Thread Calculator
Whether you are selecting a tap drill, checking thread compatibility, or programming a CNC lathe, you need accurate thread dimensions. Enter the major diameter and thread pitch to calculate the pitch diameter, minor diameter, thread height, and recommended tap drill size.
Thread Geometry Basics
All standard 60-degree threads (Unified and Metric ISO) use the same basic geometry. The key dimensions are:
- Major diameter (D): The outer diameter of the bolt or the inner diameter of the nut thread
- Pitch diameter: The diameter where the thread tooth and groove are equal width. This is the most critical dimension for fit.
- Minor diameter: The root diameter of an external thread (or the crest diameter of an internal thread)
- Thread height (H): The theoretical height of a perfect sharp V-thread: H = pitch x sqrt(3) / 2
Common Thread Sizes
Unified (Inch):
| Size | TPI (Coarse) | TPI (Fine) |
|---|---|---|
| 1/4" | 20 | 28 |
| 5/16" | 18 | 24 |
| 3/8" | 16 | 24 |
| 1/2" | 13 | 20 |
| 3/4" | 10 | 16 |
Metric:
| Size | Pitch (Coarse) | Pitch (Fine) |
|---|---|---|
| M6 | 1.0 mm | 0.75 mm |
| M8 | 1.25 mm | 1.0 mm |
| M10 | 1.5 mm | 1.25 mm |
| M12 | 1.75 mm | 1.25 mm |
Tap Drill Sizing
A 75% thread engagement is the standard target for tapped holes. Going higher than 75% does not add much strength but makes tapping significantly harder. The tap drill formula is approximately: major diameter minus one pitch.