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Millimetres to Inches

Convert millimetres to inches, with the nearest sixteenth.

Inputs
mm

The number of millimetres to convert.

Inches
3.937in
Nearest 1/16 inch
63/16
1 mm
0.0394in
10 mm
0.3937in
25.4 mm
1in
100 mm
3.937in
inches = millimetres ÷ 25.4

1 mm = 0.03937 in

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Divide millimetres by 25.4 to get inches. Workshop measurements are usually wanted as a fraction rather than a decimal, so the nearest sixteenth is shown alongside.

How to use MM to Inches Converter

  1. Enter the millimetres. Decimals are fine — calipers read to hundredths.
  2. Read the inches. To four decimals, with the nearest sixteenth underneath.

About converting millimetres to inches

Millimetres and inches meet most often in workshops, where the two systems coexist in a way they do not elsewhere. The important thing to understand is that imperial workshop measurement is fractional, not decimal. Drill sets, spanner sizes, tap sizes and sheet gauges are all marked in binary fractions of an inch, so a converted decimal is only half the answer — 4.2 mm is 0.1654 inches, which is useful only once you know it sits between 5/32 and 11/64. That is why the nearest sixteenth is shown here, and why anything finer usually needs thirty-seconds or sixty-fourths. The near-misses between the two systems are the practical hazard. A 13 mm socket is 0.5118 inches against a 1/2-inch socket at 0.5 — half a millimetre of slop, enough to round the corners of a fastener under load. M6 and 1/4-inch bolts differ by 0.35 mm in diameter and have entirely different thread pitches, so one will start in the other and then bind. Sheet metal is worse still, since gauge numbers are not a measurement of anything and vary by material. Where a part is metric, use metric tools; the conversion is for reading a drawing, not for substituting hardware.

Frequently asked questions

What is 100 mm in inches?
3.937 inches, which is very close to 3 15/16. Note it is not 4 inches — a common assumption, since 4 inches is 101.6 mm. That 1.6 mm gap is enough to matter for a fastener or a drill hole.
Why show sixteenths?
Because imperial workshop measurement is fractional rather than decimal. Rulers, drill index sets and spanner sizes are marked in halves, quarters, eighths, sixteenths and thirty-seconds, so a decimal answer still needs converting before it is usable.
Are metric and imperial fasteners interchangeable?
No, and forcing them damages both. An M6 bolt is not a 1/4-inch bolt — 6 mm against 6.35 — and the thread pitches differ entirely. Sockets are the same story: a 13 mm and a 1/2 inch are half a millimetre apart, close enough to round a nut.
How precise is a millimetre in inches?
One millimetre is 0.03937 inches, a little under a twenty-fifth. For reference, 1/32 of an inch is 0.794 mm, so a millimetre is slightly more than a thirty-second and slightly less than a sixteenth.

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