Inches to Millimeters
An instant inches-to-millimeters converter, with the formula and a reference table below.
| Inches | Millimeters |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 25 in | 635 mm |
| 50 in | 1270 mm |
| 100 in | 2540 mm |
How to use
- Enter a value in the Inches field.
- The Millimeters figure appears immediately — no button to press.
- Typing in the Millimeters field converts back to Inches the same way.
- Use the reference table for the values that come up most often.
Inches to Millimeters formula
To convert inches to millimeters, multiply by 25.4.
mm = in × 25.4
Put another way, one inch is exactly 127/5 millimeters.
Why this factor: the inch and the millimeter are both defined against the meter, so dividing one definition by the other gives 25.4 directly. Both definitions are set out in full below.
To go the other way, divide by 25.4 — or use the Millimeters to Inches converter, which is the same calculation reversed.
Worked example
Take 5 in and multiply by the factor 25.4: the result is 127 mm.
5 in × 25.4 = 127 mm
The converter above does this for any value you type, in either direction.
What is an inch?
An inch is one twelfth of a foot, used for small dimensions and for screen and wheel sizes globally.
Definition: Exactly 25.4 millimeters since the 1959 International Yard and Pound Agreement — an exact, not approximate, relation.
Where it is used: Screen diagonals, pipe and fastener sizes, tire and wheel diameters, and print layout worldwide.
What is a millimeter?
A millimeter is one thousandth of a meter — the working precision unit of manufacturing and construction.
Definition: Exactly 0.001 meter by SI prefix definition.
Where it is used: Engineering drawings, machining tolerances, timber and steel dimensions, rainfall totals.
Inches vs Millimeters
The inch is an imperial unit while the millimeter is a metric unit. That is why the factor between them is not a round number: the two systems were defined independently and only later tied together by exact agreement.
Frequently asked questions
How many millimeters are in an inch?
One inch equals 25.4 millimeters.
What is the formula to convert inches to millimeters?
Multiply the number of inches by 25.4. To reverse it, divide by 25.4 instead.
How do I convert millimeters back to inches?
Divide by 25.4, which is the same as multiplying by 0.0393700787.
How many millimeters are in 10 inches?
10 in works out to 254 mm.
How is the inch defined?
Exactly 25.4 millimeters since the 1959 International Yard and Pound Agreement — an exact, not approximate, relation.