Image tools

Millimetres to Inches Converter

Convert mm to decimal inches with full precision, plus the same length in centimetres, metres and feet.

Runs in your browser. Your image is never uploaded.

How to use MM to Inches Converter — Millimetres to Inches

  1. 1Pick the kind of measurement, then the unit you are converting from and to.
  2. 2Type your value — the result updates as you type.
  3. 3Scroll down for the same value in every other unit, and a table of common conversions.

About MM to Inches

Millimetres to inches is the conversion of the workshop and the parts catalogue: drill bits, screw lengths, sheet thickness, phone dimensions, pipe diameters. The relationship is exact — 25.4 mm to the inch — but the answers are almost always awkward decimals, which is exactly why a calculator beats mental arithmetic here.

Precision is the whole point at this scale. A 1.6% rounding error is invisible when you are converting a room, and fatal when you are converting a bearing seat. This converter keeps the exact 25.4 factor and shows up to ten significant decimals, so you can round once, at the end, to whatever your job actually needs.

If you work in fractional inches, the decimal result is still the fastest route: multiply the fractional part by 16, 32 or 64 and round to the nearest whole number. 3 mm comes out as 0.118 in, which is 1.89 sixteenths — nearest 1/8. Knowing the decimal first is what tells you how far off that nearest fraction really is.

Frequently asked questions

How many inches is 10 mm?
0.3937007874 inches — the same as 1 cm. For other common sizes: 25.4 mm is exactly 1 inch, 50 mm is 1.968504 inches, and 100 mm is 3.937008 inches.
How do I convert mm to inches by hand?
Divide by 25.4. Going back the other way, multiply inches by 25.4. Both directions are exact, because the inch is defined in terms of the millimetre rather than measured against it.
Why does precision matter for millimetres?
Because millimetres are usually used where tolerance is tight. Rounding 0.0393700787 to 0.04 is a 1.6% error — half a millimetre on a 30 mm part, which is the difference between a hole that fits and one that does not.
Does it give fractional inches?
No, the output is decimal inches. To get sixteenths, multiply the decimal part by 16 and round: 0.6875 in × 16 = 11, so 11/16. A 3 mm drill is 0.118 in, closest to 1/8 (0.125 in).
Can I use this to work out image size in pixels?
Not directly — millimetres to pixels depends on DPI, which this tool has no way of knowing. For print sizing use the DPI converter or the mm image resizer, where you can state the resolution.
Is it free, and is anything uploaded?
It is free with no sign-up, and nothing is uploaded. The conversion is JavaScript running on your device, so the numbers you type never leave the browser.