Convert between light-years, astronomical units, parsecs, and kilometers — free, using exact IAU-defined constants.
Convert between the units astronomy actually uses for distance — light-years, astronomical units (AU), parsecs, and plain kilometers or miles — using the exact, officially defined ratios between them, not rounded approximations.
An astronomical unit is the average Earth-Sun distance. A light-year is how far light travels in one Julian year — useful for interstellar distances, where kilometers become an unwieldy number of digits. A parsec (derived from "parallax of one arcsecond") is the unit professional astronomy actually uses for anything beyond the solar system, and it's larger than a light-year — one parsec equals about 3.26 light-years, a ratio fixed by its geometric definition, not measured or approximated.
Like the field it comes from, this tool works entirely with input you type in — no live astronomical data, just the fixed conversion ratios between these units.
The underlying constants (the exact length of an AU, and the exact length of a parsec) are official definitions, not measurements with error bars — so the ratios between units are as exact as floating-point arithmetic allows, though displayed results are rounded for readability.
A parsec is defined geometrically — the distance at which one AU subtends an angle of one arcsecond — and that distance happens to work out to about 3.26 light-years. It’s a coincidence of the definition, not a rounder or more fundamental unit than the light-year.
Proxima Centauri, the nearest star to the Sun besides itself, is about 4.25 light-years away — roughly 268,000 AU, or about 40 trillion kilometers.