Metre

The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second, where the second is defined by a hyperfine transition frequency of caesium.

The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle through Paris, setting 10000 km as that quarter of the Earth's polar circumference.

Over time, the definition of the metre has changed. In 1799, it was redefined using a special metal bar. Later, in 1960, it was defined using the wavelengths of light from a special type of gas called krypton-86. The current definition, adopted in 1983 and updated in 2019, keeps the metre very close in size to its original meaning. Today, the Earth's polar circumference is measured to be about 40007.863 km.

Etymology and spelling

The word metre comes from ancient Greek words meaning "to measure." It has been used in many languages for counting and measuring things.

In most English-speaking countries, we spell the unit of length metre. However, in the United States and the Philippines, people spell it meter.

History of definition

The metre is the main unit for measuring length in the International System of Units. In 2019, it was defined as the distance light travels in a vacuum in exactly one-quintillionth of a second. This new definition replaced the older one from 1791, when France decided the metre would be one ten-millionth of the distance from the North Pole to the equator through Paris.

SI prefixed forms of metre

Main article: Orders of magnitude (length)

We can use special words to show how big or small something is compared to a metre. For example, we say "30 cm" instead of "3 dm" because it is easier to understand. Very far distances are often measured in kilometres, astronomical units, light-years, or parsecs, instead of using bigger metre units.

Some older words like micron and millimicron were used for very small measurements, but we now use micrometre and nanometre instead. Using the newer words helps everyone understand better.

Equivalents in other units

Within this table, "inch" and "yard" mean "international inch" and "international yard" respectively, though approximate conversions in the left column hold for both international and survey units.

"≈" means "is approximately equal to";

"=" means "is exactly equal to".

One metre is exactly equivalent to ⁠5 000/127⁠ inches and to ⁠1 250/1 143⁠ yards.

A simple mnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3 feet 3+3⁄8 inches. This gives an overestimate of 0.125 mm.

The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm). Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly 1⁄2 toise. The Russian verst was 1.0668 km. The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.