Length Converter

The Length Converter translates distance and length values between every common metric and imperial unit in one shared interface. You meet this problem daily: a height listed in feet and inches needs to go on a metric form, a running route in kilometers needs to compare with a friend\u2019s mile total, construction drawings in millimeters need conversion for a US supplier, furniture dimensions in inches need to fit in a room measured in centimeters. Each conversion on its own is simple; doing them in your head is easy to get wrong.

This tool handles fifteen common units at once: millimeter, centimeter, meter, kilometer, inch, foot, yard, mile, nautical mile, and selected scientific units including micrometer, nanometer, astronomical unit, light year, and parsec. You type a value in any field, every other field updates instantly. Arithmetic uses IEEE 754 double precision internally, which preserves enough accuracy that round-tripping 5 miles through every other unit and back gives you exactly 5 miles at normal display precision. Feet and inches can be entered in combined form (5 feet 11 inches or 5ft 11in) and parsed automatically into the compound value most people actually use for human height.

Modern length conversion rests on the international yard and pound agreement of 1959, which defines exactly 1 yard = 0.9144 meters. Every imperial length unit derives from that: 1 inch = 0.0254 meters exactly, 1 foot = 0.3048 meters, 1 mile = 1609.344 meters. These are exact rational relationships, not approximations, so a careful converter gives exact answers within floating-point precision.

The metric unit relationships follow SI prefix rules: 1 kilometer = 1000 meters, 1 centimeter = 0.01 meters, 1 millimeter = 0.001 meters, 1 micrometer = 10^-6 meters, 1 nanometer = 10^-9 meters. These are exact decimal factors, so metric conversions never accumulate rounding error across the chain.

Nautical miles are a special case: 1 nautical mile = 1852 meters exactly, by international agreement since 1929. This matches roughly one minute of arc on a meridian, useful for aviation and maritime navigation. Nautical miles are not interchangeable with statute miles (1609.344 m), and mixing them up is a navigation-textbook error.

Astronomical units for very large distances: 1 AU = 149,597,870,700 meters (Earth-Sun distance, defined exactly since 2012), 1 light year = 9,460,730,472,580,800 meters, 1 parsec = 3.2616 light years. These conversion factors are themselves defined by other constants (speed of light, year duration), so the values carry no uncertainty at the level of meters.

Survey foot vs international foot is a pitfall specific to US land surveys. Until 2023, the US used a survey foot defined as exactly 1200/3937 m (about 0.30480061 m), very slightly longer than the international foot. This converter uses the international foot throughout since that matches what most calculators and engineering software expect. For official survey work, confirm which foot your source document uses.

Compared to typing conversions into a search engine (5ft 11in in cm), this tool shows every unit at once plus compound parsing for feet-and-inches. For a single quick conversion the search box is faster; for any real work the side-by-side view wins.

Compared to scientific calculators or specialized engineering apps, this browser tool covers the common 95% use case without setup, installation, or license fees. Professional CAD suites and unit-aware libraries (Python Pint, JavaScript convert-units) handle more exotic units and dimensional analysis, which this tool does not attempt.

Compared to phone apps, the browser version works on any device and persists no data. The conversion logic is the same (they all use the same ratios); the convenience factor is the tool being one click away in a browser tab you already have open.