Machine screw size chart
A #8 machine screw is 0.1640 in across; #8-32 UNC takes a #29 tap drill.
Numbered sizes follow the formula 0.060 + 0.013 x number, so #10 is 0.190 in. Coarse (UNC) is the default; fine (UNF) has more threads per inch. #0 only has a fine thread. Clearance holes for each size are in the drill finder.
| Size | Diameter (in) | Diameter (mm) | UNC | UNF |
|---|---|---|---|---|
| #0 | 0.0600 | 1.524 | - | 80 TPI, tap 3/64 |
| #1 | 0.0730 | 1.854 | 64 TPI, tap #53 | 72 TPI, tap #53 |
| #2 | 0.0860 | 2.184 | 56 TPI, tap #50 | 64 TPI, tap #50 |
| #3 | 0.0990 | 2.515 | 48 TPI, tap #47 | 56 TPI, tap #45 |
| #4 | 0.1120 | 2.845 | 40 TPI, tap #43 | 48 TPI, tap #42 |
| #5 | 0.1250 | 3.175 | 40 TPI, tap #38 | 44 TPI, tap #37 |
| #6 | 0.1380 | 3.505 | 32 TPI, tap #36 | 40 TPI, tap #33 |
| #8 | 0.1640 | 4.166 | 32 TPI, tap #29 | 36 TPI, tap #29 |
| #10 | 0.1900 | 4.826 | 24 TPI, tap #25 | 32 TPI, tap #21 |
| #12 | 0.2160 | 5.486 | 24 TPI, tap #16 | 28 TPI, tap #14 |
| 1/4 | 0.2500 | 6.35 | 20 TPI, tap #7 | 28 TPI, tap #3 |
| 5/16 | 0.3125 | 7.938 | 18 TPI, tap F | 24 TPI, tap I |
| 3/8 | 0.3750 | 9.525 | 16 TPI, tap 5/16 | 24 TPI, tap Q |
| 7/16 | 0.4375 | 11.112 | 14 TPI, tap U | 20 TPI, tap 25/64 |
| 1/2 | 0.5000 | 12.7 | 13 TPI, tap 27/64 | 20 TPI, tap 29/64 |
| 9/16 | 0.5625 | 14.288 | 12 TPI, tap 31/64 | 18 TPI, tap 33/64 |
| 5/8 | 0.6250 | 15.875 | 11 TPI, tap 17/32 | 18 TPI, tap 37/64 |
| 3/4 | 0.7500 | 19.05 | 10 TPI, tap 21/32 | 16 TPI, tap 11/16 |
| 7/8 | 0.8750 | 22.225 | 9 TPI, tap 49/64 | 14 TPI, tap 13/16 |
| 1 | 1.0000 | 25.4 | 8 TPI, tap 7/8 | 12 TPI, tap 59/64 |
How to read a machine screw size
A size such as #8-32 UNC has three parts: the gauge number (8), the threads per inch (32) and the thread series (UNC, unified coarse). Sizes up to #12 use gauge numbers; from 1/4 inch up they are written as fractions, for example 1/4-20. Each gauge number maps to a diameter by 0.060 + 0.013 x number, so #8 is 0.164 in.
UNC or UNF?
Coarse threads (UNC) are the default: they assemble faster and are less likely to cross-thread. Fine threads (UNF) give finer adjustment and are used where the wall is thin or the load is better spread over more threads. For the same diameter the two are not interchangeable, so match the thread count printed on the part: 1/4-20 is coarse with a #7 tap drill, 1/4-28 is fine with a #3.
Tap drill or clearance hole?
Drill the tap drill size in the part you will thread. Drill the clearance hole in the part the screw passes through. Close, normal and loose fits trade alignment against how forgiving the holes are to position. A common mistake is using the clearance drill in the part you meant to tap, which leaves nothing for the thread to cut.