Ans

Unit converter

Millimetres to inches.

Convert mm to in and in to mm. Type in either box and the other follows. Fasteners, drill bits and sheet material are specified in whichever system the factory used, so this is the conversion a workshop reaches for most.

mm → inExact definition
—
Millimetres to inches — common values, to 6 significant figures.
mmin
1 mm0.0393701 in
2 mm0.0787402 in
3 mm0.11811 in
5 mm0.19685 in
10 mm0.393701 in
15 mm0.590551 in
20 mm0.787402 in
25 mm0.984252 in
30 mm1.1811 in
50 mm1.9685 in
100 mm3.93701 in
1 mm=0.03937007874 in 1 in=25.4 mm

Definitions

  • The millimetre is a thousandth of a metre, and the unit most engineering drawings in the UK are dimensioned in.
  • The inch has been exactly 25.4 mm since the 1959 international yard and pound agreement. Screen and wheel sizes are still quoted in it.
  • This conversion is exact by definition. 1 mm = 0.03937007874 in.
  • The metre is SI; every imperial and US customary length here is defined exactly in terms of it by the 1959 international yard and pound agreement.

How it works

One direction multiplies and the other divides by the same number, so nothing is lost by going back and forth.

The value passes through m, the unit every other length & distance unit on the site is defined against, rather than a number written into this page on its own. That is why the figure here, the figure in the table above and the general length & distance converter always agree.

Display precision limits significant figures rather than decimal places, because one fixed number of decimals is useless across a range this wide. Very large and very small results switch to scientific notation with a proper superscript exponent. The editable box always holds a plain number at full working precision, so you can carry on from either side.

Common questions this answers: how many inches in a millimetre (0.0393701), how many millimetres in an inch (25.4), and what 25 mm is in inches (0.984252 in).

From the maker of Ans — a scientific calculator you can own. Same idea: a proper instrument, nothing in the way.