Screw threads are specified two different ways: imperial fasteners are called out by threads per inch (TPI), while metric fasteners use pitch in millimeters. Converting between the two — and understanding lead on multi-start threads — comes up constantly in machining, 3D-printer maintenance, and CAD work.
Why TPI and metric pitch are just two views of the same spacing
Both TPI and pitch describe the same physical property: how closely spaced the thread crests are along the screw's axis. TPI counts crests per inch; pitch measures the distance between crests directly in millimeters. Because one inch is defined as exactly 25.4 mm, the two are related by a simple reciprocal: pitch (mm) = 25.4 / TPI, and TPI = 25.4 / pitch (mm). There's no approximation here — the 25.4 mm/inch conversion factor is exact, so the math is exact too (subject only to how many decimal places you round to).
Reading pitch and TPI off real parts
A bolt callout like "1/4-20 UNC" means a 1/4-inch nominal diameter with 20 threads per inch. A metric callout like "M6 x 1.0" means a 6 mm nominal diameter with a 1.0 mm pitch. When matching or replacing a fastener across the imperial/metric divide, converting the pitch is necessary but not sufficient — the thread angle (60° for both UN and ISO metric, but not for pipe threads like NPT/BSPT) and the diameter also need to line up, or the threads won't mesh even if the pitch number is close.
When lead differs from pitch
On the overwhelming majority of fasteners, lead and pitch are the same number because the thread is single-start. Multi-start threads exist specifically to decouple "how fine/strong is each thread groove" from "how far does the part move per turn." A 2-start thread with a 1.5 mm pitch has the same thread depth and engagement as a single-start 1.5 mm-pitch thread, but advances 3.0 mm per revolution instead of 1.5 mm — useful anywhere fast axial travel matters more than maximum holding torque per turn, such as camera focus rings, quick-release clamps, and some ball-screw linear actuators.
Limits of this calculator
This tool converts pitch, TPI, and lead using the fixed 25.4 mm/inch relationship — it does not validate thread compatibility, thread angle, major/minor diameter, or class of fit (tolerance). Two threads with the same pitch can still fail to mate if their diameters, angles, or standards (UN vs. ISO metric vs. pipe thread) differ. For a firm fastener match, always confirm against a thread chart or by test-fitting, especially for load-bearing or pressure-sealing applications.