EngineeringFormulaMechanical Engineering
Thread Pitch Calculator
Convert thread pitch to TPI, TPI to pitch, calculate pitch from a measured thread sample, and find screw lead for multi-start threads.
Thread pitch is the axial distance from one thread crest to the next. Metric threads usually publish pitch directly in millimeters, while Unified and many imperial threads use TPI (threads per inch). This calculator supports both systems, measured-thread samples, and multi-start lead so the result can be used for thread gauges, tap selection, replacement screws, or motion-screw travel estimates.
TPI · Imperial thread count, such as 20 for a 1/4-20 thread.
s · Optional except in lead mode. Use 1 for ordinary screws; 2 or 3 for multi-start motion screws.
Pitch identifies thread spacing only. Confirm major diameter, thread form, hand, and tolerance class before buying taps, dies, nuts, or replacement screws.
Solution
Enter a TPI value, pitch, measured sample, or starts count to calculate thread pitch and lead.
Pmm = 25.4 / TPI
Formula Sheet
- PThread Pitch
- TPIThreads per Inch
- LMeasured Length
- NThread Intervals Counted
- sStarts
- LeadThread Lead
Common Thread Pitch References
| System | Common values | Use |
|---|---|---|
| Metric pitch | 0.5, 0.7, 1.0, 1.25, 1.5, 1.75, 2.0 mm | Metric threads are specified directly by pitch, e.g. M10 x 1.5. |
| Unified coarse/fine | 20, 18, 16, 14, 13, 12, 11.5, 11 TPI | Imperial/Unified threads are usually specified as threads per inch. |
| Single-start lead | lead = pitch | Most screws and bolts are single-start, so lead and pitch are the same. |
| Multi-start lead | lead = pitch x starts | Used for faster axial travel per turn, common in motion screws. |
Variables & Units
| Symbol | Variable | Description | Common Units |
|---|---|---|---|
| P | Thread Pitch | Axial distance between adjacent thread crests. | mm, in |
| TPI | Threads per Inch | Number of thread peaks or intervals in one inch. | threads/in |
| L | Measured Length | Length across a counted group of thread intervals. | in, mm |
| N | Thread Intervals Counted | Number of crest-to-crest intervals counted across the measured length. | intervals |
| s | Starts | Number of independent thread helices on a multi-start screw. | 1, 2, 3 |
| Lead | Thread Lead | Axial travel per one full screw revolution. | mm/rev, in/rev |
How to Use This Calculator
- 01Use TPI to Pitch when an imperial thread is labeled with threads per inch and you need the equivalent pitch.
- 02Use Pitch to TPI when a metric thread pitch is known and you want the equivalent imperial thread count.
- 03Use Measure Sample when you measure several threads over a known length. Count full crest-to-crest intervals across that length, then enter the measured length and interval count.
- 04Use Lead from Starts for multi-start screws. A single-start screw has lead equal to pitch; a 2-start screw advances two pitches per revolution.
- 05Check the nearest standard hints before choosing a tap, die, or gauge. Very close matches are helpful; final fit still depends on diameter, thread form, and tolerance class.
How the Formula Works
TPI and pitch are reciprocal measurements across one inch: pitch(in) = 1 / TPI and pitch(mm) = 25.4 / TPI.
For measured threads, pitch is measured length divided by thread count. The calculator then converts that pitch to TPI and common metric units.
Thread lead is pitch times the number of starts. Lead controls axial travel per revolution, while pitch is the spacing between adjacent thread crests.
Worked Example 01
Convert 20 TPI to pitch
Known
- Threads per inch: 20 TPI
Formula
Pmm = 25.4 / TPI
Substitution
P = 25.4 / 20
Result
Pitch = 1.27 mm = 0.050 in
This is the pitch spacing for a 20 TPI thread such as the pitch portion of a 1/4-20 callout.
Worked Example 02
Metric M10 x 1.5 pitch to TPI
Known
- Thread pitch: 1.5 mm
Formula
TPI = 25.4 / Pmm
Substitution
TPI = 25.4 / 1.5
Result
TPI ≈ 16.93 threads/in
A 1.5 mm metric pitch is close to 17 TPI numerically, but metric and Unified thread systems still differ in diameter and tolerances.
Worked Example 03
Three-start screw lead
Known
- Pitch: 2 mm
- Thread starts: 3
Formula
Lead = P x starts
Substitution
Lead = 2 x 3
Result
Lead = 6 mm/rev
A three-start screw with 2 mm pitch advances 6 mm for each full revolution.
Applications
- 01Converting 1/4-20, 10-32, or other TPI callouts to pitch
- 02Checking an M6 x 1.0 or M10 x 1.5 thread against an imperial equivalent
- 03Estimating pitch from a ruler or caliper measurement over multiple threads
- 04Finding lead for multi-start screws, lead screws, and motion-control travel per revolution
Assumptions
- 01The thread is measured along the screw axis, not around the circumference.
- 02TPI conversion uses the exact inch definition of 25.4 mm.
- 03Standard-match hints compare against common metric pitches and Unified TPI values only.
- 0460 degree thread-depth values are basic geometry context, not a manufacturing tolerance or class-of-fit calculation.
Where This Model Stops
- 01Does not identify major diameter, thread class, tolerance, flank angle, left/right hand, crest truncation, or thread form beyond simple 60 degree context.
- 02Does not replace a thread gauge, pitch micrometer, manufacturer drawing, or standard table for final fit.
- 03Metric and Unified threads can have similar pitch values but different diameter/tolerance systems, so nearest pitch alone does not identify the screw.
- 04Worn, damaged, coated, or partially threaded parts can give misleading measured-sample results.
References
- [1]
- [2]Unified Inch Screw Threads
ASME
Reference standard family for Unified inch thread terminology and TPI-based thread callouts.
Frequently Asked Questions
Is pitch the same as lead?
Only for a single-start screw. Pitch is the crest-to-crest spacing; lead is axial travel per revolution. Lead equals pitch times the number of starts.
Can this identify the exact thread size?
No. Pitch is only one part of thread identification. You also need major diameter, thread form, tolerance class, and sometimes left/right hand direction.
How should I measure thread pitch without a gauge?
Measure across many thread intervals with a ruler or caliper, then divide by the number of intervals. Counting more threads reduces error compared with measuring one crest-to-crest gap.
How is this different from a tap drill calculator?
This calculator finds pitch, TPI, lead, and basic thread geometry. A tap drill calculator uses major diameter and thread percentage to choose a drill size, which is a separate machining decision.