Length Unit Converter

How length unit conversion works

Every conversion on this page is performed by converting the source value to meters first, then dividing by the target unit's meter equivalent. For example, to convert 5 miles to kilometers: 5 × 1609.344 = 8046.72 m ÷ 1000 = 8.04672 km. All conversion factors are exact international definitions; no approximations are used in the calculation.

Results are rounded to 6 significant figures. For astronomical units (light-years, parsecs, angstroms) or sub-atomic sizes, the result is shown in scientific notation (e.g., 9.46073 × 10¹⁵).

Common length conversions

The metric system

The metric system uses powers of ten, making conversions between metric units straightforward. The base unit of length is the meter. A kilometer is 1,000 meters, a centimeter is 1/100 of a meter, and a millimeter is 1/1,000 of a meter. The metric system is the international standard and is used by the vast majority of countries worldwide for everyday measurement.

Imperial and US customary units

The imperial system (used primarily in the United States) has less regular relationships between its units. There are 12 inches in a foot, 3 feet in a yard, and 1,760 yards (5,280 feet) in a mile. The US survey system uses very slightly different foot and mile values for land surveying, but the differences are negligible for everyday use and this converter uses the international values.

Nautical miles and aviation

A nautical mile (nmi) is exactly 1,852 meters, defined as one arc-minute of latitude along any meridian of the Earth. This makes nautical miles particularly useful in navigation: a course change of one degree of latitude covers exactly 60 nautical miles. Knots (the speed unit in maritime and aviation contexts) are nautical miles per hour.

Astronomical units

For distances beyond our solar system, metric prefixes become unwieldy. Astronomers use:

• Light-year (ly): the distance light travels in one year in a vacuum: approximately 9.461 × 10¹⁵ meters.
• Parsec (pc): the distance at which one astronomical unit subtends one arc-second: approximately 3.086 × 10¹⁶ meters, or about 3.26 light-years.

The nearest star to Earth (Proxima Centauri) is about 1.3 parsecs (4.24 light-years) away. The Milky Way galaxy is about 30,000 parsecs in diameter.

Sub-atomic lengths

• Micron (µm): 10⁻⁶ meters. Used in manufacturing tolerances, cell biology (a human red blood cell is 6–8 µm), and semiconductor process nodes.
• Angstrom (Å): 10⁻¹⁰ meters. Used in chemistry and crystallography to express atomic radii, bond lengths, and crystal lattice spacings.

Metric prefix mnemonic

To remember the metric prefixes in descending order - kilo, hecto, deca, (base), deci, centi, milli - use the mnemonic:

"King Henry Died (By) Drinking Cold Milk" (Kilo · Hecto · Deca · Base · Deci · Centi · Milli)

Extending further in both directions: mega (10⁶), giga (10⁹), tera (10¹²) on the large side; micro (10⁻⁶), nano (10⁻⁹), pico (10⁻¹²) on the small side.

Historical origins of common units

• Foot: originally the length of a human foot. Charlemagne standardized it as the length of his own foot in the 8th century; the modern international foot (0.3048 m) was defined by agreement in 1959.
• Meter: defined in 1793 as one ten-millionth (10⁻⁷) of the distance from the equator to the North Pole along the Paris meridian. Today the meter is defined by the speed of light - exactly 1/299,792,458 of the distance light travels in one second.
• Mile: from the Latin mille passuum (thousand paces) - a Roman pace was two steps (~1.48 m), making a Roman mile about 1,480 m. The English statute mile of 5,280 feet was fixed by Parliament in 1593.