The defining dimensions of a thread are its major diameter (the nominal size), its pitch (TPI for inch threads, millimeters per thread for metric), and the derived pitch and minor diameters. These set what drill to use, how much thread engagement you get, and whether two parts will fit.
Unified inch threads (UNC / UNF)
| Size | Major dia (in) | TPI | Pitch dia (in) | Minor dia (in) |
|---|---|---|---|---|
| #4-40 UNC | 0.1120 | 40 | 0.0985 | 0.0822 |
| #6-32 UNC | 0.1380 | 32 | 0.1177 | 0.0974 |
| #8-32 UNC | 0.1640 | 32 | 0.1437 | 0.1234 |
| #10-24 UNC | 0.1900 | 24 | 0.1629 | 0.1359 |
| #10-32 UNF | 0.1900 | 32 | 0.1697 | 0.1494 |
| 1/4-20 UNC | 0.2500 | 20 | 0.2175 | 0.1850 |
| 1/4-28 UNF | 0.2500 | 28 | 0.2268 | 0.2036 |
| 5/16-18 UNC | 0.3125 | 18 | 0.2764 | 0.2403 |
| 3/8-16 UNC | 0.3750 | 16 | 0.3344 | 0.2938 |
| 7/16-14 UNC | 0.4375 | 14 | 0.3911 | 0.3447 |
| 1/2-13 UNC | 0.5000 | 13 | 0.4500 | 0.4001 |
| 5/8-11 UNC | 0.6250 | 11 | 0.5660 | 0.5069 |
| 3/4-10 UNC | 0.7500 | 10 | 0.6850 | 0.6201 |
Metric coarse threads
| Size | Major dia (mm) | Pitch (mm) | Pitch dia (mm) | Minor dia (mm) |
|---|---|---|---|---|
| M3 × 0.5 | 3.000 | 0.50 | 2.675 | 2.459 |
| M4 × 0.7 | 4.000 | 0.70 | 3.545 | 3.242 |
| M5 × 0.8 | 5.000 | 0.80 | 4.480 | 4.134 |
| M6 × 1.0 | 6.000 | 1.00 | 5.350 | 4.917 |
| M8 × 1.25 | 8.000 | 1.25 | 7.188 | 6.647 |
| M10 × 1.5 | 10.000 | 1.50 | 9.026 | 8.376 |
| M12 × 1.75 | 12.000 | 1.75 | 10.863 | 10.106 |
Nominal basic dimensions for external threads. Pitch and minor diameters are rounded; for tolerance classes and Go/No-Go limits consult ASME B1.1 (inch) or ISO 965 (metric).
Reading the table
Major diameter is the outside diameter of the screw. Minor diameter is the root, and it drives tap drill choice — drill close to the minor diameter for a strong thread, or a touch larger for easier tapping. Pitch diameter is where width of thread and width of groove are equal; it is the dimension thread gauges actually check.
Related: Tap Drill Chart and Drill Size Chart.