Astronomical Unit Calculator

Conversion

Convert a distance in kilometres into astronomical units (AU). One astronomical unit is the average Earth–Sun distance, defined as exactly 149,597,870.7 km, so the calculator simply divides your kilometre value by that figure. It is a handy way to express interplanetary distances in Solar-System scale.

Formula

1 AU = 149,597,870.7 km
  • One astronomical unit (AU) equals exactly 149,597,870.7 kilometres, the defined mean distance from Earth to the Sun.
  • To convert kilometres to AU, divide by 149,597,870.7. To go the other way, multiply AU by the same figure.
  • Astronomical units make Solar-System distances easier to grasp: Mars is about 1.5 AU from the Sun and Jupiter about 5.2 AU.
  • For distances beyond the Solar System, astronomers switch to light-years or parsecs, which are vastly larger than the AU.

Example: 149,597,870.7 km

Inputs
  • Kilometers: 149597870.7

Dividing 149,597,870.7 km by 149,597,870.7 gives exactly 1 AU — the average distance from Earth to the Sun, which is the definition of the astronomical unit.

Frequently asked questions

What is an astronomical unit?
It is a unit of length equal to the mean Earth–Sun distance, defined as exactly 149,597,870.7 kilometres and used mainly for distances within the Solar System.
How do I convert kilometres to AU?
Divide the distance in kilometres by 149,597,870.7. For example, 299,195,741.4 km is 2 AU.
How far is each planet in AU?
Roughly: Mercury 0.39 AU, Venus 0.72 AU, Earth 1 AU, Mars 1.52 AU, Jupiter 5.2 AU, Saturn 9.5 AU, Uranus 19.2 AU and Neptune 30.1 AU from the Sun.
Is the AU the same as a light-year?
No. A light-year is about 63,241 AU — the distance light travels in a year — so it is used for interstellar rather than interplanetary distances.
Why is the AU defined so precisely?
Since 2012 the AU has been fixed at exactly 149,597,870,700 metres by international agreement, removing dependence on changing measurements of the Earth–Sun distance.