Thread Calculator

Background.

A screw thread is a helix, and the first thing anyone needs to know about it is how tightly that helix is wound. Two conventions describe the same spacing. Metric threads quote the pitch: the distance in millimetres from one crest to the next, measured along the axis, so an M10 coarse bolt is M10×1.5. Inch-series threads count instead. A 1/4-20 UNC bolt has 20 crests in every inch of its length, and the 20 is the number printed on the box.

One inch has been exactly 25.4 mm since the 1959 international yard and pound agreement, so moving between the two conventions is a single division: pitch = 25.4 ÷ TPI, and TPI = 25.4 ÷ pitch. The relationship is reciprocal. A finer thread has a smaller pitch and a larger thread count.

The conversion comes up whenever a part from one system meets a tap, die, nut or lathe gearbox from the other. It is also how near-misses get caught. A 1.25 mm metric pitch is 20.32 TPI, not 20. A 1/2-13 UNC bolt has a pitch of about 1.954 mm, while the nearest metric size, M12 coarse, uses 1.75 mm. Threads that look identical by eye can differ by a few hundredths of a millimetre per turn, and that error adds up along every thread that is engaged.

Spacing is all the result tells you. Major diameter, flank angle (60° for Unified and ISO metric threads, 55° for Whitworth and BSP pipe threads), tolerance class and the number of starts all decide whether two parts actually mate, and none of them enters this calculation.

What is thread calculator?

Thread pitch is the axial distance between matching points on neighbouring threads, crest to crest or root to root, given here in millimetres. Threads per inch is the number of those threads in 25.4 mm of length. Each is 25.4 divided by the other, so either one fully determines the other. On an ordinary single-start thread the pitch also equals the lead, the distance a screw advances in one full turn; on a two- or three-start thread the lead is two or three times the pitch, and only the pitch is given here.

How to use this calculator.

1. 01Under Known thread measure, choose Threads per inch if your number comes from an inch designation such as 3/8-16 or 1/4-28, or Pitch in millimetres if it comes from a metric one such as M8×1.25, where the pitch is the figure after the ×.
2. 02Type the number into Known value. Fractional thread counts are accepted, which matters for pipe threads: 1-inch NPT, for example, is 11.5 TPI.
3. 03Working from an unmarked part rather than a drawing, use a thread pitch gauge if you have one. Without it, measure with calipers from one crest to the eleventh crest along, which spans ten pitches, and divide by ten to get the pitch in millimetres.
4. 04Read the pitch and TPI side by side; one of them simply repeats what you typed. Before treating the converted figure as a match for a part in the other system, confirm the diameter and thread form as well.

The formula.

Pitch (mm) = 25.4 ÷ TPI, and TPI = 25.4 ÷ pitch (mm). The 25.4 is not a rounded figure: the international inch is defined as exactly 25.4 millimetres, so the conversion adds no error of its own beyond the rounding of the displayed result. Whichever measure you enter is passed straight through to its own output, and the other is found by dividing 25.4 by it. Because the relationship is a reciprocal, its sensitivity is uneven. Going from 10 to 11 TPI shortens the pitch by about 0.231 mm, while going from 40 to 41 TPI changes it by only about 0.015 mm. That is why fine threads one count apart are so hard to tell apart without a gauge. Thread form, diameter and the number of starts play no part in the result.

A worked example.

Frequently asked questions.

References& sources.

1. [1]NIST. International yard and pound, 1959. Exact inch definition: 25.4 mm. Retrieved 2026-08-06.
2. [2]National Institute of Standards and Technology. NIST SI Guide, 2008 edition. Appendix B unit relationships. Retrieved 2026-08-06.
3. [3]International Code Council. 2021 International Residential Code. Building construction context. Retrieved 2026-08-06.