What Is TPI (Threads Per Inch)?

Definition

TPI = threads per inch. Count how many thread peaks fall within 1 inch measured along the bolt axis. For 1/4-20, there are 20 threads per inch, so pitch = 1/20 inch = 0.05 in. Metric threads use pitch in mm instead (e.g. M6 x 1.0 has 1.0 mm pitch).

TPI and pitch

Formula: pitch (mm) = 25.4 ÷ TPI. Use our Thread Pitch to TPI Converter.

Common imperial sizes

1/4-20 (coarse) and 1/4-28 (fine), 5/16-18, 3/8-16 are common. The number after the hyphen is TPI. Our Universal Screw & Bolt Size Chart lists pitch and TPI for many sizes.

FAQ

What does TPI mean?

TPI is threads per inch: the number of thread crests in one inch measured along the axis. Higher TPI means finer pitch; lower TPI means coarser.

How do you convert TPI to metric pitch?

Pitch in mm = 25.4 ÷ TPI. For example, 20 TPI = 25.4/20 = 1.27 mm. Use our Thread Pitch to TPI Converter for instant conversion.

Is higher TPI finer or coarser?

Higher TPI means finer thread (smaller distance between threads). Lower TPI means coarser thread (larger pitch).