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

How the inches to mm converter 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 convert inches to millimeters?

Multiply the number of inches by 25.4 to get millimeters.

How many millimeters are in 0.625 in?

0.625 in equals 15.875 mm. The reference table on this page lists the other values people look up most often.

How do I convert millimeters back to inches?

Divide the number of millimeters by 25.4, or use the swap control on the converter above.

Is this conversion exact?

Yes. The factor is fixed by international definition rather than measured, so the conversion is exact - only the displayed rounding limits precision.

Inches to MM Converter