TPI to MM Pitch Converter
| Thread | TPI | Pitch (mm) | Note |
|---|---|---|---|
| 1/4-20 UNC | 20 | 1.27 | coarse, the classic 1/4 |
| 1/4-28 UNF | 28 | 0.907 | fine 1/4 |
| 5/16-18 UNC | 18 | 1.411 | coarse 5/16 |
| 3/8-16 UNC | 16 | 1.587 | coarse 3/8 |
| 1/2-13 UNC | 13 | 1.954 | coarse 1/2 |
| M8 x 1.25 | 20.32 | 1.25 | metric names pitch directly |
| M10 x 1.5 | 16.93 | 1.5 | metric 10 mm |
Threads describe themselves in two vocabularies: the imperial world counts threads per inch - a 1/4-20 bolt runs 20 threads to the inch, a pitch of exactly 1.27 mm - while the metric world names the distance directly: M8x1.25 means one thread every 1.25 mm. The bridge is one division: pitch in mm = 25.4 divided by TPI. Type either number and the converter lands the other, which is the fastest way to answer the question every workshop hits eventually: will this metric bolt fit this imperial hole?
The answer to that question is almost always no - a 1.27 mm pitch and a 1.25 mm pitch differ by two percent, enough to jam after three turns and strip by the fifth - which is exactly why the conversion matters: identifying the thread you are holding before forcing it. The table carries the everyday UNC coarse, UNF fine and metric sizes; a thread pitch gauge or this converter turns a mystery bolt into a named part in seconds.
How to use
- Count the threads per inch - hold a ruler against the bolt and count crests in one inch, or in a half-inch and double it - then type the TPI.
- Or type a metric pitch in mm to read the equivalent TPI, and check whether your mystery thread is imperial after all.
- Match the table: UNC coarse threads step 20, 13, 11, 10 TPI as size climbs; UNF fine runs higher; metric pitches are round numbers by design.
Frequently asked questions
How do I convert TPI to mm pitch?
Divide 25.4 by the TPI: 20 threads per inch is 25.4 / 20 = 1.27 mm pitch, 13 TPI is 1.954 mm, 28 TPI is 0.907 mm. The 25.4 is the inch defined in millimeters - exact since 1959, no rounding anywhere. The reverse multiplies: a 1.5 mm pitch is 25.4 / 1.5 = 16.9 threads per inch, which is no standard thread and a strong hint you are holding something metric.
How do I measure the pitch of a mystery thread?
Two shop methods: hold a ruler along the bolt and count crests in one inch (count a half-inch and double for fine threads), or squeeze a thread pitch gauge onto the profile until a leaf seats. Metric bolts make it easier - the size IS the pitch, stamped right in the name: M8x1.25 is 8 mm diameter, 1.25 mm pitch. Calipers work too: measure crest-to-crest across ten crests and divide by ten.
What is the difference between UNC and UNF?
Coarse versus fine, both imperial: a 1/4-inch bolt comes in UNC at 20 TPI (1.27 mm) or UNF at 28 TPI (0.907 mm). Coarse threads assemble faster, tolerate damage and dirt, and grip softer materials; fine threads are stronger in tension, resist vibration loosening better, and allow finer adjustment. The two are not interchangeable - same diameter, different pitch, and forcing one into the other's nut wrecks both.
Why does metric name pitch while imperial names TPI?
Metric threads are specified by the distance between crests because the metric system lives in distances: M8x1.25 says everything - diameter 8, pitch 1.25 mm. Imperial threads predate that habit and count how many fit in an inch, which was easier to check with a ruler in 1864 when the Unified standard formed. The two conventions survive their standards committees; the converter is the translator.
Will a metric bolt fit an imperial thread of similar size?
Almost never, and the near-misses are the dangerous ones. M8x1.25 against 5/16-18 (1.411 mm) or 1/4-28 (0.907 mm) - diameters differ slightly and pitches differ enough that a turn or two feels right, then the threads gall and lock. The two-percent trap: 20 TPI (1.27 mm) and M8x1.25 (1.25 mm) feel identical for exactly two turns. Identify first - a gauge or this converter costs seconds; a seized bolt costs the afternoon.