Thread Pitch Calculator

Background.

Inch and metric fasteners describe the same geometry in opposite ways. A metric bolt is labelled with its pitch, the distance between neighbouring crests, so an M8 coarse thread is marked 1.25. A Unified inch bolt is labelled with a count instead: a 1/4-20 has 20 threads in every inch of length. To compare the two, or to set a lathe's gearbox, buy a tap, or match a pitch gauge, you have to turn one description into the other.

The conversion is a single division. An inch has been exactly 25.4 mm since the international yard was fixed in 1959, so the pitch in millimetres is 25.4 divided by the threads per inch, with no approximation anywhere. Coarse inch threads come out near the coarse metric series, which is where the trouble starts. A 3/8-16 has a pitch of 1.5875 mm against 1.5 mm for an M10, a gap of under a tenth of a millimetre per turn. Two threads that close can start by hand and then bind a few turns in, after the mismatch has built up to around half a millimetre, and forcing them strips the softer part.

The answer is pitch only. It says nothing about the thread's diameter, its form (60° Unified, Whitworth's 55°, the taper of a pipe thread), its tolerance class, or whether it is a multi-start thread whose lead is a multiple of the pitch. Identifying a mystery bolt still needs a caliper on the major diameter as well as the count.

What is thread pitch calculator?

Thread pitch is the axial distance from a point on one thread to the corresponding point on the next, measured parallel to the screw's axis. Threads per inch is the reciprocal of that distance expressed in inches: how many pitches fit into one inch of length. On a single-start thread the pitch is also how far the screw advances in one full turn; on a multi-start thread the advance per turn, called the lead, is the pitch multiplied by the number of starts.

How to use this calculator.

1. 01Enter the threads-per-inch figure, usually the number after the dash in an inch designation: 20 for a 1/4-20 UNC, 28 for a 1/4-28 UNF, 14 for a 1/2-inch NPT pipe thread.
2. 02If you are counting threads on an unknown bolt, lay a rule along it and count the gaps between crests over one inch, not the crests themselves; counting crests gives one too many.
3. 03For a fine thread that is hard to count, measure across ten pitches and divide by ten, or check with a pitch gauge, then convert whichever way you need.
4. 04Read the pitch in millimetres and compare it with the metric series you are matching against, remembering that the diameter must match as well.

The formula.

Pitch (mm) = 25.4 ÷ TPI. Threads per inch counts how many pitches fit in one inch, so one inch divided by that count is the length of a single pitch in inches; multiplying by 25.4 converts it to millimetres. The 25.4 is not a rounded conversion factor but the legal definition of the international inch, which is why the result carries no conversion error. Because TPI sits in the denominator, the relationship is inverse: doubling the thread count halves the pitch, and fine threads with high counts have small pitches. The same equation run backwards, TPI = 25.4 ÷ pitch, turns a metric pitch into an equivalent count, so a 1.0 mm pitch corresponds to 25.4 TPI.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ASME B1.1-2024, Unified Inch Screw Threads (BIBLIOGRAPHIC/PAYWALL).
2. [2]NIST Handbook 44, Appendix B, exact inch-to-millimetre relation.
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.