Convert inch to millimeter.

Convert inch (in) to millimeter (mm). This is a fixed-ratio conversion — multiply the input by the factor below.

Formula

mm = in × 25.4

Reference values

Things to watch for

• The international inch is exactly 25.4 mm (defined since 1959), so length conversions are exact, not approximations.
• Use mm for engineering drawings, cm for everyday measurements, m for room dimensions, km for distances.
• 1 mile = 1.609344 km (exact). Nautical miles are different: 1 nmi = 1852 m exactly (defined for nautical use).

What is a inch?

The international inch is defined as exactly 25.4 millimeters since 1959, when the US, UK, and Commonwealth countries unified their definitions. US construction lumber, screen sizes (TV diagonals, phone screens), printing measurements, and pipe sizing.

What is a millimeter?

One millimeter is exactly one-thousandth of a meter (1 mm = 0.001 m). It is the standard small-unit length in the SI metric system. Engineering drawings, machining tolerances, electronics dimensions, manufactured-part specifications, and rainfall measurement.

How big is one in in real terms?

If the unit feels abstract, comparing it to known objects often helps. 36 in is one yard — about the length of a baseball bat; 1 in is about the diameter of a US quarter; and 3 in is about the length of an iPhone's width.

How to convert inch to millimeter

To convert a value from inch (in) to millimeter (mm), apply the conversion factor shown in the formula above. The calculation is the same whether you do it by hand, in a spreadsheet, or with the live converter on this page.

Steps:

1. Take your input value in in.
2. Apply the formula mm = in × 25.4.
3. The result is your value in mm.

For repeated calculations, save the formula in a spreadsheet or use the live converter at the top of this page — it handles the math automatically and displays the result as you type.

Frequently asked questions

How do I convert in to mm?

Apply the formula in → mm shown above, or just type your value into the converter at the top of this page. The result updates instantly.

Which is bigger, in or mm?

This depends on the conversion ratio in the formula. If the factor multiplied by your value gives a larger number, then mm is the smaller unit (so it takes more of them to express the same quantity). If the result is smaller, then mm is the larger unit.

When would I use in versus mm?

Both units measure length, so the choice depends on context. Inch is typically used for engineering specs, construction, navigation, and everyday measurement; millimeter similarly. Most professional fields standardize on one or the other based on regional conventions or technical tradition.

How precise is this in to mm conversion?

The conversion factor shown is the internationally defined exact value (or the best-published approximation if the relationship is irrational, like degrees-to-radians). The live converter on this page uses double-precision floating-point math, accurate to about 15 significant digits — far beyond any practical engineering need.

Is the conversion ratio exact, or an approximation?

Most unit conversions between SI metric units, and between SI and US customary units, have been formally defined as exact values since the 1959 international yard-pound agreement and subsequent SI redefinitions. Exceptions are unit pairs that involve irrational numbers (radians, e.g.) or empirical conversions (like food calories, which depend on temperature). When in doubt, consult the formula at the top of this page.

Related conversions

See also