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 is defined as the distance light travels in a very small fraction of a second. This fraction is based on a special property of the element caesium.

Long ago, in 1791, people tried to define the metre using the Earth. They said it was one part of the distance from the equator to the North Pole.

The way we define the metre has changed over time. In 1799, they used a metal bar. In 1960, they used light from a gas called krypton-86. The definition we use today was chosen in 1983 and updated in 2019. It stays very close to the original idea.

Etymology and spelling

The word metre comes from ancient Greek words meaning "to measure." People use it to count and measure things.

In most English-speaking countries, we spell the unit of length metre. But 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. At that time, 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 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. Using these newer words helps everyone understand better.

Equivalents in other units

This table shows how a metre compares to other units of length.

"≈" means "is approximately equal to";

"=" means "is exactly equal to".

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

A simple mnemonic to help with conversion is "three 3s": 1 metre is nearly equal to 3 feet 3+3⁄8 inches.

The ancient Egyptian cubit was about half a metre. The Scottish and English ell was a little less than a metre. The ancient Parisian toise was made exactly equal to 2 metres in the mesures usuelles system. The Russian verst was about one kilometre. The Swedish mil was changed to exactly 10 kilometres.