Threads per inch to millimeters
Type a value in TPI to get it in mm instantly.
1.27mm
That's 1.27× as coarse as an M6 bolt, while an M10 bolt is 1.18× coarser. Ten millimeters of travel takes about 7.874 turns.
Convert the other way: mm to TPI
What it means
How coarse is that thread?
Pitch is only half the story: a thread also has a diameter and a flank angle. Metric threads use 60°, as do Unified threads, but British Whitworth uses 55°, so they never interchange.
The math
How to convert TPI to mm
Pitch and threads per inch are reciprocals: mm = 25.4 ÷ TPI, and TPI = 25.4 ÷ mm. A finer thread means more threads per inch and a smaller pitch.
Example: 20 TPI = 1.27 mm.
Every unit
Your pitch in every unit
Tables
TPI to mm conversion table
| TPI | mm |
|---|---|
| 4 TPI | 6.35 mm |
| 6 TPI | 4.23333 mm |
| 8 TPI | 3.175 mm |
| 10 TPI | 2.54 mm |
| 11 TPI | 2.30909 mm |
| 13 TPI | 1.95385 mm |
| 14 TPI | 1.81429 mm |
| 16 TPI | 1.5875 mm |
| 18 TPI | 1.41111 mm |
| 20 TPI | 1.27 mm |
| 24 TPI | 1.05833 mm |
| 28 TPI | 0.907143 mm |
| 32 TPI | 0.79375 mm |
| 40 TPI | 0.635 mm |
mm to TPI conversion table
| mm | TPI |
|---|---|
| 4 mm | 6.35 TPI |
| 6 mm | 4.23333 TPI |
| 8 mm | 3.175 TPI |
| 10 mm | 2.54 TPI |
| 11 mm | 2.30909 TPI |
| 13 mm | 1.95385 TPI |
| 14 mm | 1.81429 TPI |
| 16 mm | 1.5875 TPI |
| 18 mm | 1.41111 TPI |
| 20 mm | 1.27 TPI |
| 24 mm | 1.05833 TPI |
| 28 mm | 0.907143 TPI |
| 32 mm | 0.79375 TPI |
| 40 mm | 0.635 TPI |
Quick answers
Quick answers
- How many mm is 1 TPI?
- 1 TPI is 25.4 mm.
- How many mm is 10 TPI?
- 10 TPI is 2.54 mm.
- How many mm is 20 TPI?
- 20 TPI is 1.27 mm.
The units
About these units
Threads per inch (TPI)
The imperial measure: how many threads fit in one inch.
Millimeters (mm)
The metric pitch: how far a bolt advances in one turn.
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