The millimeter (metric) and the inch (US/UK customary) both have exact definitions, making this a precise length conversion.
How it works
Dividing millimeters by 25.4 gives inches; multiplying inches by that same constant gives millimeters back. Type into either field and the other updates instantly.
What this does not include
This does not include centimeters directly — for that, use this site’s cm-to-in calculator instead, though the same 25.4 constant underlies both.
How to use this calculator
- Enter a value in either millimeters or inches — the other field fills in automatically.
Frequently asked questions
Why is 1 inch exactly 25.4 mm?
It’s the exact international definition set by a 1959 agreement standardizing the inch.
Where is this conversion most commonly needed?
Precision engineering and machining specs, screw and fastener sizes, and reading product dimensions from a different country’s measurement system.
Which is more precise for small measurements, mm or fractional inches?
Millimeters — a metric ruler’s smallest division (1 mm) is finer and easier to read precisely than typical fractional-inch markings.
