Tap drill sizes
The drill to use before tapping a unified thread, for every size from 0-80 to 1-12. Each size has its own page with the arithmetic, the drill at other engagements, and the nearest metric equivalent.
Every figure here is computed rather than transcribed: the tap drill is the major diameter less 0.974 divided by the threads per inch, which is 75% of the thread height for a 60° form. The build checks that rule against all 39 published chart values and fails if any of them disagree.
UNC — coarse threads
The default in general work. Coarse threads tap faster, tolerate a knock better and hold better in aluminium, plastic and other soft material.
| Tap | Drill | Inches | Millimetres |
|---|---|---|---|
| 1-64 | #53 | 0.0595" | 1.51 mm |
| 2-56 | #50 | 0.07" | 1.78 mm |
| 3-48 | #47 | 0.0785" | 1.99 mm |
| 4-40 | #43 | 0.089" | 2.26 mm |
| 5-40 | #38 | 0.1015" | 2.58 mm |
| 6-32 | #36 | 0.1065" | 2.71 mm |
| 8-32 | #29 | 0.136" | 3.45 mm |
| 10-24 | #25 | 0.1495" | 3.8 mm |
| 12-24 | #16 | 0.177" | 4.5 mm |
| 1/4-20 | #7 | 0.201" | 5.11 mm |
| 5/16-18 | F | 0.257" | 6.53 mm |
| 3/8-16 | 5/16" | 0.3125" | 7.94 mm |
| 7/16-14 | U | 0.368" | 9.35 mm |
| 1/2-13 | 27/64" | 0.4219" | 10.72 mm |
| 9/16-12 | 31/64" | 0.4844" | 12.3 mm |
| 5/8-11 | 17/32" | 0.5313" | 13.49 mm |
| 3/4-10 | 21/32" | 0.6563" | 16.67 mm |
| 7/8-9 | 49/64" | 0.7656" | 19.45 mm |
| 1-8 | 7/8" | 0.875" | 22.23 mm |
UNF — fine threads
Used where the material is thin, where vibration would work a coarse thread loose, or where a finer adjustment is wanted. Automotive and aerospace work is largely fine.
| Tap | Drill | Inches | Millimetres |
|---|---|---|---|
| 0-80 | 3/64" | 0.0469" | 1.19 mm |
| 1-72 | #53 | 0.0595" | 1.51 mm |
| 2-64 | #50 | 0.07" | 1.78 mm |
| 3-56 | #45 | 0.082" | 2.08 mm |
| 4-48 | #42 | 0.0935" | 2.37 mm |
| 5-44 | #37 | 0.104" | 2.64 mm |
| 6-40 | #33 | 0.113" | 2.87 mm |
| 8-36 | #29 | 0.136" | 3.45 mm |
| 10-32 | #21 | 0.159" | 4.04 mm |
| 12-28 | #14 | 0.182" | 4.62 mm |
| 1/4-28 | #3 | 0.213" | 5.41 mm |
| 5/16-24 | I | 0.272" | 6.91 mm |
| 3/8-24 | Q | 0.332" | 8.43 mm |
| 7/16-20 | 25/64" | 0.3906" | 9.92 mm |
| 1/2-20 | 29/64" | 0.4531" | 11.51 mm |
| 9/16-18 | 33/64" | 0.5156" | 13.1 mm |
| 5/8-18 | 37/64" | 0.5781" | 14.68 mm |
| 3/4-16 | 11/16" | 0.6875" | 17.46 mm |
| 7/8-14 | 13/16" | 0.8125" | 20.64 mm |
| 1-12 | 59/64" | 0.9219" | 23.42 mm |
Choosing an engagement
75% is the chart figure and the right default in mild steel, aluminium and plastic. Drop to 65% in stainless, tool steel or anything else that fights the tap: the thread still holds about 95% of the load and the tap is much less likely to snap. Going above 75% buys almost no strength and a great deal of torque.