Thread Pitch Calculator: TPI, Metric Pitch & Lead

Published September 22, 2026 · MfgWorkbench editorial reference

Convert the distance between neighboring thread crests to threads per inch, or calculate metric pitch from TPI. Lead is shown separately so a multi-start thread is not mistaken for a single-start thread.

Pitch, TPI and lead are different quantities

Pitch is the axial spacing between adjacent thread forms. TPI counts those spacings per inch: pitch in mm = 25.4/TPI, and TPI = 25.4/pitch in mm. The exact length conversion is 1 inch = 25.4 mm.

Lead is the axial travel in one revolution. For a thread with s starts, lead = s × pitch. The helix count must be known independently; converting TPI cannot tell you how many starts a screw has. The calculator accepts integer start counts and rejects a zero pitch or TPI.

Worked examples

A 20 TPI thread has a pitch of 1.27 mm. If it has one start, it advances 1.27 mm per revolution. A two-start thread with that same pitch advances 2.54 mm per revolution. A 1.5 mm metric pitch corresponds mathematically to 16.933333 TPI, but that does not turn it into a standard 17 TPI thread.

When measuring with a ruler, measure across several complete pitches and divide by the number of intervals, not the number of visible crests. Eleven crests span ten pitch intervals. A thread pitch gauge is usually easier to compare than a short ruler measurement.

What this conversion cannot identify

Diameter, flank angle, taper, hand, thread class and sealing method are outside this conversion. Similar pitch does not establish interchangeability. In particular, matching TPI on two pipe fittings is not proof that NPT and BSP parts fit safely. Use the thread designation, appropriate gauges and manufacturer documentation. For tapping-hole selection, use the separate tap drill tool; pitch alone is not a hole specification.