UnitPairs

inches to millimetres converter

1 in = 25.4 mm. Type a value in either box — the other side updates instantly, using the exact definition factor, not a rounded shortcut. Fasteners, drill bits and engineering drawings: inch sizes on one side of the Atlantic, millimetres on the other.

Rate: 1 in = 25.4 mm · 1 mm = 0.0393700787 in · As a tape-measure fraction:

Inches as fractions

Tape measures and rulers are marked in sixteenths of an inch, not decimals. For the worked example on this page, 0.5 in is closest to 1/2 in — the fraction is an approximation, off by 0.00e+0 in (about 0 mm). The fraction readout in the converter above updates with whatever you type, so you can read any result straight onto a ruler.

Formula and a worked example

millimetres = inches × 25.4. Worked example: 0.5 in × 25.4 = 12.7 mm. To go back, divide by the same factor (or multiply by 0.0393700787).

Where the factor comes from: The inch is defined as exactly 2.54 centimetres (25.4 mm) under the 1959 international yard and pound agreement. Factors on UnitPairs follow the NIST definitions, so results agree between pages to the last digit shown.

inches to millimetres chart

inches (in)millimetres (mm)
0.256.35
0.512.7
125.4
250.8
376.2
4101.6
6152.4
8203.2
10254
12304.8

Questions

How many millimetres are in one inche?

Exactly 25.4 mm — that is the rate this converter applies in both directions.

How do I convert in to mm by hand?

Multiply the inches value by 25.4. For a rough mental estimate, 25.4 is usually close enough — but use the full factor for anything you plan to build, buy or dose.

How accurate is this converter?

The factors are the exact international definitions, so the only rounding is in the display — results are shown to six decimal places, and the underlying value is never pre-rounded.

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