The metre (Commonwealth English [1]) or meter (American English [2]) (symbol: m) is the SI base unit of length. It is defined as the length of the path travelled by light [3] in absolute vacuum during a time [4] interval of 1/299,792,458 of a second [5].

## SI prefixes applied to the metre

Adding SI prefixes to metre creates multiples and submultiples; for example kilometre (1000 metres; kilo- = 1000) and millimetre (one thousandth of a metre; milli- = 1 / 1 000).

The metre may be used with SI prefixes as shown.

Multiple Name Symbol Multiple Name Symbol
100 metre m
101 decametre dam 10−1 decimetre dm
102 hectometre hm 10−2 centimetre cm
103 kilometre km 10−3 millimetre mm
106 megametre Mm 10−6 micrometre µm
109 gigametre Gm 10−9 nanometre nm
1012 terametre Tm 10−12 picometre pm
1015 petametre Pm 10−15 femtometre fm
1018 exametre Em 10−18 attometre am
1021 zettametre Zm 10−21 zeptometre zm
1024 yottametre Ym 10−24 yoctometre ym

## Conversions

1 metre is equivalent to:

• exactly 1/0.9144 yards [6] (approximately 1.0936 yards)
• exactly 1/0.3048 feet [7](approximately 3.2808 feet)
• exactly 10000/254 inches [8] (approximately 39.370 inches)

## History

See History at [9]

### Timeline of definition

• May 8, 1790 — The National Assembly (French Revolution) [10] decides that the length of the new metre would be equal to the length of a pendulum with a half-period of one second.
• March 30, 1791 — The French National Assembly accepts the proposal by the French Academy of Sciences [11] that the new definition for the metre be equal to one ten-millionth of the length of the earth's meridian (geography)[12] along a quadrant (one-fourth the polar circumference of the earth).
• 1795 — Provisional metre bar constructed of brass [13].
• December 10, 1799 — The French National Assembly specifies that the platinum metre bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.
• September 28, 1889 — The first Conférence Générale des Poids et Mesures [14] defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium [15], measured at the melting point of ice.
• October 6, 1927 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmospheric pressure [16] and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
• October 20, 1960 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths [17] in vacuum of the electromagnetic radiation [18] corresponding to the transition between the 2p10 and 5d5 quantum levels of the krypton-86 atom.
• October 21, 1983 — The seventeenth CGPM defines the length to be distance travelled by light [19] in vacuum during a time interval of 1/299 792 458 of a second [20].