Astronomical Distance Converter
Convert between kilometers, astronomical units, light-years, and parsecs instantly.
Type in any field and the others update automatically.
Type in any field — the others update instantly.
Astronomical distances get so large that ordinary kilometers stop being useful, so astronomers switch units to match the scale of the problem. Inside the solar system they use the astronomical unit; for stars and galaxies, the light-year and the parsec.
Key conversions:
- 1 AU (astronomical unit) = 149,597,870.7 km (the average Earth-to-Sun distance)
- 1 light-year = 9,460,730,472,580.8 km (how far light travels in one year)
- 1 parsec = 30,856,775,814,913.7 km (about 3.2616 light-years)
For scale:
- Earth to the Moon: about 384,400 km (0.00257 AU)
- Earth to the Sun: 1 AU
- Proxima Centauri, the nearest star: 4.24 light-years
- The center of the Milky Way: about 26,000 light-years
The light-year is the intuitive one. Light is fast, so a light-year is a staggering distance, and the name does the explaining. The parsec is less obvious. It comes from geometry: the distance at which one astronomical unit would span one arcsecond of sky. Professional astronomers prefer it because it falls straight out of how stellar distances are actually measured, by parallax.
The classic mistake is treating a light-year as a unit of time. It isn’t. It’s a distance, the road light covers in a year. The twist is that looking far also means looking back: a star “4 light-years away” appears as it did 4 years ago, and that part genuinely is about time.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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