Thread Pitch Calculator
Convert thread pitch both directions: enter threads per inch (TPI) to get metric pitch in millimeters, or enter a metric pitch in mm to get TPI. Also shows pitch in inches and the lead if the thread has multiple starts.
Example: with Convert TPI → metric pitch (mm) · Value (TPI or mm) 20 · Number of starts 1 → Converted pitch: 1.270 mm per thread.
- Pitch in inches0.0500 in (20.00 TPI = 1.270 mm)
- Lead (advance per turn)1.270 mm (0.0500 in) per turn
Computed by the calculator below using its default values. Change any input to see your own numbers.
Pitch (mm) = 25.4 ÷ TPI and TPI = 25.4 ÷ pitch, from the exact definition 1 in = 25.4 mm. Lead = pitch × number of starts.
TPI and metric pitch measure the same thing backwards
Inch threads count crests per inch (TPI); metric threads state the distance between crests in millimeters. One is the reciprocal of the other through the exact conversion 1 inch = 25.4 mm: pitch in mm = 25.4 ÷ TPI. So a 1/4-20 bolt (20 TPI) has a 1.27 mm pitch, and an M6×1.0 works out to 25.4 TPI. Higher TPI means a finer thread; higher metric pitch means a coarser one — an easy sign flip to trip over.
Pitch is also why 'close enough' fasteners strip. M8×1.25 (20.3 TPI) looks like it might mate with 5/16-18, but the 11% pitch mismatch means only the first threads engage before binding. Measure with a pitch gauge, or count full threads against a ruler over an inch (or 10 mm) of length.
How it’s calculated
Pitch (mm) = 25.4 ÷ TPI; TPI = 25.4 ÷ pitch (mm); pitch (in) = 1 ÷ TPI. The inch-millimeter factor is exact by definition (NIST: 1 in = 25.4 mm). Lead = pitch × number of starts; a single-start thread advances one pitch per revolution.
Converts pitch geometry only — it does not check whether diameters, thread angles, or classes of fit make two fasteners actually compatible.
Pitch equivalents for common sizes
| Thread | Pitch | Equivalent |
|---|---|---|
| 1/4-20 UNC | 0.0500 in | 1.270 mm |
| 5/16-18 UNC | 0.0556 in | 1.411 mm |
| M6 × 1.0 | 1.0 mm | 25.40 TPI |
| M8 × 1.25 | 1.25 mm | 20.32 TPI |
| M10 × 1.5 | 1.5 mm | 16.93 TPI |
| 1/2-13 UNC | 0.0769 in | 1.954 mm |
Computed with pitch = 25.4 ÷ TPI (exact inch definition); standard UNC/ISO coarse sizes.
Common mistakes
- Reading a metric pitch number as TPI — a 1.5 pitch is coarse, but 1.5 TPI would be a thread two-thirds of an inch long.
- Assuming near-matches interchange: M8×1.25 and 5/16-18 differ by 11% in pitch and will gall after two turns.
- Confusing pitch with lead on multi-start threads — a 2-start thread advances two pitches per revolution.
- Counting threads over a half inch and forgetting to double before calling it TPI.
Frequently asked questions
How do I convert TPI to metric pitch?
Divide 25.4 by the TPI: pitch (mm) = 25.4 ÷ TPI. A 20 TPI thread has a 25.4 ÷ 20 = 1.27 mm pitch. The reverse works the same way: TPI = 25.4 ÷ pitch.
What is the difference between pitch and TPI?
Pitch is the crest-to-crest distance (used by metric threads, in mm); TPI is how many crests fit in one inch (used by Unified threads). They are reciprocals through 25.4 mm per inch.
What is lead, and when does it differ from pitch?
Lead is how far the nut advances in one full turn. On common single-start threads lead equals pitch; on a 2-start thread the nut rides two parallel helixes and advances two pitches per turn.
How do I measure the pitch of an unknown bolt?
Use a thread pitch gauge, or hold the bolt against a ruler: count full threads in exactly one inch for TPI, or measure the span of 10 threads in mm and divide by 10 for metric pitch. Then convert here.