Inches to MM Converter

How to convert inches to millimeters?

To convert inches to millimeters, multiply by 25.4 — the exact factor defined by the 1959 international yard-and-pound agreement. This conversion is mathematically exact, with no measurement uncertainty. NIST, ISO 80000, and ASME Y14.5 all use this value.

d_(mm) = d_(in) × 25.4

Example

Convert 10 inches to millimeters:

d_(mm) = 10in × 25.4 = 254mm

How many millimeters in an inch?

Exactly 25.4 mm, by international definition since 1959.

1 in = 25.4 mm

How many inches in a millimeter?

Exactly 1/25.4 in, or approximately 0.03937008 in.

1 mm = 0.03937008 in

Is the inch-to-mm conversion exact?

Yes. The 1959 international yard-and-pound agreement defines 1 inch = 25.4 mm exactly, harmonizing US and British imperial units to the SI meter. No measurement uncertainty is introduced by the conversion itself.

What is an Inch?

The inch is the principal small-length unit of the US customary and imperial systems, defined as 25.4 mm exactly. It descends from the Roman uncia (twelfth) and remains dominant in US construction, electronics, PCB layout, and fastener threads.

What is a Millimeter?

The millimeter is one one-thousandth of a meter, the SI base unit of length. It is the working unit of mechanical engineering drawings worldwide outside the US, and of all medical-imaging, CNC-machining, and 3D-printing pipelines.

Popular inches to millimeters conversion table