Definition

The metre is the base unit of length in the SI metric system of units and is defined as being the length of the path travelled by light in a vacuum during a time period of 1/299,792,458th of a second.

Usage

The metre and its derivative measures (kilometres, centimetres, millimetres etc) are the legal units of length and distance across most of the world apart from the US. However usage of the metric units of length, height or distance is not complete in countries other than the US, with the United Kingdom and British Commonwealth countries, that used to use Imperial measures, still formally and informally retaining some non-metric measures. For instance most people in Great Britain would not be able to tell you their height in metres.

Table of Metric Length Units

The following table gives metric length units, the metre and its derivatives (using standard SI metric prefixes), with their equivalent in yards.

Unit	Abbreviation	As Metres	As Yards	Notes
micrometre	μm	0.000001	0.000001093613	Used in precision engineering
millimetre	mm	0.001	0.001093613	Used in fine measurements and the building trade
centimetre	cm	0.01	0.01093613	Often used when referring to a persons height
decimetre	dm	0.1	0.1093613	Rarely used
metre	m	1	1.093613	Commonly used for many measurements
decametre	dam	10	10.93613	Rarely used
hectometre	hm	100	109.3613	Rarely used
kilometre	km	1,000	1,093.6138	Used on most road signs
megametre	Mm	1,000,000	1,093,613.8	Rarely used

History & Background

The metre in the original French metric system was designed to be 1/10,000,000th of the distance from the equator to the north pole around the earth's circumference at sea level. In 1799 a prototype metre bar made of platinum was produced. This protoype was superseded by a platinum-iridium bar in 1889, when the first General Conference on Weights and Measures (CGPM) added a temperature constraint into the definition.

At the 7th CGPM in 1927, atmospheric pressure was added in to the definition. After this future definitions of the metre were no longer to use a physical object and in 1960 at the 11th CGPM the metre was defined in terms of wavelengths of light from a specified transition in krypton-86.

In 1983 the current definition of the metre in terms of the length of the path travelled by light in a vacuum was introduced at the 17th CGPM. More detail on the history of the metre can be found here.

Conversion Formulae for Metres

To Imperial - Yards

• 1 yard = 0.9144 metres
• 1 metre = 1.093613 yards

To Imperial - Feet

• 1 foot = 0.3048 metres
• 1 metre = 2.280840 feet

To Imperial - Inches

• 1 inch = 0.0254 metres
• 1 metre = 39.370079 inches

Imperial to metric conversions are exact (as the yard was defined in terms of the metre in 1959), however the metric to imperial conversions are approximate and given to 6 decimal places.