Convert lengths between metric (mm, cm, m, km), imperial (inch, foot, yard, mile), nautical miles, and astronomical units.
nm
µm
mm
cm
dm
m
dam
hm
km
Mm
Gm
Tm
thou
in
ft
yd
fathom
furlong
mi
nmi
Unit used in navigation, based on one minute of latitude (1852 m).
au
Average distance between the Earth and the Sun.
ly
The distance that light travels in vacuum in one Julian year.
pc
Unit of distance used in astronomy (approx. 3.26 light years).
Two Systems on the Same Ruler
Length is the easiest physical quantity to measure and the messiest to standardize. Most of the world has settled on the metric system — a base‑10 decimal ladder defined since 1983 by how far light travels in 1/299,792,458 of a second. The United States, the United Kingdom (informally), and a handful of others still routinely use the imperial / US‑customary family, which descends from medieval body-based units (the foot, the yard, the inch).
This converter handles both systems plus astronomical scales (light-year, AU, parsec) so you can move between them without keeping a conversion table at hand.
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
When Each Unit Shows Up
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.
Disclaimer
Converted values are rounded for display and may carry small floating-point error. For engineering, surveying, or legal purposes where precision matters, verify against a primary standard.