Length Converter.

Convert between metric, imperial, nautical, and astronomical units instantly. From millimetres to light-years, with a full breakdown of every calculation.

✓ Free 🌍 International 21 units

Length Converter

Convert any distance or measurement.

Unit system

Value

From

Result

Helpful tips

✈️

Aviation uses feet worldwide

Even in fully metric countries, aircraft altitude is measured in feet. It's a safety standard enforced by ICAO worldwide.

📏

The inch is defined by the metre

Since 1959, one inch has been defined as exactly 25.4 mm. Imperial units are officially metric under the hood.

👕

Clothing sizes mix both systems

EU sizes use centimetres for body measurements; US and UK use inches. When ordering internationally, always check the size chart.

🔬

Nanometres for the very small

Chip processes, wavelengths, and molecules are measured in nm. A human hair is about 80,000 nm wide, to give you a sense of scale.

How length conversion works

The short version, for people
who just want the number

Every length conversion boils down to a single principle: all units can be expressed as a fixed multiple of metres. To convert from one unit to another, you first convert the input to metres, then divide by the target unit's metre equivalent. That's it. There's no approximation involved, because the relationships between units are exact by definition.

The metric system (SI) is built around the metre, with prefixes scaling it up or down in powers of ten. A kilometre is exactly 1,000 metres; a centimetre is exactly 0.01 metres. Imperial units have been officially defined in metric terms since 1959, when one inch was fixed at exactly 25.4 millimetres. This means every foot, yard, and mile is ultimately a precise fraction of a metre.

Nautical units are defined by the geometry of the Earth. One nautical mile equals exactly one arcminute of latitude, a relationship that made it the universal standard for navigation long before GPS. The metre itself was originally defined the same way: as one ten-millionth of the distance from the equator to the North Pole.

Metric Result = Value × (from_m ÷ to_m)

Imperial 1 in = 25.4 mm · 1 ft = 0.3048 m

Nautical 1 nmi = 1,852 m exactly

FAQ

Common questions

Mostly inertia, cost, and culture. The US started a metrication process in the 1970s but it was never made mandatory, and public resistance was strong. The UK officially adopted metric for trade and most official purposes but retained miles for road signs and pints for draught beer as cultural exceptions. In practice, both countries operate a mixed system: science, medicine, and manufacturing use metric almost universally, while everyday life sticks to familiar imperial units. Liberia and Myanmar are the only other countries that have not formally adopted the metric system.

Yes, and they have been since 1959. The International Yard and Pound Agreement defined one inch as exactly 25.4 millimetres. Every other imperial length unit cascades from that: one foot is 304.8 mm, one yard is 914.4 mm, and one mile is 1,609.344 metres. There is no approximation. This means a length conversion between metric and imperial is always perfectly precise, not a rounded estimate. Survey measurements in the US historically used a slightly different "survey foot" (1200/3937 metres), but it was officially retired in 2023.

Aviation altitude is measured in feet worldwide because the standards were set by ICAO (the International Civil Aviation Organisation) when the US dominated civil aviation in the mid-20th century. American aircraft, instruments, and procedures were already built around feet, so feet became the global standard for vertical separation. Switching would require recertifying every altimeter, retraining every pilot and air traffic controller, and updating every procedure simultaneously across every country at once. The safety risk of a fragmented transition is considered higher than the inconvenience of an unusual unit. China and Russia used metres for a period but have since aligned with the international feet standard.

A nautical mile is defined as exactly 1,852 metres, which corresponds to one arcminute of latitude on the Earth's surface. This made it enormously practical for navigation: on a nautical chart, one degree of latitude is always 60 nautical miles, so you can measure distance directly from the chart's latitude scale without any conversion. Speed at sea is measured in knots, where one knot equals one nautical mile per hour. Both the navy and commercial aviation use nautical miles globally, regardless of whether a country otherwise uses metric or imperial units.

A light-year is the distance light travels in one year, about 9.461 × 10¹⁵ metres, or roughly 9.46 trillion kilometres. To put that in perspective: the Sun is about 8 light-minutes away. The nearest star, Proxima Centauri, is 4.24 light-years away. The Milky Way is roughly 100,000 light-years across. Astronomers also use the parsec (about 3.26 light-years) for stellar distances, and the astronomical unit (AU), which is the average Earth-Sun distance of about 150 million kilometres, for distances within our solar system. The converter supports all three.

All metric and imperial conversions use exact, internationally defined values, so there is no rounding in the conversion factors themselves. Astronomical unit values (AU, light-year, parsec) use the most current IAU-defined constants. The only limitation is floating-point precision in the browser's JavaScript engine, which is accurate to roughly 15 to 17 significant digits, far beyond anything you'd need in practice. Results are displayed to a maximum of 10 significant figures. For scientific or engineering work requiring more precision than that, a dedicated computational tool would be more appropriate.