Convert millimetres to inches
1 mm = 0.0393701 in
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How many inches are in a millimetre?
One millimetre is 0.0393700787 inches. To convert any amount, multiply by 0.0393700787. Going the other way, one inch is 25.4 millimetres.
What is the formula to convert mm to in?
Multiply the millimetre figure by 0.0393700787. So in = mm × 0.0393700787. For a quick mental estimate, dividing by about 25.4 gets you close enough for most everyday purposes.
What does a millimetre actually look like?
One millimetre is roughly the thickness of a credit card, which is 0.76 mm. The millimetre is the working unit of engineering drawings and machining tolerances almost everywhere metric is used, because it avoids decimals for most part dimensions.
And what is a inch?
One inch is roughly the width of an adult thumb. The inch has been exactly 25.4 millimetres since 1959. Screen sizes, pipe diameters and tyre rims are still specified in inches almost universally.
Is this conversion exact?
Yes. Since the 1959 international yard and pound agreement, the imperial and metric systems are related by exact definitions rather than measurements — the inch is exactly 25.4 mm and the pound exactly 0.45359237 kg. Everything derived from those, including this conversion, is therefore exact. Round at the end rather than partway through, since rounding mm before converting introduces an error the conversion then multiplies.
About mm to in
How to convert millimetres to inches
Multiply by 0.0393700787. One millimetre is 0.0393700787 inches, and one inch is 25.4 millimetres.
in = mm × 0.0393700787
Worked example
10 mm = 0.3937008 in. For a sense of scale, one millimetre is roughly the thickness of a credit card, which is 0.76 mm, and one inch is roughly the width of an adult thumb.
About the millimetre
The millimetre is the working unit of engineering drawings and machining tolerances almost everywhere metric is used, because it avoids decimals for most part dimensions.
About the inch
The inch has been exactly 25.4 millimetres since 1959. Screen sizes, pipe diameters and tyre rims are still specified in inches almost universally.
Quick reference
| millimetres | inches |
|---|---|
| 1 mm | 0.0393701 in |
| 2 mm | 0.0787402 in |
| 5 mm | 0.19685 in |
| 10 mm | 0.393701 in |
| 25 mm | 0.984252 in |
| 50 mm | 1.9685 in |
| 100 mm | 3.93701 in |
| 1000 mm | 39.3701 in |
Getting it right
This conversion is exact by definition, so the only limit on accuracy is how many digits you keep. Round at the end rather than partway through — rounding mm to a whole number before converting introduces an error that the conversion then multiplies.
Other length conversions
Convert inches back to millimetres, or pick any other pair with the full unit converter. Related: mm to km, mm to m, mm to cm.