This focused launch chart covers common shop sizes only. Each entry is tied to manufacturer documentation and an applicable standard identity; it is not a reproduction of ASME tolerance tables.
UNC and UNF cutting-tap drill chart
These are manufacturer recommendations for cutting taps. Diameter conversions are display aids; the named drill designation remains the lookup result.
| Thread | Pitch | Recommended drill | Diameter | Converted diameter |
|---|---|---|---|---|
| #4-40 | 40 tpi | #43 | 0.0890 in | 2.261 mm |
| #6-32 | 32 tpi | #36 | 0.1065 in | 2.705 mm |
| #8-32 | 32 tpi | #29 | 0.1360 in | 3.454 mm |
| #10-24 | 24 tpi | #26 | 0.1470 in | 3.734 mm |
| #10-32 | 32 tpi | #21 | 0.1590 in | 4.039 mm |
| 1/4-20 | 20 tpi | #7 | 0.2010 in | 5.105 mm |
| 1/4-28 | 28 tpi | #3 | 0.2130 in | 5.410 mm |
| 5/16-18 | 18 tpi | F | 0.2570 in | 6.528 mm |
| 3/8-16 | 16 tpi | 5/16 in | 0.3125 in | 7.938 mm |
| 3/8-24 | 24 tpi | Q | 0.3320 in | 8.433 mm |
| 1/2-13 | 13 tpi | 27/64 in | 0.421875 in | 10.716 mm |
| #4-48 | 48 tpi | #42 | 0.0935 in | 2.375 mm |
| #6-40 | 40 tpi | #33 | 0.1130 in | 2.870 mm |
| #8-36 | 36 tpi | #29 | 0.1360 in | 3.454 mm |
| #12-24 | 24 tpi | #16 | 0.1770 in | 4.496 mm |
| #12-28 | 28 tpi | #15 | 0.1800 in | 4.572 mm |
| 5/16-24 | 24 tpi | I | 0.2720 in | 6.909 mm |
| 7/16-14 | 14 tpi | U | 0.3680 in | 9.347 mm |
| 7/16-20 | 20 tpi | 25/64 in | 0.390625 in | 9.922 mm |
| 1/2-20 | 20 tpi | 29/64 in | 0.453125 in | 11.509 mm |
| 9/16-12 | 12 tpi | 31/64 in | 0.484375 in | 12.303 mm |
| 9/16-18 | 18 tpi | 33/64 in | 0.515625 in | 13.097 mm |
| 5/8-11 | 11 tpi | 17/32 in | 0.53125 in | 13.494 mm |
| 5/8-18 | 18 tpi | 37/64 in | 0.578125 in | 14.684 mm |
| 3/4-10 | 10 tpi | 21/32 in | 0.65625 in | 16.669 mm |
| 3/4-16 | 16 tpi | 11/16 in | 0.6875 in | 17.463 mm |
| 7/8-9 | 9 tpi | 49/64 in | 0.765625 in | 19.447 mm |
| 7/8-14 | 14 tpi | 13/16 in | 0.8125 in | 20.638 mm |
| 1-8 | 8 tpi | 7/8 in | 0.8750 in | 22.225 mm |
| 1-12 | 12 tpi | 59/64 in | 0.921875 in | 23.416 mm |
How to use the chart
Match the full callout, including diameter, TPI and series. A 1/4-20 UNC and a 1/4-28 UNF thread share the same nominal diameter but require different tap drills.
The recommendation is for a conventional cutting tap. Forming taps displace material instead of cutting it and normally need a larger, manufacturer-specific hole.
- Confirm the thread family and pitch.
- Check the tap maker’s chart for the tap and work material.
- Drill, deburr and gauge the result before committing a production batch.
Why recommendations differ
Tap drill charts target a practical thread engagement rather than the theoretical sharp-V profile. Material strength, chip behavior, tap geometry, coating and tolerance class all influence a suitable hole.
A nearby number, letter or fractional drill may change tapping torque substantially even when the diameter difference looks small.
Do not use straight-thread rules for NPT or BSP taper threads. Pipe-thread geometry, gauging and engagement are different.
Technical references
- Cle-Line C01112 Tap and Drill Recommendations — Greenfield Industries / Cle-Line. Accessed 2026-11-09.
- Decimal Equivalent & Drill Size Chart — Travers Tool Co.. Accessed 2026-11-09.
- ASME B1.1-2024 Unified Inch Screw Threads — ASME. Accessed 2026-11-10.
Standards references identify scope and terminology; protected dimensional tables are not reproduced.