Inches to MM Converter

Convert inches to millimeters instantly, with a reference table covering drill sizes, fasteners and fractional inches.

Inches

Millimeters

1 in = 25.4 mm

Multiply inches by 25.4 to get millimeters.

The link carries the values you entered, so whoever opens it sees the same result. Nothing is stored on our side — the numbers travel in the link itself.

Inches to millimeters reference table

The conversions people look up most often, rounded to 3 decimal places.

Common inches to millimeters conversions
Inch (in)	Millimeter (mm)
0.125 in	3.175 mm
0.25 in	6.35 mm
0.375 in	9.525 mm
0.5 in	12.7 mm
0.625 in	15.875 mm
0.75 in	19.05 mm
1 in	25.4 mm
1.5 in	38.1 mm
2 in	50.8 mm
3 in	76.2 mm
4 in	101.6 mm
6 in	152.4 mm
12 in	304.8 mm

The inches people look up most

0.5 in = 12.7 mm: Why a half-inch bolt is not a metric bolt - the nearest spanner is a third of a millimetre oversize, and that slack is what rounds the head off.
1 in = 25.4 mm: Fixed by the 1959 international yard and pound agreement, and every other inch figure derives from this one.
2 in = 50.8 mm: Nominal sizes are not real measurements - a 2-inch board finishes well under the 50.8 mm the conversion gives.
3 in = 76.2 mm: Standard US casing and trim widths, which have no clean metric equivalent at all.
6 in = 152.4 mm: The point most rulers mark as halfway, and the shortest rule most people actually own.
12 in = 304.8 mm: A foot, 304.8 mm. Worth memorising, because it converts a lot of drawings in your head.

How the inches to mm calculation works

One inch is exactly 25.4 millimeters:

millimeters = inches × 25.4

This is the workshop conversion. Millimetres are the working unit for anything machined, and inches are still the working unit for a great deal of American hardware, so the two meet constantly on the same bench.

Fractional inches convert cleanly, and are worth memorising:

Fraction	Decimal	Millimeters
1/16 in	0.0625	1.5875
1/8 in	0.125	3.175
1/4 in	0.25	6.35
3/8 in	0.375	9.525
1/2 in	0.5	12.7
3/4 in	0.75	19.05
1 in	1.0	25.4

Enter fractions as decimals above — 0.375 rather than "3/8".

The practical warning: metric and imperial fasteners are close enough to damage each other. A 13 mm socket will slip onto a 1/2-inch bolt head (12.7 mm) and round it off. A 10 mm socket and a 3/8-inch (9.525 mm) are similarly mismatched. The 0.3–0.5 mm of slop is exactly enough to ruin a corner under load.

Drill sizes have the same problem: a 6 mm bit in a hole specified as 1/4 inch leaves 0.35 mm of play, which is loose for a dowel and tight for a bolt.

To convert back, divide by 25.4.

Frequently asked questions

How do I enter a fraction like 7/16 in the converter?

Divide the top number by the bottom and type the decimal: 7 ÷ 16 is 0.4375, which converts to 11.11 mm. If you handle sixteenths often it is quicker to remember that one sixteenth is 1.5875 mm and multiply up - seven sixteenths is 7 × 1.5875, the same 11.11 mm. Eighths, quarters and halves are just multiples of the same figure.

Why do metric drill sets step in 0.5 mm and imperial ones in 64ths?

A sixty-fourth of an inch is 0.397 mm, so a fractional index steps more finely than a 0.5 mm metric one. Between 6 mm and 7 mm a fractional set offers 1/4 in at 6.35 mm and 17/64 in at 6.75 mm, while a metric set offers 6.5 mm alone. The two indexes never coincide, which is why a workshop that works in both ends up owning both.

Do imperial and metric threads interchange if the diameters convert?

The diameters convert and the threads do not. A 1/4 in UNC bolt has a 6.35 mm major diameter, close to an M6, but the pitches differ and nothing will interchange. Older British Whitworth threads use a 55-degree thread form against the 60 degrees of both metric and Unified, so they will not even start in each other’s holes. Match the thread standard, not the converted size.

Inches to MM Converter