Length is one of the seven SI base quantities, defined in the International System of Units by the metre (m). Since 1983 the metre has been defined as the distance light travels in vacuum in 1/299,792,458 of a second, making every length conversion a multiplication or division by an exact factor of this single standard. Every other length unit on this page — inch, foot, mile, nautical mile, light-year, chi, cun — is expressed as an exact or conventionally fixed factor of the metre.
Metric prefixes let the metre cover everything from atomic scales to interplanetary distances without changing unit: 1 picometre (pm) = 10⁻¹² m is roughly the size of an atomic nucleus, while 1 kilometre (km) = 10³ m handles everyday travel. Going further, 1 astronomical unit (AU) ≈ 1.496 × 10⁸ km is the Earth-Sun distance, and 1 light-year (ly) ≈ 9.461 × 10¹² km crosses to stellar scales. This tool lets you jump freely between any of these scales.
Imperial and US customary units are still common in the United States and partially in the UK. The international inch is defined as exactly 0.0254 m since the 1959 international yard and pound agreement; 1 foot = 12 inches = 0.3048 m; 1 yard = 3 ft = 0.9144 m; 1 mile = 1760 yd = 1609.344 m. The nautical mile (1852 m) is independent and is used for marine and aviation navigation because it corresponds to one minute of arc of latitude.
Traditional Chinese length units date back thousands of years. The late-Qing / Republican convention standardised 1 chi (尺) = 1/3 m, 1 cun (寸) = 1/10 chi, 1 zhang (丈) = 10 chi ≈ 3.33 m and 1 li (里) = 1800 chi = 500 m. They remain common in informal use for body height ('1 m 75' is often spoken as '五尺三' or similar), traditional medicine dosage descriptions and historical/literature contexts. This tool follows the modern convention so your conversions to metric are exact.
Astronomical Unit (AU) and light-year (ly) are the two scales that matter beyond the solar system. 1 AU is defined as exactly 149,597,870,700 m since 2012 — the rough Earth-Sun distance used for spacecraft navigation and orbital mechanics. 1 light-year = 9,460,730,472,580,800 m, defined as the distance light travels in one Julian year. For perspective, the nearest star system (Proxima Centauri) is about 4.24 ly away, and our galaxy is roughly 100,000 ly across.
All the conversions on this page are pure linear scaling — no offset is involved, unlike temperature. So a single factor suffices: target_value = source_value × (source_factor / target_factor). The factors are precomputed once and stored; each conversion is a single multiplication and division at full double precision. Display formatting uses exponential notation only when the magnitude would otherwise be unreadable.
