Metric ↔ Imperial Thread Converter
Convert metric pitch to imperial threads per inch, or TPI back to metric pitch — either direction, instantly.
Pitch (mm) → TPI
TPI → Pitch (mm)
Why this is a straight division, not a lookup table
TPI = 25.4 ÷ pitch (mm), and pitch (mm) = 25.4 ÷ TPI — both directions are the same relationship, converting between "threads per inch" and "distance per thread in mm" through the fixed 25.4 mm-per-inch constant. Unlike matching a metric bolt to a "nearest" imperial size (which is genuinely approximate and depends on which property you're trying to match), pitch and TPI describe the exact same physical spacing, just measured in different units.
This converts the thread's pitch, not its diameter
A metric M8×1.25 thread and an imperial 5/16-24 thread happen to be close in outside diameter, but that doesn't mean their pitches convert to each other — this tool only converts the pitch number itself between mm-per-thread and threads-per-inch, not the whole thread designation. Don't use a pitch conversion here as a substitute for finding an actual equivalent fastener size.
Where this feeds into other calculators
If you have a metric print calling out pitch in mm but need TPI for an imperial-only tap chart, or the reverse, convert it here first, then carry the result into the Tap Drill Size Calculator or Tap Feed Calculator.