Tap drill chart
Which drill to use before tapping — and which diameter to bore if you single-point the internal thread on a lathe.
Quick answer
1/4-20 UNC → #7 drill (0.201″), 1/2-13 UNC → 27/64″ (about 75% thread). Metric: drill = D − P, M10×1.5 → 8.5 mm. To single-point the thread, bore to D1 = D − 1.0825 × P: 8.376 mm for M10×1.5.
The tap drill for a metric thread is the diameter minus the pitch: d = D − P, about 77 % of thread — the ISO 2306 / DIN 336 series. Inch sizes use the standard drills for about 75 % of thread. Thebore for single-point threading is the basic nut minor diameter D1 = D − 1.0825 · P: the passes start from it and the thread comes out full-form.
Metric coarse threads
| Thread | Pitch, mm | Tap drill, mm | Bore D1, mm |
|---|---|---|---|
| M3×0.5 | 0.5 | 2.5 | 2.46 |
| M4×0.7 | 0.7 | 3.3 | 3.24 |
| M5×0.8 | 0.8 | 4.2 | 4.13 |
| M6×1 | 1 | 5.0 | 4.92 |
| M8×1.25 | 1.25 | 6.8 | 6.65 |
| M10×1.5 | 1.5 | 8.5 | 8.38 |
| M12×1.75 | 1.75 | 10.3 | 10.11 |
| M14×2 | 2 | 12.0 | 11.84 |
| M16×2 | 2 | 14.0 | 13.84 |
| M18×2.5 | 2.5 | 15.5 | 15.29 |
| M20×2.5 | 2.5 | 17.5 | 17.29 |
| M22×2.5 | 2.5 | 19.5 | 19.29 |
| M24×3 | 3 | 21.0 | 20.75 |
| M27×3 | 3 | 24.0 | 23.75 |
| M30×3.5 | 3.5 | 26.5 | 26.21 |
| M33×3.5 | 3.5 | 29.5 | 29.21 |
| M36×4 | 4 | 32.0 | 31.67 |
Metric fine threads
| Thread | Pitch, mm | Tap drill, mm | Bore D1, mm |
|---|---|---|---|
| M8×1 | 1 | 7.0 | 6.92 |
| M10×1.25 | 1.25 | 8.8 | 8.65 |
| M10×1 | 1 | 9.0 | 8.92 |
| M12×1.5 | 1.5 | 10.5 | 10.38 |
| M12×1.25 | 1.25 | 10.8 | 10.65 |
| M14×1.5 | 1.5 | 12.5 | 12.38 |
| M16×1.5 | 1.5 | 14.5 | 14.38 |
| M18×1.5 | 1.5 | 16.5 | 16.38 |
| M20×1.5 | 1.5 | 18.5 | 18.38 |
| M22×1.5 | 1.5 | 20.5 | 20.38 |
| M24×2 | 2 | 22.0 | 21.84 |
| M24×1.5 | 1.5 | 22.5 | 22.38 |
| M27×2 | 2 | 25.0 | 24.84 |
| M30×2 | 2 | 28.0 | 27.84 |
| M30×1.5 | 1.5 | 28.5 | 28.38 |
UNC / UNF
| Thread | Tap drill | Decimal, in | mm | % thread | Bore D1, in |
|---|---|---|---|---|---|
| #4-40 UNC | #43 | 0.0890 | 2.26 | 71% | 0.0849 |
| #6-32 UNC | #36 | 0.1065 | 2.71 | 78% | 0.1042 |
| #8-32 UNC | #29 | 0.1360 | 3.45 | 69% | 0.1302 |
| #10-24 UNC | #25 | 0.1495 | 3.80 | 75% | 0.1449 |
| #10-32 UNF | #21 | 0.1590 | 4.04 | 76% | 0.1562 |
| 1/4-20 UNC | #7 | 0.2010 | 5.11 | 75% | 0.1959 |
| 1/4-28 UNF | #3 | 0.2130 | 5.41 | 80% | 0.2113 |
| 5/16-18 UNC | F | 0.2570 | 6.53 | 77% | 0.2524 |
| 5/16-24 UNF | I | 0.2720 | 6.91 | 75% | 0.2674 |
| 3/8-16 UNC | 5/16 | 0.3125 | 7.94 | 77% | 0.3073 |
| 3/8-24 UNF | Q | 0.3320 | 8.43 | 79% | 0.3299 |
| 7/16-14 UNC | U | 0.3680 | 9.35 | 75% | 0.3602 |
| 7/16-20 UNF | 25/64 | 0.3906 | 9.92 | 72% | 0.3834 |
| 1/2-13 UNC | 27/64 | 0.4219 | 10.72 | 78% | 0.4167 |
| 1/2-20 UNF | 29/64 | 0.4531 | 11.51 | 72% | 0.4459 |
| 9/16-12 UNC | 31/64 | 0.4844 | 12.30 | 72% | 0.4723 |
| 9/16-18 UNF | 33/64 | 0.5156 | 13.10 | 65% | 0.5024 |
| 5/8-11 UNC | 17/32 | 0.5312 | 13.49 | 79% | 0.5266 |
| 5/8-18 UNF | 37/64 | 0.5781 | 14.68 | 65% | 0.5649 |
| 3/4-10 UNC | 21/32 | 0.6562 | 16.67 | 72% | 0.6418 |
| 3/4-16 UNF | 11/16 | 0.6875 | 17.46 | 77% | 0.6823 |
| 7/8-9 UNC | 49/64 | 0.7656 | 19.45 | 76% | 0.7547 |
| 7/8-14 UNF | 13/16 | 0.8125 | 20.64 | 67% | 0.7977 |
| 1-8 UNC | 7/8 | 0.8750 | 22.22 | 77% | 0.8647 |
| 1-12 UNF | 59/64 | 0.9219 | 23.42 | 72% | 0.9098 |
In gummy materials (stainless, copper, aluminum) taps cut easier with a drill a few thousandths larger — slightly less thread, still plenty strong, and fewer broken taps.
For single-point threading bore D1 to size or a hair over: smaller and the tool will shave the crests of the nut profile.
Hole depth with the drill point is in the drill point depth calculator, the threading passes in the thread calculator.