Meter

The meter (American spelling; metre in British/international) is the SI base unit of length. One meter equals 100 centimeters or about 39.37 inches.

The definition has evolved four times:

• 1793: one ten-millionth of the distance from equator to North Pole through Paris. Physically meaningful but hard to remeasure.
• 1889: the distance between two scratches on a platinum-iridium bar stored in Sèvres, France. Stable but only one definitive copy.
• 1960: 1,650,763.73 wavelengths of a specific krypton-86 emission line. Reproducible in any lab with the right equipment.
• 1983 (current): the distance light travels in vacuum in 1/299,792,458 of a second. Defines the meter in terms of time, fixing the speed of light by definition.

The current definition reflects the modern SI’s strategy: define base units in terms of fundamental physical constants rather than artifacts. The same revolution redefined the kilogram in 2019 to be based on Planck’s constant rather than a chunk of metal.

Practical equivalents: 1 inch ≈ 2.54 cm exactly (defined relation since 1959); 1 foot = 0.3048 m; 1 yard = 0.9144 m; 1 mile = 1,609.344 m.