What is the difference between pitch and TPI?

Pitch is the distance from one thread crest to the next, used in metric threads in millimeters. TPI counts how many threads fall in one inch, used in inch threads. They are reciprocals scaled by 25.4: pitch in mm equals 25.4 divided by TPI.

How are the pitch and minor diameters found?

For 60-degree Unified and metric threads the pitch diameter is the major diameter minus 0.6495 times the pitch, and the basic minor diameter is the major minus 1.2269 times the pitch. These constants come from the standard thread-form geometry.

Why do Acme threads use different constants?

Acme threads have a 29-degree included angle and a square-ish profile, so their pitch and minor diameters subtract roughly half and one full pitch respectively rather than the 60-degree factors used for UN and metric threads.

Can I use this to set up a lathe?

Yes. The pitch tells you the leadscrew setting, and the pitch and minor diameters give the target you cut to and the depth to feed in. Always verify a finished thread with a gauge or mating part before relying on the numbers.

Why is the Buttress profile marked approximate?

Buttress threads come in several pressure-flank angle variants and the exact diameters depend on the standard chosen. The tool uses a representative single-depth approximation, so confirm against the specific Buttress standard for critical work.

Thread Pitch Calculator

Thread pitch ties together gauge selection, lathe setup, and inspection. This calculator converts cleanly between threads-per-inch and metric pitch, and then derives the pitch and minor diameters from the thread profile so you have the exact targets needed to cut or measure a thread.

How it works

Pitch and TPI are reciprocals once you account for the inch-to-millimeter factor:

pitch (mm) = 25.4 / TPI
TPI        = 25.4 / pitch (mm)

For a 60-degree Unified or metric profile the diameters follow from the pitch:

pitch diameter = major − 0.6495 × pitch
minor diameter = major − 1.2269 × pitch

Acme and Buttress profiles use their own depth factors because their flank angles differ from the 60-degree form.

Example and tips

A 1/4-20 thread has a pitch of 1/20, or 0.05 inch (1.27 mm). On a 0.25 inch major diameter the pitch diameter comes out at 0.2175 inch and the minor at about 0.1887 inch, matching the published basic dimensions. When single-pointing on a lathe, feed to the pitch-diameter target on the compound and confirm fit with the mating part, since real threads carry tolerance allowances the basic dimensions do not show.