Last updated: 2026-04-30

Length Units

The metric system measures length on a base‑10 decimal ladder; the meter has been defined since 1983 by how far light travels in 1/299,792,458 of a second. Most of the world uses metric as the primary standard. The United States, the United Kingdom (informally), and a handful of others also use the imperial / US‑customary family, which descends from body-based historical units (the foot, the yard, the inch).

This converter handles both systems plus astronomical scales (light-year, AU, parsec).

Conversion Reference

All conversions are computed against the SI meter exactly. Imperial units are defined relative to the meter by international agreement (1 inch = 0.0254 m exactly).

From	To meters	Notes
1 inch	0.0254	Exact, by 1959 international yard agreement
1 foot	0.3048	12 inches
1 yard	0.9144	3 feet
1 mile	1,609.344	5,280 feet
1 nautical mile	1,852	One minute of latitude (exact, by definition)
1 fathom	1.8288	6 feet — used for water depth
1 furlong	201.168	220 yards — horse racing
1 light year	9.461 × 10¹⁵	Distance light travels in one Julian year
1 astronomical unit	1.496 × 10¹¹	Mean Earth–Sun distance
1 parsec	3.086 × 10¹⁶	≈3.26 light years

Usage by Context

Centimeters and millimeters dominate clothing sizes, paper formats, and most of the world's engineering drawings. Meters and kilometers are the everyday outdoor scale — running distances, road signs, construction drawings.

Inches still rule consumer screens (the "27-inch monitor"), pipe diameters, and US construction. Feet appear in aviation altitude (flight levels are in 100s of feet worldwide, even in metric countries), height in person-to-person conversations in the US/UK, and ceiling heights. Miles and yards are highway and short-distance US conventions; nautical miles are universal for ships and aircraft navigation because one minute of latitude is one nautical mile by definition — the unit makes chart work straightforward.

Light years, astronomical units, and parsecs are the astronomy ladder. AU for things inside the solar system, light years for popular‑science distances, parsecs for professional astronomy (it falls naturally out of stellar parallax measurement).

Frequently Asked Questions (FAQ)

Are these conversions exact or approximate?

Imperial units (inch, foot, yard, mile) and the nautical mile are exact by international definition. Astronomical units are by their nature approximations of physical phenomena.

Why does 1 inch equal exactly 0.0254 m?

The 1959 International Yard and Pound Agreement, signed by the US, UK, Canada, Australia, New Zealand, and South Africa, redefined the inch in metric terms to harmonize previously slightly different national versions. Before 1959, the US "survey inch" and the British "imperial inch" differed by about two parts per million.

When should I use a nautical mile vs. a statute mile?

Nautical miles are used for sea and air navigation because one minute of arc along a meridian is, by definition, one nautical mile (1852 m exactly). This makes chart work and great-circle calculations straightforward. The "ordinary" statute mile (1609.344 m) is used for road distances, especially in the US and UK.