Common length units:

• Meter (m): SI base unit
• Foot (ft): imperial unit
• Inch (in): 1/12 of a foot
• Kilometer (km): 1000 meters
• Mile (mi): 5280 feet
• Yard (yd): 3 feet
• Centimeter (cm): 1/100 of a meter
• Millimeter (mm): 1/1000 of a meter

Key relationships:

• 1 m = 3.281 ft = 39.37 in
• 1 ft = 0.3048 m = 12 in
• 1 km = 0.6214 mi = 3281 ft
• 1 mi = 1.609 km = 5280 ft

The Meter:

Originally defined in 1793 during the French Revolution as one ten-millionth of the distance from the equator to the North Pole. Today it is defined by the speed of light: the distance light travels in 1/299,792,458 of a second.

The Foot:

One of the oldest units of measurement, based on the length of a human foot. Standardized in 1959 as exactly 0.3048 meters by international agreement between the US, UK, and other countries.

The Mile:

Derived from the Roman "mille passus" (a thousand paces). The statute mile of 5,280 feet was established by an act of the English Parliament in 1593.

Metric units (m, km, cm) are used for:

• Science and engineering worldwide
• Road distances (most countries)
• Construction (metric countries)
• Athletics and sports (track events)

Imperial units (ft, mi, in) are used for:

• Construction and real estate (US, UK)
• Road distances (US, UK)
• Aviation altitude (feet worldwide)
• Personal height measurement (US)

Other units:

• Nautical miles: marine and air navigation
• Yards: sports (football, golf)
• Micrometers/nanometers: science and manufacturing