Length Converter

Why inch to centimetre is exact

Since 1959 the international yard is defined as exactly 0.9144 m, so 1 inch is exactly 2.54 cm, 1 foot exactly 0.3048 m and 1 mile exactly 1609.344 m. The older US survey foot (1200/3937 m) differed by about 2 parts per million and was retired by NIST at the end of 2022.

Mil is not millimetre

A mil (thou) is one thousandth of an inch, 25.4 µm. Film and coating thicknesses in mils are often confused with millimetres, which are about 39 times larger.

Length unit definitions

meter (m)

The metre (m) is the SI base unit of length, defined since 1983 as the distance light travels in vacuum in 1/299 792 458 of a second.

kilometer (km)

The kilometre (km) is 1000 metres. It is the standard unit for road distances and speed limits outside the US and UK.

centimeter (cm)

The centimetre (cm) is one hundredth of a metre. It is the everyday metric unit for body height, clothing and small objects.

millimeter (mm)

The millimetre (mm) is one thousandth of a metre. It is the standard unit on engineering drawings in metric countries.

micrometer (µm)

The micrometre (µm), often called a micron, is one millionth of a metre. A human hair is roughly 50–100 µm thick.

nanometer (nm)

The nanometre (nm) is one billionth of a metre. Visible light spans roughly 380–780 nm.

angstrom (Å)

The ångström (Å) is 10⁻¹⁰ m, or 0.1 nm, a convenient size for atoms and chemical bonds.

mile (mi)

The mile (mi) is 5280 feet, exactly 1609.344 metres. It is the road-distance unit in the United States and United Kingdom. Reference: The international yard and pound agreement (1959) fixed 1 yard = 0.9144 m exactly, so the inch, foot and mile are exact.

yard (yd)

The yard (yd) is 3 feet or 36 inches, exactly 0.9144 m. Reference: The international yard and pound agreement (1959) fixed 1 yard = 0.9144 m exactly, so the inch, foot and mile are exact.

foot (ft)

The international foot (ft) is exactly 0.3048 m. Since 2023 the US survey foot is retired, so the international foot is the only US foot. Reference: The international yard and pound agreement (1959) fixed 1 yard = 0.9144 m exactly, so the inch, foot and mile are exact.

inch (in)

The inch (in) is exactly 2.54 centimetres. It is 1/12 of a foot. Reference: The international yard and pound agreement (1959) fixed 1 yard = 0.9144 m exactly, so the inch, foot and mile are exact.

mil (mil)

The mil, or thou, is one thousandth of an inch: exactly 25.4 µm. It is not a millimetre.

nautical mile (nmi)

The nautical mile is exactly 1852 metres, close to one minute of latitude. Speed in nautical miles per hour is the knot.

All length conversions

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Sources

• The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures.
• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology.

Last reviewed: 2026-10-15.