Convert metric pitch into threads per inch and estimate the basic ISO pitch diameter from major diameter and pitch.
| TPI | – |
| Pitch diameter | – |
Use this calculator when you can measure a bolt's major diameter and pitch, or read them from a stamp such as M8×1.25.
TPI = 25.4 ÷ pitch. Pitch diameter ≈ major − 0.6495 × pitch, the basic ISO metric relation. Major diameter and pitch can be entered in millimetres (mm) or inches (in). Pitch diameter follows the same unit. TPI is threads per inch and is not converted.
This is a basic-size estimate. It is not a full thread-inspection layout, a tolerance class, or a go/no-go gauge plan.
Major diameter: Outside (major) diameter, in mm or in. An M10 is nominally 10 mm.
Pitch: Distance between threads, in mm or in. Coarse M10 is 1.5 mm; fine might be 1.25 or 1.0 mm.
TPI: Threads per inch, from 25.4 mm ÷ pitch.
Pitch diameter: Estimated basic pitch diameter.
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