Convert inches to mm

One inch is defined as exactly 25.4 millimetres. That definition has been exact since 1959, so converting inches to mm is pure multiplication by 25.4 — no rounding involved. Example: 10 in × 25.4 = 254 mm.

Type a value and get the result with its full working shown — the coefficient, the multiplication and the rounding you choose.

EXACT COEFFICIENT BIDIRECTIONAL

inch → millimeter

in
mm

Decimals

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inches to mm conversion table

inmm
1 in25.4000 mm
2 in50.8000 mm
3 in76.2000 mm
4 in101.6000 mm
5 in127.0000 mm
6 in152.4000 mm
7 in177.8000 mm
inmm
8 in203.2000 mm
9 in228.6000 mm
10 in254.0000 mm
25 in635.0000 mm
50 in1270.0000 mm
100 in2540.0000 mm

Rounded to 4 decimals · exact values in the calculator above.

The exact formula

mm = in × 25.4

The millimetre is 1/1000 of a metre, the SI base unit of length. Engineering, machining and 3D printing work almost exclusively in mm. Multiply by 25.4 (in → mm); reverse: multiply by 0.039370078740157.

SOURCE: NIST Handbook 44 / NIST SP 811

Unit definitions

inch (in)
The international inch is exactly 25.4 mm. Before 1959 the US and UK inches differed slightly; the modern value is exact worldwide. Source: NIST Handbook 44 / NIST SP 811.
millimeter (mm)
The millimetre is 1/1000 of a metre, the SI base unit of length. Engineering, machining and 3D printing work almost exclusively in mm. Source: BIPM SI Brochure, 9th ed.（1 mm = 0.001 m）.

Frequently asked questions

How many mm is 1 inch?

Exactly 25.4 mm. The inch has been defined as exactly 25.4 millimetres since the 1959 international agreement.

How many mm is 10 inches?

10 in = 254 mm. Multiply by 25.4: 10 × 25.4 = 254 mm exactly.

Is 1 inch exactly 25.4 mm?

Yes. Since 1959 the international inch is defined as exactly 25.4 mm, replacing the older US (25.4000508 mm) and UK definitions.

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