If you do not have a thread gauge, count the thread crests over a known length with a ruler or caliper. Metric threads are specified by pitch in millimetres; unified (inch) threads by threads per inch. Counting over a longer length gives a more accurate result.
How to use the Thread Pitch Calculator
- Measured length
- Number of threads counted
- Every result field is editable: type into Thread pitch and Number of threads counted plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the measured length and the number of complete threads in it. Type a pitch into the result to find how many threads fit in the length.
Inputs explained
| Field | What to enter |
|---|---|
| Measured length | Units: mm, in |
| Number of threads counted | A number |
What each result means
- Thread pitch = [Measured length]÷[Number of threads counted]
- Threads per inch (TPI) = 0.0254÷[Thread pitch]
Example values
How the main result changes when Number of threads counted changes and everything else stays at the example values above:
| Number of threads counted | Thread pitch (mm) |
|---|---|
| 5 | 3 |
| 7.5 | 2 |
| 10 | 1.5 |
| 12.5 | 1.2 |
| 15 | 1 |
| 20 | 0.75 |
Formula
Pitch P = length ÷ number of threads
TPI = threads ÷ length in inches = 25.4 ÷ P (mm)
Worked example
Inputs: Measured length: 15 mm; Number of threads counted: 10.
Results: Thread pitch = 1.5 mm; Threads per inch (TPI) = 16.93.
Frequently asked questions
How do I convert TPI to pitch?
Pitch (mm) = 25.4 ÷ TPI. 20 TPI = 1.27 mm.
Is ¼-20 the same as M6 × 1?
No. ¼ in is 6.35 mm and 20 TPI is 1.27 mm pitch; they are not interchangeable.
How accurate is counting threads?
Counting 10 or more threads over a measured length usually identifies the standard pitch exactly.