Tap drill chart
The recommended drill bit for cutting an internal thread with a tap, sized for roughly 75% thread engagement: strong enough for full-strength fasteners, loose enough to keep tapping torque sane.
UNC — Unified National Coarse
| Thread | TPI | Tap drill | Decimal (in) | mm |
|---|---|---|---|---|
| #4-40 | 40 | #43 | 0.0890 | 2.26 |
| #5-40 | 40 | #38 | 0.1015 | 2.58 |
| #6-32 | 32 | #36 | 0.1065 | 2.71 |
| #8-32 | 32 | #29 | 0.1360 | 3.45 |
| #10-24 | 24 | #25 | 0.1495 | 3.80 |
| 1/4-20 | 20 | #7 | 0.2010 | 5.11 |
| 5/16-18 | 18 | F | 0.2570 | 6.53 |
| 3/8-16 | 16 | 5/16" | 0.3125 | 7.94 |
| 7/16-14 | 14 | U | 0.3680 | 9.35 |
| 1/2-13 | 13 | 27/64" | 0.4219 | 10.72 |
| 9/16-12 | 12 | 31/64" | 0.4844 | 12.30 |
| 5/8-11 | 11 | 17/32" | 0.5312 | 13.49 |
| 3/4-10 | 10 | 21/32" | 0.6562 | 16.67 |
UNF — Unified National Fine
| Thread | TPI | Tap drill | Decimal (in) | mm |
|---|---|---|---|---|
| #4-48 | 48 | #42 | 0.0935 | 2.37 |
| #5-44 | 44 | #37 | 0.1040 | 2.64 |
| #6-40 | 40 | #33 | 0.1130 | 2.87 |
| #8-36 | 36 | #29 | 0.1360 | 3.45 |
| #10-32 | 32 | #21 | 0.1590 | 4.04 |
| 1/4-28 | 28 | #3 | 0.2130 | 5.41 |
| 5/16-24 | 24 | I | 0.2720 | 6.91 |
| 3/8-24 | 24 | Q | 0.3320 | 8.43 |
| 7/16-20 | 20 | 25/64" | 0.3906 | 9.92 |
| 1/2-20 | 20 | 29/64" | 0.4531 | 11.51 |
| 9/16-18 | 18 | 33/64" | 0.5156 | 13.10 |
| 5/8-18 | 18 | 37/64" | 0.5781 | 14.68 |
| 3/4-16 | 16 | 11/16" | 0.6875 | 17.46 |
Metric coarse (ISO)
| Thread × pitch | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M1.6×0.35 | 0.35 | 1.25 mm |
| M2×0.4 | 0.40 | 1.6 mm |
| M2.5×0.45 | 0.45 | 2.05 mm |
| M3×0.5 | 0.50 | 2.5 mm |
| M3.5×0.6 | 0.60 | 2.9 mm |
| M4×0.7 | 0.70 | 3.3 mm |
| M5×0.8 | 0.80 | 4.2 mm |
| M6×1.0 | 1.00 | 5.0 mm |
| M8×1.25 | 1.25 | 6.8 mm |
| M10×1.5 | 1.50 | 8.5 mm |
| M12×1.75 | 1.75 | 10.2 mm |
| M16×2.0 | 2.00 | 14.0 mm |
Converting between the systems
A metric drill can substitute for an inch tap drill when the sizes are close. As a rule, stay within ±0.1 mm (about ±0.004") of the recommended diameter: smaller risks tap breakage, larger thins the threads. To check a substitute, convert the inch recommendation with inches to millimeters, or compare the thread's pitch directly using TPI to thread pitch.
Frequently asked questions
Where does the tap drill rule of thumb come from?
For a 75%-thread hole in inch threads, subtract the thread pitch from the major diameter: drill = D − 1/TPI. For 1/4-20 that is 0.250" − 1/20 = 0.200", and the nearest standard drill is #7 (0.201"). The values in the table above are the standard published recommendations, which follow that rule.
Why not drill the minor diameter exactly?
A tapped hole needs some material for the tap to cut into, but too much thread engagement increases torque and risk of tap breakage while adding little strength beyond about 75% engagement in steel. The 75% hole is the accepted shop compromise; 50–60% is often used in tough or gummy materials.
What about metric taps?
Metric coarse taps use the same logic with millimeter numbers: drill ≈ nominal diameter − pitch. M6×1.0 → 6 − 1 = 5.0 mm. The table lists the ISO-recommended drills.
Do I need a different drill for cast iron or aluminum?
The drill size stays the same; what changes is how much engagement you want. In soft aluminum many shops tap at 65–70% (a slightly larger drill) to reduce loading, while in hard alloys they stay at 75% for strength.
Sources: UNC/UNF pairs per ASME B1.1 thread data as tabulated in Machinery's Handbook; metric pairs per ISO general-purpose metric-coarse tap practice. See our sources log for details.