What is an astronomical unit?
The astronomical unit (AU) is a unit of length used mainly in astronomy, roughly equal to the average distance from Earth to the Sun. Since 2012, the International Astronomical Union (IAU) defines it as an exact value: 1 AU = 149,597,870.7 kilometers (about 92,955,807.3 miles). Before 2012 the AU was tied to the Gaussian gravitational constant and varied slightly with refinements in measurement; it is now a fixed, agreed-upon length so calculations across astronomy are consistent.
The conversion formulas
All conversions on this page reduce to a single fixed factor, applied directly or inverted:
- AU to kilometers: km = AU × 149,597,870.7
- Kilometers to AU: AU = km ÷ 149,597,870.7
- AU to miles: mi = AU × 149,597,870.7 × 0.621371192
- Miles to AU: AU = (mi ÷ 0.621371192) ÷ 149,597,870.7
- AU to light-seconds: light-seconds = (AU × 149,597,870.7) ÷ 299,792.458 (the speed of light in km/s)
This calculator uses those exact factors directly, so a value of 1 AU always converts to 149,597,870.7 km, no matter which direction you start from.
Why the AU is used instead of kilometers or miles
Distances within the solar system are enormous compared to everyday units, so astronomers use the AU as a convenient yardstick. Mars orbits at roughly 1.52 AU from the Sun, Jupiter at about 5.20 AU, and Neptune at about 30.1 AU — numbers that are far easier to work with than their kilometer equivalents (227 million km, 778 million km, and 4.5 billion km respectively). The AU also appears throughout orbital mechanics, exoplanet science, and spacecraft navigation, where distances are naturally expressed relative to Earth's orbit.
Reference distances in astronomical units
Some commonly cited benchmarks help put the AU in context:
- Earth to Sun (average): 1 AU = 149,597,870.7 km
- Mercury to Sun (average): about 0.387 AU
- Mars to Sun (average): about 1.524 AU
- Jupiter to Sun (average): about 5.203 AU
- Neptune to Sun (average): about 30.07 AU
- Voyager 1 (as of 2025, roughly): about 165 AU from the Sun
- Light travel time across 1 AU: about 499 seconds (8 minutes 19 seconds), which is why sunlight takes about 8 minutes to reach Earth
How to use this calculator
Enter the value you want to convert, choose the conversion direction from the dropdown (AU to km, AU to miles, km to AU, miles to AU, or AU to light-seconds), and press Calculate. The result card shows the converted value along with its equivalent in kilometers for quick cross-checking. Use Reset to return to the default of 1 AU converted to kilometers.
Frequently asked questions
How many kilometers are in one astronomical unit? Exactly 149,597,870.7 km, by IAU definition.
How many miles are in one astronomical unit? About 92,955,807.3 miles (149,597,870.7 km × 0.621371192).
Is the AU exactly Earth's distance from the Sun? It is close to the average (mean) Earth–Sun distance, but Earth's orbit is slightly elliptical, so the actual distance ranges from about 147.1 million km at perihelion (closest approach) to about 152.1 million km at aphelion (farthest point). The AU itself is now a fixed constant rather than a literal daily measurement.
Why was the AU redefined in 2012? Older definitions depended on the Gaussian gravitational constant and assumptions about solar mass, which introduced tiny variability as measurement precision improved. The IAU fixed the AU at an exact value in 2012 so it functions as a stable unit of length rather than a measured quantity.
How is the AU related to a light-year and a parsec? A light-year (about 9.461 trillion km) equals roughly 63,241 AU. A parsec (about 3.0857 × 10^13 km) equals about 206,265 AU — the distance at which 1 AU subtends an angle of one arcsecond.