MM to Inches
This free mm to inches converter is built for the small, exact measurements where a rounding error actually matters: camera sensor sizes, thread pitches, jewelry gauges, 3D-print tolerances and machining drawings.
How to use
- Type a value in millimeters into the Value box.
- The equivalent length in decimal inches appears instantly as you type.
- Swap the From and To selects to convert inches back to millimeters.
- Copy the full equation shown under the result into your drawing or document.
Frequently asked questions
Is 1 inch exactly 25.4 mm?
Yes — exactly. The international inch is defined as 25.4 millimeters (2.54 cm × 10) with zero rounding, which is why dividing millimeters by 25.4 gives the exact inch value used in engineering and manufacturing.
What is the formula for mm to inches?
inches = mm ÷ 25.4. For example, 100 mm ÷ 25.4 = 3.937007874 in. The reverse direction multiplies by 25.4: 3 in × 25.4 = 76.2 mm.
How many mm is a quarter inch?
A quarter inch is 25.4 ÷ 4 = 6.35 mm exactly, a half inch is 12.7 mm and three quarters of an inch is 19.05 mm. These exact benchmarks are handy for quick sanity checks on drawings.
Do you store the measurements I convert?
No. The conversion runs entirely in your browser and nothing is uploaded, stored or logged. Close the page and nothing remains.
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About this tool
This free mm to inches converter is built for the small, exact measurements where a rounding error actually matters: camera sensor sizes, thread pitches, jewelry gauges, 3D-print tolerances and machining drawings. Because the inch is internationally defined as exactly 25.4 millimeters, the conversion is a single division — inches = mm ÷ 25.4 — with no approximate factor hiding in the middle. The result updates on every keystroke and the complete equation is printed beneath it, so you can drop the exact numbers straight into a spec sheet, a support ticket or a homework answer without re-deriving anything. Fraction-loving contexts get served too: the result gives you the decimal inches that calipers and CAD expect, and common benchmarks are easy to spot — 25.4 mm is one inch, 12.7 mm is half, 6.35 mm is a quarter. Everything is computed locally in your browser as you type: no uploads, no history, no logging of any dimension you enter, which matters when the numbers belong to an unreleased product or a client drawing covered by an NDA. To convert the other direction, swap the From and To selects and the page becomes an inches to mm converter instantly.