Reference
Tapping drill chart
A cut tap in steel is often drilled for about 75% thread. That is a shop approximation, not a promise, and it is the wrong chart for a roll form tap.
The formula
With the major diameter and the pitch in the same unit:
drill = major − (0.01299 × percent of thread × pitch)
For inch threads, pitch is 1 / TPI. For ISO metric coarse and fine threads below, the major diameter is the nominal size. At 75% the subtraction is 0.974 × pitch. The percent you actually get is whatever stock drill you pick:
percent = (major − drill) / (0.01299 × pitch)
A smaller hole is a higher percent of thread, a stronger thread, and a tap that is more likely to break. Under about M6, and in stainless or prehard steel, use the tap manufacturer's chart instead of rounding to the nearest drill and hoping.
Form taps (roll taps) need a larger hole, often near the pitch diameter. The chart on this page is for cut taps only. The tap maker publishes the drill for that tap, that material, and the percent of thread they want.
About 75% thread
"Stock drill" is the nearest number, letter, or 1/64 in fraction for inch threads, and the nearest 0.1 mm for metric. The percent column is what that drill actually produces with the formula above. A few famous charts list a slightly different metric drill (M12×1.75 is often printed as 10.2 mm, which is closer to 79% than to 75%).
| Thread | 75% drill | Nearest stock drill | Thread at stock drill |
|---|---|---|---|
| #4-40 | 0.0876 in (2.23 mm) | #43 · 0.089 in (2.26 mm) | 70.8% |
| #6-32 | 0.1076 in (2.73 mm) | #36 · 0.1065 in (2.71 mm) | 77.6% |
| #8-32 | 0.1336 in (3.39 mm) | #29 · 0.136 in (3.45 mm) | 69% |
| #10-24 | 0.1494 in (3.79 mm) | #25 · 0.1495 in (3.8 mm) | 74.8% |
| 1/4-20 | 0.2013 in (5.11 mm) | #7 · 0.201 in (5.11 mm) | 75.4% |
| 5/16-18 | 0.2584 in (6.56 mm) | F · 0.257 in (6.53 mm) | 76.9% |
| 3/8-16 | 0.3141 in (7.98 mm) | 5/16 · 0.3125 in (7.94 mm) | 77% |
| 7/16-14 | 0.3679 in (9.34 mm) | U · 0.368 in (9.35 mm) | 74.9% |
| 1/2-13 | 0.4251 in (10.8 mm) | 27/64 · 0.421875 in (10.72 mm) | 78.2% |
| 9/16-12 | 0.4813 in (12.23 mm) | 31/64 · 0.484375 in (12.3 mm) | 72.2% |
| 5/8-11 | 0.5364 in (13.63 mm) | 17/32 · 0.53125 in (13.49 mm) | 79.4% |
| 3/4-10 | 0.6526 in (16.58 mm) | 21/32 · 0.65625 in (16.67 mm) | 72.2% |
| 7/8-9 | 0.7668 in (19.48 mm) | 49/64 · 0.765625 in (19.45 mm) | 75.8% |
| 1-8 | 0.8782 in (22.31 mm) | 7/8 · 0.875 in (22.23 mm) | 77% |
| #10-32 | 0.1596 in (4.05 mm) | #21 · 0.159 in (4.04 mm) | 76.4% |
| 1/4-28 | 0.2152 in (5.47 mm) | #3 · 0.213 in (5.41 mm) | 79.8% |
| 5/16-24 | 0.2719 in (6.91 mm) | I · 0.272 in (6.91 mm) | 74.8% |
| 3/8-24 | 0.3344 in (8.49 mm) | Q · 0.332 in (8.43 mm) | 79.4% |
| 7/16-20 | 0.3888 in (9.88 mm) | 25/64 · 0.390625 in (9.92 mm) | 72.2% |
| 1/2-20 | 0.4513 in (11.46 mm) | 29/64 · 0.453125 in (11.51 mm) | 72.2% |
| 5/8-18 | 0.5709 in (14.5 mm) | 37/64 · 0.578125 in (14.68 mm) | 65% |
| 3/4-16 | 0.6891 in (17.5 mm) | 11/16 · 0.6875 in (17.46 mm) | 77% |
| M3 × 0.5 | 0.0989 in (2.51 mm) | 2.5 mm · 0.0984 in (2.5 mm) | 77% |
| M4 × 0.7 | 0.1306 in (3.32 mm) | 3.3 mm · 0.1299 in (3.3 mm) | 77% |
| M5 × 0.8 | 0.1662 in (4.22 mm) | 4.2 mm · 0.1654 in (4.2 mm) | 77% |
| M6 × 1.0 | 0.1979 in (5.03 mm) | 5.0 mm · 0.1969 in (5 mm) | 77% |
| M8 × 1.25 | 0.267 in (6.78 mm) | 6.8 mm · 0.2677 in (6.8 mm) | 73.9% |
| M10 × 1.5 | 0.3362 in (8.54 mm) | 8.5 mm · 0.3346 in (8.5 mm) | 77% |
| M12 × 1.75 | 0.4053 in (10.3 mm) | 10.3 mm · 0.4055 in (10.3 mm) | 74.8% |
| M16 × 2.0 | 0.5532 in (14.05 mm) | 14.1 mm · 0.5551 in (14.1 mm) | 73.1% |
| M20 × 2.5 | 0.6915 in (17.56 mm) | 17.6 mm · 0.6929 in (17.6 mm) | 73.9% |
| M24 × 3.0 | 0.8298 in (21.08 mm) | 21.1 mm · 0.8307 in (21.1 mm) | 74.4% |
| M8 × 1.0 | 0.2766 in (7.03 mm) | 7.0 mm · 0.2756 in (7 mm) | 77% |
| M10 × 1.25 | 0.3458 in (8.78 mm) | 8.8 mm · 0.3465 in (8.8 mm) | 73.9% |
| M12 × 1.5 | 0.4149 in (10.54 mm) | 10.5 mm · 0.4134 in (10.5 mm) | 77% |
| M12 × 1.25 | 0.4245 in (10.78 mm) | 10.8 mm · 0.4252 in (10.8 mm) | 73.9% |
Thread proportions for a gear are not a tap drill. Use gearcalculators.com for the gear math, and Evolvent Design if the gear needs to be cut.