The astronomical unit (AU) is the standard yardstick for measuring distances within the Solar System. This converter turns AU into kilometres and kilometres into AU using the exact value fixed by the International Astronomical Union (IAU) in 2012. It is handy for students, astronomy enthusiasts and anyone making sense of planetary distances.
How it works
One AU is the average distance from Earth to the Sun, defined as exactly 149,597,870.7 km. The conversion is a simple multiplication or division:
- AU to km: multiply the number of AU by 149,597,870.7.
- km to AU: divide the distance in km by 149,597,870.7.
Type a value into either box and the other updates instantly using this exact factor.
Example
Jupiter orbits the Sun at about 5.2 AU. In kilometres:
5.2 × 149,597,870.7 ≈ 777.9 million km.
| Object | Distance from Sun (AU) | Distance (million km) |
|---|---|---|
| Mercury | 0.39 | 57.9 |
| Earth | 1.00 | 149.6 |
| Mars | 1.52 | 227.4 |
| Jupiter | 5.20 | 777.9 |
| Saturn | 9.58 | 1,433.5 |
| Uranus | 19.2 | 2,872.5 |
| Neptune | 30.1 | 4,495.1 |
Why “average” distance?
Planetary orbits are ellipses, not perfect circles, so the Earth–Sun distance varies across the year. At perihelion (closest approach, around January) Earth is about 0.983 AU from the Sun; at aphelion (farthest, around July) it is about 1.017 AU. The AU is defined as the mean semi-major axis of Earth’s orbit — essentially the long-radius average — not the minimum or maximum.
Comparing AU to other astronomical distance units
The AU is the right unit for distances within the Solar System, but it becomes unwieldy for larger scales. Astronomers use other units accordingly:
| Scale | Appropriate unit | Relation to AU |
|---|---|---|
| Solar System | AU | 1 AU = 149,597,870.7 km |
| Nearby stars | light-year | 1 ly ≈ 63,241 AU |
| Nearby stars | parsec | 1 pc ≈ 206,265 AU |
| Galaxies | kiloparsec | 1 kpc ≈ 206,265,000 AU |
The nearest star system, Alpha Centauri, is about 4.24 light-years away — roughly 268,000 AU. At that scale, the AU becomes as unwieldy as measuring a trans-Atlantic flight in millimetres.
Historical context
Before 2012, the AU was defined by astronomical measurements and slightly adjusted over time. The 2012 IAU resolution fixed it as an exact number (149,597,870,700 metres) to eliminate uncertainty and simplify calculations. The previous value agreed to many decimal places but was not exact; the new definition removes any ambiguity.
The conversion is pure arithmetic that runs entirely in your browser, with no network requests.