Thread Dimensions and Tap Drill Reference
Dimensions, tolerance limits and tap drills for 214 screw and pipe threads: ISO metric, Unified inch (UNC, UNF, UNEF), NPT and BSP. Every value is checked against the governing standard and shown in both millimetres and inches.
- ISO metric coarse36 sizes, M1 to M68. The default metric thread.
- ISO metric fine56 sizes, M3×0.35 to M48×3.
- UNC, Unified coarse33 sizes, #1-64 to 4-4.
- UNF, Unified fine24 sizes, #0-80 to 1-1/2-12.
- UNEF, Unified extra fine25 sizes, #12-32 to 1-11/16-18.
- NPT pipe threads19 sizes, 1/16 to 12 inch, taper.
- BSP pipe threads21 sizes, G1/16 to G4, parallel.
- Tap drill chartEvery tap drill on one page, with % thread.
- Thread identifierEnter a diameter and pitch to find the thread.
The parts of a thread
Three diameters describe a thread. The major diameter is measured over the crests of the bolt and gives the thread its name. The minor diameter is the bore of the nut, and sets the tap drill. The pitch diameter lies between them, where ridge and groove are equally wide. The pitch is the distance from one crest to the next; inch threads state it as threads per inch.
Common metric threads
| Thread | Pitch (mm) | Pitch Ø (mm) | Tap drill (mm) | Clearance hole (mm) | Stress area (mm²) |
|---|---|---|---|---|---|
| M3×0.5 | 0.5 | 2.675 | 2.5 | 3.4 | 5.03 |
| M4×0.7 | 0.7 | 3.545 | 3.3 | 4.5 | 8.78 |
| M5×0.8 | 0.8 | 4.480 | 4.2 | 5.5 | 14.2 |
| M6×1 | 1 | 5.350 | 5 | 6.6 | 20.1 |
| M8×1.25 | 1.25 | 7.188 | 6.8 | 9 | 36.6 |
| M10×1.5 | 1.5 | 9.026 | 8.5 | 11 | 58 |
| M12×1.75 | 1.75 | 10.863 | 10.2 | 13.5 | 84.3 |
| M16×2 | 2 | 14.701 | 14 | 17.5 | 157 |
| M20×2.5 | 2.5 | 18.376 | 17.5 | 22 | 245 |
| M24×3 | 3 | 22.051 | 21 | 26 | 353 |
Common inch threads
| Thread | TPI | Major Ø (in) | Pitch Ø (in) | Tap drill | Drill (mm) |
|---|---|---|---|---|---|
| #6-32 UNC | 32 | 0.1380 | 0.1177 | #36 | 2.71 |
| #8-32 UNC | 32 | 0.1640 | 0.1437 | #29 | 3.45 |
| #10-24 UNC | 24 | 0.1900 | 0.1629 | #25 | 3.80 |
| 1/4-20 UNC | 20 | 0.2500 | 0.2175 | #7 | 5.11 |
| 5/16-18 UNC | 18 | 0.3125 | 0.2764 | F | 6.53 |
| 3/8-16 UNC | 16 | 0.3750 | 0.3344 | 5/16" | 7.94 |
| 1/2-13 UNC | 13 | 0.5000 | 0.4500 | 27/64" | 10.72 |
| 5/8-11 UNC | 11 | 0.6250 | 0.5660 | 17/32" | 13.49 |
| 3/4-10 UNC | 10 | 0.7500 | 0.6850 | 21/32" | 16.67 |
| 1-8 UNC | 8 | 1.0000 | 0.9188 | 7/8" | 22.22 |
Guides
Where the numbers come from
The diameters of metric and Unified threads are computed from the profile formulas of ISO 724 and ASME B1.1 and then compared with the tables printed in those standards. Tolerance limits follow ISO 965-1 and ASME B1.1. Pipe thread dimensions are taken from ASME B1.20.1 and ISO 228-1. Tap drills are conventions, not calculations, so they are taken from published drill tables and the source is named where tables disagree.