Online Tool Store
Screw Thread Size Converter
Screw Thread Size Converter is for identifying and comparing threads, not for justifying a substitution — the nearest equivalent is almost never a mating part.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Unit & Converters
Screw Thread Size Converter
Frontend preview — no upload or external service.
Thread conversion
M8x1.25 has a 1.25 mm pitch, equivalent to about 20.3 TPI. The nearest imperial size is 5/16-18 UNC — close, but not interchangeable.
How the Screw Thread Size Converter works
- Enter the thread designation as it appears on the fastener or drawing.
- Choose the thread system it belongs to — coarse and fine differ in pitch, not diameter.
- Read the nearest equivalent, and check the pitch difference before assuming the two will mate.
The method
Metric threads are specified by diameter and pitch in millimetres; imperial threads by diameter and threads per inch.
TPI = 25.4 / pitch in mm
The two systems almost never produce an exact match, which is why a near equivalent is a substitution rather than an interchange.
FAQ
Can I substitute a metric bolt for an imperial one?
Rarely. Even where diameters look close, the pitch differs, so the threads will cross-thread and strip. Nearest-equivalent figures are for identifying an unknown fastener, not for mixing systems.
What is the difference between coarse and fine threads?
Pitch. A fine thread has more threads per unit length, which gives greater tensile strength and better resistance to vibration loosening, at the cost of being easier to cross-thread and slower to run down.
How do I identify an unknown thread?
Measure the outside diameter with calipers and the pitch with a thread gauge. Diameter alone is not enough, because coarse and fine threads of the same diameter look similar and will not mate.
How we compare
| Feature | Online Tool Store | A search-engine answer box | A reference table |
|---|---|---|---|
| Both systems side by side | ✓ | One at a time | ✓ |
| Pitch and TPI shown | ✓ | ✗ | ✓ |
| Warns about non-interchangeable pairs | ✓ | ✗ | ✗ |
| No install | ✓ | ✓ | ✗ |
Screw Thread Size Converter is for identifying and comparing threads, not for justifying a substitution — the nearest equivalent is almost never a mating part.