Donaldson R F Astronomical Distance Converter
Kilometres are too small for space. Convert between kilometres, astronomical units, light-years and parsecs.
The four units
A kilometre needs no introduction. An astronomical unit (AU) is the average Earth–Sun distance, exactly 149,597,870.7 km by definition — a natural yardstick for the Solar System. A light-year is the distance light travels in one year (about 9.46 trillion km) — useful for distances to other stars. A parsec is defined geometrically: it is the distance at which 1 AU, seen edge-on, would span an angle of exactly one arcsecond (1/3600 of a degree) — about 3.26 light-years, and the unit professional astronomers use most, because it falls directly out of how stellar distances are actually measured (by parallax).
Frequently asked questions
Why do parsecs come from an angle?
Distances to nearby stars are measured by parallax: how much a star appears to shift against the background sky as Earth orbits the Sun. A star exactly 1 parsec away shows a parallax shift of exactly 1 arcsecond, which is why the unit was defined that way — it turns a measured angle directly into a distance.
How far is the nearest star in these units?
This tool does not include specific star distances, since those change slightly as better measurements are published; use it to convert whatever figure you already have.
Related tools: Light Travel Time Calculator, Planet Facts Table, Unit Converter.
Source / Last verified: astronomical unit (149,597,870.7 km, IAU-defined) from NASA/JPL Solar System Dynamics, "au (astronomical unit)" (ssd.jpl.nasa.gov/glossary/au.html), read 27 September 2026. Light-year and parsec are both geometric/definitional (light-year = speed of light × one Julian year; parsec = the distance at which 1 AU subtends 1 arcsecond), computed from that AU value and the exact SI speed of light rather than taken from a second source.