By Zencalcy Editorial TeamLast reviewed About us
Imperial threads count threads per inch; metric threads state the distance between threads. One is the reciprocal of the other, scaled by 25.4. That conversion is enough to identify most fasteners and to pick a tap or die.
Convert between threads per inch and metric pitch.
pitch (mm) = 25.4 ÷ TPI TPI = 25.4 ÷ pitch
UNC uses coarse pitches: 1/4-20, 5/16-18, 3/8-16, 1/2-13. UNF uses fine: 1/4-28, 5/16-24, 3/8-24, 1/2-20. Metric coarse pitches: M6 × 1, M8 × 1.25, M10 × 1.5, M12 × 1.75.
| Symbol | Meaning |
|---|---|
| TPI | Threads per inch. |
| P | Pitch in millimetres. |
Pitch = 25.4 ÷ 13 = 1.954 mm, which is close to but not interchangeable with M12 × 1.75.
Continue with the Beam Deflection Calculator, Beam Load Calculator, Bending Stress Calculator, or the Wood Beam Span Calculator — or browse more structural & workshop and all construction calculators.
No. 1/4-20 is 1.27 mm pitch on a 6.35 mm diameter; M6 is 1.0 mm pitch on 6 mm. They will not thread together properly.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026