El metro (símbolo m) es la principal unidad de longitud del Sistema Internacional de Unidades. Un metro es la distancia que recorre la luz en el vacío en un intervalo de 1/299 792 458 de segundo. Su símbolo es m (adviértase que no es una abreviatura: no admite mayúscula, punto ni plural).

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The metre, or meter (American spelling), (from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the distance travelled by light in a vacuum in 1/299 792 458 seconds. The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1799, it was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889 and 1927). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted. The imperial inch is defined as 0.0254 metres (2.54 centimetres or 25.4 millimetres). One metre is about  3 3⁄8 inches longer than a yard, i.e. about  39 3⁄8 inches.

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