Reference Table
Thread Sizes Reference
Common thread standards — metric (M), BSP, NPT, UNF, UNC — with pitch, tap drill sizes, and applications. Essential for maintenance and fabrication.
5 min readUpdated July 2026
Metric Coarse Threads (ISO)
| Thread | Pitch (mm) | Tap drill (mm) | Clearance drill (mm) |
|---|---|---|---|
| M3 | 0.5 | 2.5 | 3.3 |
| M4 | 0.7 | 3.3 | 4.3 |
| M5 | 0.8 | 4.2 | 5.3 |
| M6 | 1.0 | 5.0 | 6.5 |
| M8 | 1.25 | 6.8 | 8.5 |
| M10 | 1.5 | 8.5 | 10.5 |
| M12 | 1.75 | 10.2 | 12.5 |
| M16 | 2.0 | 14.0 | 16.5 |
| M20 | 2.5 | 17.5 | 21.0 |
| M24 | 3.0 | 21.0 | 25.0 |
BSP (British Standard Pipe) Threads
| BSP size | Threads per inch | OD (mm) | Tap drill (mm) |
|---|---|---|---|
| 1/8" | 28 | 9.73 | 8.8 |
| 1/4" | 19 | 13.16 | 11.8 |
| 3/8" | 19 | 16.66 | 15.3 |
| 1/2" | 14 | 20.96 | 19.0 |
| 3/4" | 14 | 26.44 | 24.5 |
| 1" | 11 | 33.25 | 30.5 |
| 1 1/4" | 11 | 41.91 | 39.0 |
| 1 1/2" | 11 | 47.80 | 45.0 |
| 2" | 11 | 59.61 | 56.5 |
NPT (National Pipe Taper) Threads
| NPT size | Threads per inch | OD (mm) | Tap drill (mm) |
|---|---|---|---|
| 1/8" | 27 | 10.29 | 8.7 |
| 1/4" | 18 | 13.72 | 11.1 |
| 3/8" | 18 | 17.15 | 14.5 |
| 1/2" | 14 | 21.34 | 18.0 |
| 3/4" | 14 | 26.67 | 23.0 |
| 1" | 11.5 | 33.40 | 29.0 |
| 1 1/4" | 11.5 | 42.16 | 38.0 |
| 1 1/2" | 11.5 | 48.26 | 44.0 |
| 2" | 11.5 | 60.33 | 56.0 |
BSP vs NPT are not compatible
BSP and NPT threads have different thread angles (55° vs 60°) and different pitches. They cannot be mixed. A BSP fitting in an NPT thread will leak. Always verify the thread standard before assembly.
How CHI can help
Ask CHI about thread sizes and to recommend the correct tap drill size for any thread standard. The CHI assistant can look up specific values, explain what each table entry means, and help you cross-reference between different standards and unit systems.