Inches to Millimeters Converter

How many millimeters are in an inch?

There are 25.4 millimeters in one inch, exactly. The inch is formally defined through the millimeter, so this is the number that every other inch-to-metric factor is built from: 2.54 cm and 0.0254 m are just the same figure shifted by powers of ten.

millimeters = inches × 25.4Reverse: inches = millimeters ÷ 25.4.

Since millimeters are small, the answers are usually comfortable whole or one-decimal values. A ten-inch pipe is 254 mm, and a one-inch bolt head is 25.4 mm across.

Why the millimeter is the unit of choice for precision work

Machinists, designers and makers in metric countries specify parts in millimeters because the unit is fine enough to avoid fractions in most cases. Imported components, however, often come with inch dimensions: bolt lengths, drill bits, pipe fittings, lumber thicknesses and the thickness of sheet metal gauges. Converting to millimeters lets you compare them directly with a metric drawing or a pair of calipers.

Shop drawings also use the thou (or mil), which is a thousandth of an inch. One thou is 0.0254 mm, so a tolerance of 0.005 in works out to 0.127 mm. Entering 0.005 in the converter returns 0.127.

How to convert inches to millimeters

1. Turn any fraction into a decimal first, for example 3/4 becomes 0.75.
2. Enter the decimal inches in the first box.
3. Read the millimeters in the second box; it updates as you type.

Example 1. A three-quarter-inch socket: 0.75 × 25.4 = 19.05 mm.

Example 2. A 2.5 inch wood screw: 2.5 × 25.4 = 63.5 mm.

Example 3. A 1/16 inch drill bit: 1 ÷ 16 = 0.0625, and 0.0625 × 25.4 = 1.5875 mm.

For lengths above a few hundred millimeters it can be easier to think in centimeters or meters; the inches to centimeters converter is one step up.

Inches to millimeters conversion table

Common pitfalls

• Using 25 as a shortcut. It is fine for a rough guess, but over 100 inches it is off by 40 mm.
• Forgetting nominal sizes. A 2x4 board is not 2 by 4 inches once it is milled, and a 1/2 inch pipe has a different actual diameter. Convert the real dimension, not the trade name.
• Confusing mm with cm. 25.4 mm and 2.54 cm are the same length. Check which unit your drawing uses before you cut.

To go from a metric size back to inches, use the millimeters to inches converter.