ThreadSizeGuideThreads · Drills · Fasteners

Machining fundamentals

Thread Pitch vs TPI: The Difference and Exact Conversion

Pitch is the distance from one thread point to the next. TPI is the number of thread intervals per inch. Convert with TPI = 25.4 ÷ pitch_mm and pitch_mm = 25.4 ÷ TPI.

1.0 mm pitch25.4 TPI
1.25 mm pitch20.32 TPI
20 TPI1.27 mm pitch
32 TPI0.79375 mm pitch

The equations use exactly 25.4 millimeters per inch. The displayed result is rounded; the calculation engine retains an exact rational value until formatting.

Metric pitch distance compared with threads per inch count
Original ThreadSizeGuide diagram. Pitch measures distance per thread; TPI counts threads per inch. They are reciprocal descriptions.

Conversion is comparison, not identity

A converted TPI value does not make a metric thread interchangeable with an inch thread. Diameter, profile, tolerances and standard remain different.

Measure several intervals

When a pitch gauge is unavailable, measuring across several thread intervals and dividing reduces the effect of reading error. Do not count crest endpoints as extra full intervals.

Important limitation
Never force a nearly matching thread because the converted pitch looks close.