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.

What is the Thread Pitch Calculator?

Convert threads-per-inch to pitch in millimeters or metric pitch to TPI, and calculate the major, pitch, and minor diameters for 60-degree Unified and metric, 29-degree Acme, and Buttress thread profiles. Useful for lathe thread cutting and gauge selection. It runs free in your browser on Gera Tools, with nothing uploaded.

Thread Pitch Calculator

Name: Thread Pitch Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Thread profiles and when they matter

Unified (UN/UNF) and metric (M) threads share the 60-degree flank angle, so their depth constants are identical. The difference is only the unit system: a 1/4-20 UNC and an M6×1.0 both follow the 60-degree geometry.

Acme threads (29-degree included angle) are designed for power transmission — lead screws, jacks, and vises. Their shallower, wider profile is stronger under axial load than a 60-degree thread would be. The depth constants are different because the thread depth is roughly half the pitch, not the 0.6495 factor used for 60-degree forms.

Buttress threads carry load in one direction only, with a near-perpendicular pressure flank and a sloped relief flank. They appear in gun breeches, bottle caps, and pipe couplings where the force is axial in one direction. The tool uses a representative approximation; for critical work confirm against the specific Buttress standard.

Worked example: 1/4-20 UNC

TPI: 20
Pitch: 25.4 ÷ 20 = 1.27 mm (or 0.050 inch)
Major diameter: 0.2500 inch
Pitch diameter: 0.2500 − (0.6495 × 0.050) = 0.2500 − 0.0325 = 0.2175 inch
Minor diameter: 0.2500 − (1.2269 × 0.050) = 0.2500 − 0.0613 = 0.1887 inch

These match the published basic dimensions for 1/4-20 UNC. Note that real threads have tolerance bands applied on top of these basic dimensions — the tool gives the nominal (zero-tolerance) values, which are the mid-point targets for machining.

Practical use on the lathe

When single-pointing, set your compound to cut toward the pitch diameter. Use a thread micrometer or thread gauge to check where you are; a simple thread mic reads the pitch diameter directly. Stop cutting when the pitch diameter reaches the nominal value and the thread fits the mating part — do not try to hit the minor diameter exactly, since the root form is usually left by the tool profile.

For gauge selection, the pitch is the key dimension: match it to the pitch of your thread micrometer or ring gauge. Many gauges are labelled by pitch in millimetres or by TPI, so the conversion this tool provides also tells you which gauge to reach for.