# Kilometres - History & Definition

The kilometre is defined as being 10^{3} metres. The metre being the base unit of length in the SI system of units
used internationally.

The metre is currently defined as:

The length of the path travelled by light in a vacuum during a time interval of 1/299792458 of a second

Historically the metre has had a number of different definitions including:

- The length of a pendulum with a half period of 1 second. (1790)
- One ten millionth of the length of the earth's meridian along a quadrant through Paris, France. (1791)
- The distance between two lines on a standard bar of an alloy of platinum with 10% iridium measured at the melting point of ice. (1889)
- 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p
^{10}and 5d^{5}quantum levels of the krypton-86 atom (1960)

More detailed information on the definition of the metre is available at Wikipedia.

Metre is the correct spelling for the unit of length and its derivatives such as millimetre, centimetre and kilometre in all English speaking countries apart from America where meter is used.

Full kilometres to miles conversion tables can be found on the following pages:

Alternatively you can use our Quick Converter.