Imperial screws are designated by their diameter (in inches) and pitch (revolutions per inch). For instance, ¼-20 means: diameter 0.25”, 20 threads per inch. Smaller screws are designated by a numerical gauge, larger numbers correspond to larger screws.
On this page:
- Drills to use for making screw holes
- Actual diameters of imperial screws
- Dimensions of the heads of imperial screws
Drills for screw holes
| Screw size | Drill for 75% thread | Drill for 50% thread | Drill for forming tap | Drill for clearance |
|---|---|---|---|---|
| #000-120 | 71 | 69 | (70) | 62–65 |
| #00-90 | 65 | 60 | (62) | 54–55 |
| #0-80 | 3/64” | 55 | (54) | 52–50 |
| #2-56 | 50 | 49 | (5/64”) | 43–41 |
| #4-40 | 43 | 41 | 39 | 32–30 |
| #6-32 | 36 | 32 | 1/8” | 27–25 |
| #8-32 | 29 | 27 | 25 | 18–16 |
| #10-24 | 25 | 20 | 11/64” | 9–7 |
| #10-32 | 21 | 18 | 16 | 9–7 |
| #12-24 | 16 | 12 | 9–8 | 2–1 |
| 1/4”-20 | 7 | 7/32” | 1 | 17/64”–9/32” |
Use 75% thread for aluminum, 50% for steel. Forming taps are specified for 65% thread.
Parentheses indicate forming taps not available from McMaster.
Minor and major diameters of US screws
| Screw size | Minor diam. (inch) | Minor diam. (mm) | Major diam. (inch) | Major diam. (mm) |
|---|---|---|---|---|
| #000-120 | 0.0232” | 0.59 | 0.034” | 0.86 |
| #00-90 | 0.0326” | 0.83 | 0.047” | 1.19 |
| #0-80 | 0.045” | 1.14 | 0.060” | 1.52 |
| #2-56 | 0.070” | 1.78 | 0.086” | 2.18 |
| #4-40 | 0.089” | 2.26 | 0.112” | 2.84 |
| #6-32 | 0.100” | 2.54 | 0.138” | 3.51 |
| #8-32 | 0.126” | 3.20 | 0.164” | 4.17 |
| #10-24 | 0.139” | 3.53 | 0.190” | 4.83 |
| #10-32 | 0.152” | 3.86 | 0.190” | 4.83 |
| #12-24 | 0.165” | 4.19 | 0.216” | 5.49 |
| 1/4”-20 | 0.189” | 4.80 | 0.250” | 6.35 |
Head diameters of US screws
| Screw size | Socket cap (inch) | Socket cap (mm) | Flat head (inch) | Flat head (mm) |
|---|---|---|---|---|
| #000 | – | – | 0.061” | 1.55 |
| #00 | – | – | 0.089” | 2.26 |
| #0 | 0.096” | 2.44 | 0.112” | 2.84 |
| #2 | 0.140” | 3.56 | 0.162” | 4.11 |
| #4 | 0.183” | 4.65 | 0.212” | 5.38 |
| #6 | 0.226” | 5.74 | 0.262” | 6.65 |
| #8 | 0.270” | 6.86 | 0.312” | 7.92 |
| #10 | 0.312” | 7.92 | 0.362” | 9.19 |
| #12 | 0.324” | 8.23 | 0.412” | 10.46 |
| 1/4” | 0.375” | 9.52 | 0.477” | 12.12 |
Flat head diameters apply to both 82° and 100° head angles.
Socket cap head height is the same as major diameter for all sizes.
Sources
- Forming taps: Jarvis Cutting Tools
- Regular taps: shender4.com
- Additional information from McMaster, Newman Tools, and Ames Web