Thread Pitch Calculator

Convert TPI to metric pitch, and get the basic pitch diameter, minor diameter and wire measurement for a 60° thread.

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Pitch

Threads per inch
Pitch (inch)
Pitch (mm)
Pitch diameter
Basic minor diameter
Thread height H
Best wire size
Over three wires
Nearest Standard

Saved Threads

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Name	TPI	Pitch (mm)	Pitch Dia.	Remove

How to Calculate Thread Pitch

Pitch is the distance from one thread to the next. Inch threads are named by threads per inch (TPI), metric threads by pitch in millimeters. They convert through 25.4 mm per inch.

Pitch (in) = 1 ÷ TPI Pitch (mm) = 25.4 ÷ TPI TPI = 25.4 ÷ pitch (mm)

Unified (UN) and ISO metric threads share the same 60° profile, so the basic dimensions follow the same formulas from the pitch P and major diameter D:

Pitch diameter = D − 0.6495 P Basic minor diameter = D − 1.0825 P Sharp-V height H = 0.8660 P

Best wire size = 0.57735 P Measurement over three wires = pitch diameter + 3 × wire − 0.86603 P

Worked Example

Close Inch and Metric Pitches

TPI	Pitch (mm)	Nearest Metric Pitch

Tips

A thread pitch gauge is the quickest way to identify a thread. Some inch and metric pitches are close enough to start by hand and then bind: 1 mm (25.4 TPI) against 24 or 26 TPI, and 1.25 mm (20.3 TPI) against 20 TPI. These are basic sizes; real parts have tolerance classes (2A/2B, 6g/6H) that make them slightly smaller or larger.