Quick answer
Thread pitch in millimetres = 25.4 ÷ threads per inch (TPI). 20 TPI is a pitch of 1.27 mm.
Choose the direction and enter threads per inch or pitch. The calculator returns the other.
Related tools: Roof Pitch Calculator: Roof pitch as rise per 12 inches, angle, slope percent and rafter length factor. Ohm's Law Calculator: Solve voltage, current or resistance.
How to use the Thread Pitch Calculator
- Choose convert: Threads per inch → pitch (mm) and Pitch (mm) → threads per inch.
- Enter value (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Convert | Choose one: Threads per inch → pitch (mm); Pitch (mm) → threads per inch | Starts at Threads per inch → pitch (mm) |
| Value | A number | > 0 |
Formula and method
Formula
pitch (mm) = 25.4 ÷ TPI
IEEE-754 double precision; shown to the decimals indicated.
1 inch = 25.4 mm exactly.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Convert = Threads per inch → pitch (mm); Value = 20.
| Result | Value |
|---|---|
| Pitch | 1.27 mm |
Example 2
Inputs: Convert = Pitch (mm) → threads per inch; Value = 1.5.
| Result | Value |
|---|---|
| Threads per inch | 16.9333 TPI |
Example 3
Inputs: Convert = Threads per inch → pitch (mm); Value = 13.
| Result | Value |
|---|---|
| Pitch | 1.9538 mm |
The first line is the headline result (pitch). IEEE-754 double precision; shown to the decimals indicated.
Common pairs
1/4-20 UNC has 20 TPI (1.27 mm), 3/8-16 has 16 TPI (1.5875 mm) and M10 coarse has 1.5 mm pitch. A 1.5 mm pitch is 16.93 TPI, close to but not the same as 16 TPI.
Not interchangeable
Metric and unified threads can look similar but will damage each other if forced. Use a thread gauge to check pitch before buying fasteners.
How the result changes with value
Holding every other input at the first worked example, this table changes only value. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Value | Pitch |
|---|---|
| 10 | 2.54 mm |
| 20 | 1.27 mm |
| 40 | 0.635 mm |
| 80 | 0.3175 mm |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Value: entering "0" shows "Value must be greater than 0." beside that field and no result.
Choosing between the Thread Pitch Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Bolt Torque Calculator](/engineering/bolt-torque-calculator/) | nut factor K, nominal diameter and target preload | Torque |
| [Roof Pitch Calculator](/construction/roof-pitch-calculator/) | i know the and value | Pitch |
| [Ohm's Law Calculator](/engineering/ohm-s-law-calculator/) | Current | |
| [Voltage Drop Calculator](/engineering/voltage-drop-calculator/) | current, one-way length and wire gauge | Voltage drop |
| [Gear Ratio Calculator](/engineering/gear-ratio-calculator/) | driving gear teeth and driven gear teeth | Gear ratio |
Background: structural and wiring estimates
Structural and electrical results are only as good as their assumptions. Bending stress is the bending moment divided by the section modulus, and an allowable stress is the yield strength divided by a safety factor that depends on the governing design code and the type of load. For a single-phase or DC run the voltage drop uses the round-trip length: V = 2 × L × R per unit length × I.
Recommended limits, such as keeping branch-circuit voltage drop around 3%, differ between sources and code editions. These pages are estimates: follow the applicable code and a qualified engineer or electrician for real work.
Assumptions and limitations
- Metric and unified threads are not always interchangeable
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for value to confirm this.
Frequently asked questions
How do I measure thread pitch?
Use a thread gauge or count threads over an inch.
What is a fine thread?
A smaller pitch for the same diameter.
What is TPI?
TPI is threads per inch, the number of thread crests along one inch; pitch in mm is 25.4 ÷ TPI.
What does the Thread Pitch Calculator calculate?
Convert between threads per inch and pitch in millimeters. It returns Pitch.
What formula does the Thread Pitch Calculator use?
pitch (mm) = 25.4 ÷ TPI. 1 inch = 25.4 mm exactly.
What do I need to enter in the thread Pitch Calculator?
You enter Convert and Value.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Metric and unified threads are not always interchangeable.
What does the “Convert” option change?
It sets how the thread Pitch is worked out. The choices are Threads per inch → pitch (mm) and Pitch (mm) → threads per inch.
What is an example of a thread Pitch result?
With convert Threads per inch → pitch (mm) and value 20, the pitch is 1.27 mm.
Where does the method come from?
The method follows: International Yard and Pound Agreement (1959).
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Metric and unified threads are not always interchangeable.
Related calculators
- Roof Pitch Calculator — Roof pitch as rise per 12 inches, angle, slope percent and rafter length factor.
- Ohm's Law Calculator — Solve voltage, current or resistance.
- Gear Ratio Calculator — Gear ratio, output speed and torque multiplication.
- Power-to-Weight Ratio Calculator — Power per unit weight of a vehicle.
Sources
- International Yard and Pound Agreement (1959)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.