600" | 80 | 0.0478 | 1.215 | 0.3175 |
| 1-72 | 0.0730" | 72 | 0.0595 | 1.510 | 0.3528 |
| 2-64 | 0.0860" | 64 | 0.0708 | 1.798 | 0.3969 |
| 3-56 | 0.0990" | 56 | 0.0816 | 2.073 | 0.4536 |
| 4-48 | 0.1120" | 48 | 0.0917 | 2.329 | 0.5292 |
| 5-44 | 0.1250" | 44 | 0.1029 | 2.613 | 0.5773 |
| 6-40 | 0.1380" | 40 | 0.1136 | 2.887 | 0.6350 |
| 8-36 | 0.1640" | 36 | 0.1369 | 3.478 | 0.7056 |
| 10-32 | 0.1900" | 32 | 0.1596 | 4.053 | 0.7937 |
| 1/4"-28 | 0.2500" | 28 | 0.2152 | 5.466 | 0.9071 |
| 5/16"-24 | 0.3125" | 24 | 0.2719 | 6.906 | 1.0583 |
| 3/8"-24 | 0.3750" | 24 | 0.3344 | 8.494 | 1.0583 |
| 7/16"-20 | 0.4375" | 20 | 0.3888 | 9.875 | 1.2700 |
| 1/2"-20 | 0.5000" | 20 | 0.4513 | 11.463 | 1.2700 |
| 9/16"-18 | 0.5625" | 18 | 0.5084 | 12.913 | 1.4111 |
| 5/8"-18 | 0.6250" | 18 | 0.5709 | 14.500 | 1.4111 |
| 3/4"-16 | 0.7500" | 16 | 0.6891 | 17.503 | 1.5875 |
| 7/8"-14 | 0.8750" | 14 | 0.8054 | 20.457 | 1.8143 |
| 1"-12 | 1.0000" | 12 | 0.9188 | 23.338 | 2.1167 |
| 1-1/4"-12 | 1.2500" | 12 | 1.1688 | 29.688 | 2.1167 |
| 1-1/2"-12 | 1.5000" | 12 | 1.4188 | 36.038 | 2.1167 |
Formulas used
Tap drill: TD = D − 1.299038 × (%/100) × pitch (metric); TD = D − (1.299038 × %/100) / TPI (imperial)
Example: M10×1.5 at 75%: 10 − 1.299×0.75×1.5 = 8.54 → standard 8.5 mm drill
Frequently asked questions
What size drill for an M10×1.5 tap?
8.5 mm at 75% engagement (calculated 8.54 mm). For 65% engagement use 8.6 mm; for 80% use 8.4 mm.
What size drill for a 1/4-20 tap?
At 75%: 0.2010 in — a #7 drill (0.2010 in) exactly; 13/64 in (0.2031 in) gives about 70%.
Why not drill at 100% thread?
Doubling engagement from 50% to 100% roughly doubles tapping torque while adding almost no joint strength — the bolt, not the internal thread, controls assembly strength. 60–75% is the practical range.