TPI to pitch converter
Enter threads per inch or the pitch in millimeters and the other one is calculated. You also see the nearest standard pitch and how far the thread drifts over your length if you cut with it.
Quick answer
Pitch P = 25.4 / TPI mm (or 1 / TPI inch), back TPI = 25.4 / P. Example: 13 TPI (1/2-13 UNC) → P ≈ 1.9538 mm = 0.076923″; a 1.5 mm pitch ≈ 16.93 TPI.
Result
- Pitch in mm
- —
- Pitch in inches
- —
- Nearest metric pitch
- —
- Nearest standard TPI
- —
- K word for G33
- —
TPI and pitch chart
Click a row to load it into the calculator.
| TPI | Pitch, mm | Pitch, in | Nearest metric |
|---|---|---|---|
| 4 | 6.3500 | 0.25000 | 6 |
| 4.5 | 5.6444 | 0.22222 | 5.5 |
| 5 | 5.0800 | 0.20000 | 5 |
| 6 | 4.2333 | 0.16667 | 4 |
| 7 | 3.6286 | 0.14286 | 3.5 |
| 8 | 3.1750 | 0.12500 | 3 |
| 9 | 2.8222 | 0.11111 | 3 |
| 10 | 2.5400 | 0.10000 | 2.5 |
| 11 | 2.3091 | 0.09091 | 2.5 |
| 11.5 | 2.2087 | 0.08696 | 2 |
| 12 | 2.1167 | 0.08333 | 2 |
| 13 | 1.9538 | 0.07692 | 2 |
| 14 | 1.8143 | 0.07143 | 1.75 |
| 16 | 1.5875 | 0.06250 | 1.5 |
| 18 | 1.4111 | 0.05556 | 1.5 |
| 19 | 1.3368 | 0.05263 | 1.25 |
| 20 | 1.2700 | 0.05000 | 1.25 |
| 24 | 1.0583 | 0.04167 | 1 |
| 27 | 0.9407 | 0.03704 | 1 |
| 28 | 0.9071 | 0.03571 | 1 |
| 32 | 0.7937 | 0.03125 | 0.8 |
| 36 | 0.7056 | 0.02778 | 0.7 |
| 40 | 0.6350 | 0.02500 | 0.6 |
| 44 | 0.5773 | 0.02273 | 0.6 |
| 48 | 0.5292 | 0.02083 | 0.5 |
| 56 | 0.4536 | 0.01786 | 0.45 |
| 64 | 0.3969 | 0.01563 | 0.4 |
| 72 | 0.3528 | 0.01389 | 0.35 |
| 80 | 0.3175 | 0.01250 | 0.3 |
Who needs it and why
Inch threads are given in threads per inch, metric ones by pitch in millimeters. The machine and the program work with the pitch, so TPI has to be converted — accurately enough, or the error builds up over a long thread.
- Cutting inch threads — on a metric machine: the pitch in mm and the G33 K word with enough decimals.
- Repairs — when you need a mating part for an American or British assembly and only know the pitch.
- Deciding whether to substitute — a close metric thread for an inch one (or back) and how much error builds up over your length.
- Pipe threads — 11.5 and 27 TPI (NPT), 19 TPI (BSP) — all in the chart.
The formula
pitch, mm = 25.4 / TPI, and back TPI = 25.4 / pitch. For example, 13 TPI = 1.9538 mm, and a 1.5 mm pitch = 16.93 TPI.
Can you cut an inch thread with a metric pitch
On a CNC lathe you don't have to: the pitch is any number in the program. But when the mating part already exists and the pitches are close, the error matters: over a thread or two you won't see it, but on a long nut even a few hundredths of a millimeter make the thread bind. The calculator shows that error over your length.
Why K needs six decimals and what else changes in G20 is covered in Inch threads in LinuxCNC. Thread depth and passes are in the thread calculator.