Mm to Inches Calculator

Convert millimeters to inches — and inches back to millimeters — with the exact 1 inch = 25.4 mm formula. See decimal inches plus a nearest 1/64 inch fraction. Everything runs in the browser.

1 in = 25.4 mm
mm ↔ inches
Stays on your device

An mm to inches calculator answers a length question: if a part, drawing, or product is listed in millimeters, what is that size in inches? Enter millimeters to see decimal inches, or switch directions and enter inches to see millimeters. The same international inch is used both ways.

The formula is inches = millimeters ÷ 25.4 and millimeters = inches × 25.4. Example: 25.4 mm is exactly 1 in. 12.7 mm is 0.5 in (1/2 in). 10 mm is about 0.393701 in, shown as 25/64 in when rounded to the nearest sixty-fourth. Use the decimal result when a 1/64 step is not tight enough.

Results are educational conversions, not a calibrated measurement or an engineering spec. Tape marks, shop drawings, and tolerances can differ. Everything runs in your browser. Totals never leave the device, and there is no account.

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How to use the mm to inches calculator

Choose millimeters → inches (default) or inches → millimeters.
Enter a length of 0 or more in the selected unit.
Read the converted decimal value and the nearest 1/64 inch fraction.
Remember 1 inch = 25.4 mm exactly — there is no extra rounding in the formula.
Use the decimal inches when a sixty-fourth step is not precise enough.

Frequently asked questions

How do I convert millimeters to inches?

Divide millimeters by 25.4. The international inch is defined as exactly 25.4 mm, so 25.4 mm = 1 in, 12.7 mm = 0.5 in, and 10 mm ≈ 0.393701 in. This mm to inches calculator also shows a nearest 1/64 inch fraction.

What is the exact mm to inches formula?

Inches = millimeters ÷ 25.4. Millimeters = inches × 25.4. There is no rounding in the SI definition of the inch — 1 inch is exactly 25.4 millimeters. Example: 50.8 mm ÷ 25.4 = 2 in.

How many millimeters are in an inch?

There are exactly 25.4 millimeters in one inch. That is the international yard-and-pound definition used for science, manufacturing, and this converter. Older U.S. survey inches differed slightly and are not used here.

Can I convert inches back to millimeters?

Yes. Switch to inches → mm and enter a decimal inch value. The page multiplies by 25.4. 1 in is 25.4 mm, 2 in is 50.8 mm, and 0.5 in is 12.7 mm. The same formula runs both directions in the browser.

How do fractional inches work on this calculator?

After the decimal conversion, the page rounds to the nearest 1/64 inch and reduces the fraction. 12.7 mm is exactly 1/2 in. 10 mm is about 0.393701 in, shown as 25/64 in. Use the decimal result when you need more precision than a 1/64 step.

What numbers can I enter?

Millimeters and inches must be finite and 0 or more. Empty, infinite, or negative values are rejected. Commas in thousands (1,000) are accepted. Results are a unit conversion only — they are not a calibrated measurement or an engineering spec.

Does this mm to inches calculator upload my numbers?

No. Millimeters, inches, and the 25.4 formula are calculated in your browser. Nothing is sent to a server or stored on our side, and there is no account.

Is this a measurement or engineering specification?

No. Results are an educational conversion from the numbers you enter using 1 inch = 25.4 mm. Shop drawings, tape-measure marks, and machining tolerances can differ. Check the drawing or a qualified spec before you cut or order material.