AU Calculator: Convert Astronomical Units to km, miles & ly
An astronomical unit (AU) equals the average Earth-Sun distance. Convert AU to kilometers, miles, and light-years using the IAU's exact 2012 definition.
Astronomical Unit Calculator
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About Astronomical Units
An Astronomical Unit (AU) is a unit of length used to measure distances within our solar system. One AU is defined as the average distance between Earth and the Sun.
Astronomers use AU as a convenient way to express distances within our solar system. For example, Mercury is about 0.4 AU from the Sun, while Neptune is about 30 AU away.
For distances beyond our solar system, light-years are typically used instead of AU, as they represent much larger distances.
Documentation
What is an astronomical unit?
An astronomical unit (AU) is a unit of length equal to 149,597,870.7 kilometers (92,955,807.3 miles). It is close to the average distance between Earth and the Sun. Astronomers use it to describe distances inside the solar system, where kilometers and miles produce numbers too large to read comfortably.
The International Astronomical Union fixed this value by definition in 2012. Before then, the AU was calculated from the Sun's mass and the strength of gravity, so its exact value could shift slightly as measurements improved. Fixing it removed that uncertainty. The defined value stays extremely close to Earth's real average distance from the Sun, which varies a little because Earth's orbit is an ellipse, not a perfect circle.
Astronomical unit formula
This calculator converts between astronomical units and kilometers, miles, or light-years using the constants below.
AU to kilometers
d(km) = d(AU) × 149,597,870.7
AU to miles
d(mi) = d(AU) × 92,955,807.3
AU to light-years
d(ly) = d(AU) × 0.000015812507409
To convert the other way, divide instead of multiply. For example, to turn a distance in kilometers into AU:
d(AU) = d(km) ÷ 149,597,870.7
Example calculation
Mars orbits the Sun at an average distance of about 1.52 AU. To find that distance in kilometers:
1.52 × 149,597,870.7 = 227,388,763 km (about 141.3 million miles)
To check the answer, divide back: 227,388,763 ÷ 149,597,870.7 = 1.52 AU.
A second example: a near-Earth asteroid is reported to pass 7,500,000 km from Earth. In astronomical units, that is 7,500,000 ÷ 149,597,870.7 ≈ 0.05 AU, closer than Mercury ever gets to the Sun.
Distances of the planets in AU
| Planet | Average distance from Sun (AU) |
|---|---|
| Mercury | 0.39 |
| Venus | 0.72 |
| Earth | 1.00 |
| Mars | 1.52 |
| Jupiter | 5.20 |
| Saturn | 9.54 |
| Uranus | 19.19 |
| Neptune | 30.07 |
Rocky planets sit close together near the Sun, while the gas and ice giants are spread far apart. The Kuiper belt, a region of icy bodies past Neptune, extends to roughly 50 AU. The Oort cloud, a distant shell of comets thought to surround the solar system, may reach 100,000 AU or more. Past that scale, astronomers switch to light-years, since one light-year equals about 63,241 AU.
Voyager 1, launched in 1977, is the most distant human-made object. It left the heliosphere and entered interstellar space in 2012 at about 121 AU from the Sun. As of 2026 it is roughly 170 AU out, or about 25.4 billion kilometers, moving outward at close to 3.5 AU per year.
How the astronomical unit was first measured
The ancient Greek astronomer Aristarchus of Samos tried to measure the Sun's distance around 270 BCE by comparing angles to the Moon at exactly half phase. His method was sound geometry, but the measurement was nearly impossible to make precisely by eye. He concluded the Sun was about twenty times farther than the Moon; it is actually about four hundred times farther.
In the early 1600s, Johannes Kepler's laws of planetary motion revealed the relative spacing of the planets, showing, for instance, that Mars orbits about 1.5 times farther from the Sun than Earth does. That gave astronomers ratios but not a distance in miles or kilometers.
The first real breakthrough came from transits of Venus, when Venus passes directly between Earth and the Sun. Astronomers timed these transits from widely separated points on Earth in 1761 and 1769, sending expeditions to places such as Siberia and Tahiti. Small differences in the timing, caused by parallax, let them calculate the Earth-Sun distance to within a few percent of the modern value.
In the 1960s, radar astronomy improved the measurement enormously. Scientists bounced radio waves off Venus and Mercury and timed the echo, giving a distance accurate to within a few kilometers. By 2012, the value was known precisely enough that the IAU could simply define it as a fixed constant.
Frequently asked questions
What is an astronomical unit used for? It gives a convenient, human-scale way to describe distances inside the solar system. Saying Neptune orbits at 30 AU is easier to picture than saying it orbits at 4.5 billion kilometers.
How many kilometers are in one astronomical unit? Exactly 149,597,870.7 kilometers, or about 92,955,807 miles, by the IAU's 2012 definition.
How is the AU related to the light-year? One light-year equals about 63,241 AU. Light takes about 8 minutes and 19 seconds to cross one AU, but a full year to cross one light-year, which is why light-years are used for distances between stars.
Is one AU exactly the same as Earth's distance from the Sun? Not exactly. Earth's orbit is an ellipse, so its distance from the Sun ranges from about 0.983 AU in January to about 1.017 AU in July. The AU is close to the average of that range but is now defined as a fixed number rather than measured directly.
Who first measured the astronomical unit accurately? Early estimates from transits of Venus in the 18th century came within a few percent of today's value. Radar measurements in the 1960s made it far more precise, and the IAU fixed its exact value in 2012.
Does the calculator work in both directions? Yes. Enter a value in AU to see it in kilometers, miles, or light-years, or enter a value in one of those units to see the equivalent in AU.