Last updated October 12, 2026 · Report an error
Distances in space are so large that everyday units like kilometers quickly become unwieldy, so astronomers use the astronomical unit (AU — the average Earth-Sun distance), the light-year (the distance light travels in one Julian year), and the parsec (based on stellar parallax). This tool converts between all of them and familiar terrestrial units.
How it works
Enter a value, choose the unit you're converting from, and the unit you want to convert to. Every unit is defined by its exact multiplier relative to meters, the shared base unit — the astronomical unit and light-year use their IAU-exact defined values.
- Enter value.
- Enter from.
- Enter to.
- Click Calculate to see your results.
Examples
Earth to the Sun
1 astronomical unit (AU) — the average Earth-Sun distance — equals exactly 149,597,870,700 meters by IAU definition.
Who should use it
- Converting a textbook astronomical distance into more familiar units.
- Space and astronomy education.
Industry applications
- Astronomy and astrophysics
- Science education
- Aerospace
Advantages
- Covers both everyday units (km, miles) and specialized astronomical units.
- Uses IAU-exact defined values for AU and light-year.
Limitations
- Does not include angular-diameter or apparent-magnitude-based distance estimates — pure unit conversion only.
Common mistakes to avoid
- Confusing light-years (a distance) with actual elapsed time — light-years measure how far, not how long.
Best practices
- Use AU for solar-system-scale distances, light-years or parsecs for interstellar and galactic distances.
Tips
- For distances within the solar system, AU is usually the most intuitive unit; for anything beyond, light-years or parsecs are standard.