inches to mm
Unit Converters

inches to mm

Convert inches to millimeters (or millimeters to inches) instantly. Type a value in either field and the other updates live; use the swap button to flip the direction.

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How It Works

One inch is exactly 25.4 millimeters, so to convert inches to millimeters you multiply the number of inches by 25.4. To go the other way, divide millimeters by 25.4.

Frequently Asked Questions

How many millimeters are in one inch?
One inch is exactly 25.4 millimeters. This is the official international definition, so multiplying inches by 25.4 gives the exact millimeter equivalent.
Why is the inch defined as exactly 25.4 mm?
In 1959, the international yard and pound agreement redefined the inch as exactly 25.4 millimeters to unify imperial and metric measurements worldwide.
Is this the same conversion used in engineering?
Yes. The 25.4 mm per inch definition is used universally in engineering, manufacturing, and scientific work, regardless of whether the country uses metric or imperial units.

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