Thread Pitch Conversion Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 10/11/2026
Thread pitch in mm is calculated with 25.4 ÷ TPI (threads per inch), since one inch is exactly 25.4 mm. A 20 TPI thread (a common UNF standard) corresponds to a pitch of 1.27 mm.
Explanation
A thread's pitch is the distance between two consecutive threads, a quantity that determines compatibility between a screw and its nut or tapped hole: two threads of the same diameter but different pitch cannot assemble together. Countries using the imperial system (notably the United States and United Kingdom) traditionally express this pitch in TPI (threads per inch), the number of threads counted over one inch of length, while the metric system expresses it directly in millimeters between two threads. The conversion between the two is a purely arithmetic relationship, with no approximation at all: since one inch is exactly 25.4 mm by international definition, a 20 TPI thread has threads spaced 25.4 ÷ 20 = 1.27 mm apart. This calculator converts only the thread PITCH (a precise physical quantity), never the nominal diameter of a commercial designation like "M6" or "1/4 inch": unlike pitch, these metric and imperial nominal diameters never match exactly (an M6 measures 6 mm in diameter, while a 1/4 inch measures 6.35 mm — two neighboring but distinct standards, not an exact conversion of one another), which would make such a correspondence misleading if presented as a precise conversion rather than a mere table proximity. For a given length measurement, this same gap between inches and millimeters also explains why a cable sized in AWG never matches a round metric cross-section exactly.
Example: a 20 TPI UNF thread
Inputs
Threads per inch: 20 TPI.
Calculation
Pitch = 25.4 ÷ 20 = 1.27 mm.
Result
This 20 TPI thread has a pitch of 1.27 mm between two consecutive threads.
Frequently asked questions
Why does a higher TPI mean a smaller pitch?
Because TPI counts the number of threads over a fixed length of one inch: if more threads are packed into that same length, each thread is necessarily closer to the next one, so the pitch (the distance between two threads) is smaller. An 80 TPI thread (very fine, used for example in watchmaking or precision instrumentation) thus has a much smaller pitch than a 10 TPI thread (coarse).
Does this calculator tell me if a metric and an imperial screw are interchangeable?
It lets you compare their respective pitches once converted to the same unit, but real mechanical compatibility also depends on the nominal diameter, which never converts exactly between the two systems (an M6 and a 1/4 inch are close but different diameters, 6 mm versus 6.35 mm). Even with an identical pitch after conversion, two threads with slightly different diameters generally won't assemble properly.
What's the difference between a UNC and a UNF thread?
Both are imperial thread standards with a constant pitch for a given diameter: UNC (Unified Coarse) has a larger pitch (lower TPI, threads farther apart), generally used for quick assembly and disassembly; UNF (Unified Fine) has a smaller pitch (higher TPI), offering more precise tightening and better resistance to loosening from vibration, but slower assembly.