Thread Pitch Diameter Calculator
Calculate pitch diameter, minor diameter, and tap drill size for metric and imperial screw threads.
Understanding thread geometry:
Every screw thread has three critical diameters: major (the outer diameter you measure with calipers), pitch (the effective diameter where the thread flanks engage), and minor (the root diameter at the bottom of the thread grooves). The pitch diameter is the most important for thread fit — it determines whether a bolt threads into a nut smoothly or binds.
Metric thread formulas (ISO 68-1):
For a standard 60° V-thread profile:
Pitch diameter (d₂) = Major diameter - 0.6495 × Pitch
Minor diameter (d₁) = Major diameter - 1.0825 × Pitch
Tap drill size = Major diameter - Pitch
Thread depth (H) = 0.6134 × Pitch
Imperial (Unified) thread formulas:
Pitch = 1 / TPI (threads per inch)
Pitch diameter = Major diameter - 0.6495 × Pitch
Minor diameter = Major diameter - 1.0825 × Pitch
Common metric thread reference:
| Thread | Major (mm) | Pitch (mm) | Pitch Dia (mm) | Minor Dia (mm) | Tap Drill (mm) |
|---|---|---|---|---|---|
| M3 | 3.000 | 0.50 | 2.675 | 2.459 | 2.50 |
| M4 | 4.000 | 0.70 | 3.545 | 3.242 | 3.30 |
| M5 | 5.000 | 0.80 | 4.480 | 4.134 | 4.20 |
| M6 | 6.000 | 1.00 | 5.350 | 4.917 | 5.00 |
| M8 | 8.000 | 1.25 | 7.188 | 6.647 | 6.80 |
| M10 | 10.000 | 1.50 | 9.026 | 8.376 | 8.50 |
| M12 | 12.000 | 1.75 | 10.863 | 10.106 | 10.20 |
| M16 | 16.000 | 2.00 | 14.701 | 13.835 | 14.00 |
| M20 | 20.000 | 2.50 | 18.376 | 17.294 | 17.50 |
Example calculation (M10 × 1.5):
- Major diameter: 10.000 mm
- Pitch: 1.5 mm
- Pitch diameter: 10.0 - (0.6495 × 1.5) = 10.0 - 0.974 = 9.026 mm
- Minor diameter: 10.0 - (1.0825 × 1.5) = 10.0 - 1.624 = 8.376 mm
- Tap drill: 10.0 - 1.5 = 8.5 mm
- Thread depth: 0.6134 × 1.5 = 0.920 mm
Thread engagement and strength:
For a standard bolt-nut connection, 60–75% thread engagement provides full rated strength. The tap drill formula above gives approximately 75% engagement. Deeper engagement (smaller drill) increases tapping torque and tap breakage risk without significant strength gain.
Fine vs. coarse threads:
Fine pitch threads have smaller pitch for the same major diameter, giving a larger minor diameter (stronger in tension), better vibration resistance, and finer adjustment. Coarse threads are easier to assemble, more tolerant of damaged threads, and standard for general fastening.