Inches to millimetres Converter

How many millimetres are in a inch?

One inch is 25.4 millimetres. The full conversion is millimetres = inches × 25.4, and going the other way you divide instead of multiply.

A few anchor values worth memorising: 25.4 mm to the inch, 254 mm to ten, and 2,540 mm to a hundred. The calculator above handles the rest, including the awkward decimals.

The reference table under the converter lists the values people look up most often, so for a common figure you can read the answer off directly rather than typing anything.

Worked examples

Three worked examples, each multiplying by 25.4:

2.5 in × 25.4 = 63.5 mm. 25 in × 25.4 = 635 mm. 250 in × 25.4 = 6,350 mm.

To reverse any of them, divide instead of multiplying — or use the swap button above, which does the same thing without the arithmetic. Reversing a converted value should return you to exactly what you started with; if it does not, something has been rounded too early.

Doing this conversion in your head

The exact factor is 25.4, which nobody multiplies by in their head. It is very close to 127/5, so multiplying by 127 and dividing by 5 lands within 0% of the correct answer — accurate enough for anything short of engineering work.

Since the factor is greater than one, the number always gets bigger: a inch is smaller than a millimetre, so any quantity converts to more of them. If your answer came out smaller, you have divided when you should have multiplied.

That sanity check catches the majority of real conversion errors. Before trusting any converted figure, ask whether the direction of change makes sense — it takes a second and it is the single most effective safeguard there is.

When you need to convert inches to millimetres

Most people arrive at this conversion from one of a few directions, each with its own tolerance for error.

On the source side, inches are screen diagonals, tyre and wheel sizes, US lumber and plumbing dimensions, and paper measurements. On the target side, millimetres turn up in engineering drawings, screw and drill sizes, rainfall figures and anything measured with callipers. The conversion is essentially a translation between those two worlds — most often because a figure produced in one context has to be understood, submitted or acted on in the other.

Where the converted figure will be cut, dosed, billed or filed, round once at the end rather than at each step. For a rough sense of scale, round freely.

All of it happens in the page. There is no upload, no queue and no record of what you converted once the tab is closed.

What a inch and a millimetre actually are

The inch (in). Defined by international agreement in 1959 as exactly 25.4 millimetres.

The millimetre (mm). The working unit of engineering drawings and machine shops almost everywhere outside the United States.

Because both measure length and their definitions are set by standards bodies, the factor between them never changes. It is not an approximation that improves with better instruments; it is a definition.

Common mistakes when converting inches to millimetres

Significant figures are worth deciding in advance. A conversion can hand you eight decimal places, and reporting all of them implies a precision your original measurement almost certainly did not have. Match the output to the accuracy of the input.

If you are converting many values, change the source unit in the dropdown rather than converting through an intermediate unit. Every extra hop is another rounding opportunity.