Small units, precise work
Millimeters are where measurements get serious. When a centimeter is too coarse, you reach for the millimeter, and that’s exactly the scale where the metric-imperial clash gets annoying. The fix is one ratio: there are 25.4 millimeters in an inch. Flip it and one millimeter is 0.0393701 inches.
Type your value, read the result the instant it appears. Everything calculates locally in your browser, so there’s no waiting and nothing uploaded anywhere.
Hardware, bolts, and drill bits
This is the conversion every maker and DIYer needs. Fasteners are a mess of mixed standards. A drill bit might be sold as 6 mm in one shop and 15/64 inch in another, and those are nearly the same hole (6 mm is 0.2362 inches). Wrench sizes, socket sets, sheet metal thickness: half of it is metric, half imperial, and nobody labels both.
Wire and sheet gauges add another layer. A 1.5 mm² cable, a 0.8 mm steel panel, an M4 screw with a 4 mm shank: knowing those in inches helps when your tools or parts come from the other side of the standards divide. Drop the number in and you’ll see whether two parts actually match or just look close.
Rainfall and the tiny stuff
Weather reports outside the US give rainfall in millimeters. “25 mm of rain overnight” sounds modest until you convert it: that’s almost exactly one inch falling across every square foot. A 50 mm downpour is about two inches, which is a lot of water.
Small physical dimensions live here too. Phone thickness (often 7 to 9 mm), the diameter of a coin, the lead in a mechanical pencil (0.5 mm is 0.0197 inches). When a spec sheet only gives one system, this gets you the other.
Some useful values:
- 1 mm = 0.0393701 in
- 5 mm = 0.19685 in
- 10 mm = 0.393701 in (and 10 mm is 1 cm)
- 25.4 mm = 1 in exactly
- 100 mm = 3.93701 in
Running the tool
Enter millimeters, and the inch figure updates live with no button to click. The dropdown lets you change either unit, so switching to centimeters or going the reverse direction stays on this page. A reference table sits below the input for the values folks check most often.
Results round to a sensible precision for general use. If you’re machining to a thousandth of an inch, work from the exact 25.4 mm per inch ratio yourself, since that figure carries no rounding error.
Questions people ask
How many mm in an inch?
25.4 millimeters, exactly. It’s a defined value, so there’s no approximation involved. That makes inch-to-mm conversions clean.
What is 1 mm in inches?
0.0393701 inches. A rough rule of thumb: 1 mm is about 1/25 of an inch, so 25 mm gets you close to a full inch.
How do I convert mm to inches by hand?
Divide the millimeters by 25.4. So 38 mm divided by 25.4 is about 1.496 inches, basically 1.5 inches. Multiplying by 0.0393701 gives the same answer.
Is 10 mm the same as 1 cm in inches?
It is. 10 mm equals 1 cm, and both convert to 0.393701 inches. The metric system stacks neatly in tens, which is why this works out evenly.
How much is 25 mm of rain in inches?
About 0.98 inches, so call it an inch. A solid day of steady rain. 25.4 mm would be exactly one inch.
Does the calculation happen on your servers?
No. It’s plain JavaScript running in your browser. Nothing about your input is sent anywhere, and it keeps working even if you go offline after loading.