Millimeters to inches converter
Convertersconversion pair
Convert millimeters to inches instantly: 1 mm is 0.03937 inches. Bolts, paper and small parts, with the exact math and a copy button.
25.4 mm is an inch — the one number metric and imperial agreed to share.
work surface
Math, not advice — results are indicative. Read the disclaimer.
pro tips
Rough math: 25 mm ≈ 1 inch, so 100 mm is about 4 inches.
A 0.8 mm pen tip is 0.031 inches — the decimal where handwriting lives.
Paper is about 0.1 mm thick, so 254 sheets stack to roughly one inch of paperwork.
what it does
One millimeter is 0.03937 inches. That's the whole deal: divide by 25.4 and the inches appear. The number 25.4 is the peace treaty between the two systems, signed in 1959 and somehow still holding.
It shows up wherever small things get measured twice: a 12 mm bolt (0.47 in, the size of nothing dramatic), a 0.8 mm pen tip (0.03 in, the width of a signature), a 3.5 mm headphone jack, paper thickness in tenths of a millimetre, and every spec sheet that prints mm while the wrench in your hand is in fractions of an inch.
The mental shortcut is generous: 25 mm is roughly an inch. So 50 mm is about two inches, 100 mm about four, and anything under 10 mm is just "small" in both languages. For anything that has to actually fit, though, do it exactly — millimetres are where "close enough" turns into a wobble.
Type any number and the widget answers in inches on the spot, with the math in the hint. Click the result to copy it, and hit swap when you're heading the other way — the inches-to-millimeters version covers that side.
quick answers
How many millimeters are in an inch?
Exactly 25.4. It's the one imperial number the metric world adopted without arguing.
Is 10 mm the same as a centimeter?
Yes — 10 mm is 1 cm. When a spec flips between them, nothing changed except the label.
Why is 25.4 so oddly precise?
Because in 1959 the inch was officially defined as exactly 25.4 mm, ending decades of near-misses and mildly furious machinists.