Astronomical Unit Calculator

This astronomical unit calculator converts a distance given in astronomical units into the everyday and scientific units astronomers reach for: kilometres, metres, miles, the time light takes to travel that far, light-years and parsecs. One astronomical unit, written AU, is the average distance from the Earth to the Sun, fixed by definition at exactly 149,597,870.7 km. It is the natural yardstick for the Solar System, where quoting planetary distances in kilometres quickly becomes unwieldy: Mars sits about 1.52 AU from the Sun, Jupiter about 5.2 AU, and Neptune around 30 AU. Enter any number of AU and the calculator does the arithmetic instantly, so you can see how far a spacecraft has travelled, how long a radio signal will take to reach it, or how a Solar System distance compares with the vast gaps between stars. Because the Sun sits roughly 8 light-minutes away, distances in AU also translate neatly into light travel time, which is often the most intuitive way to grasp them. Use it for homework, for planning an astronomy lesson, or just to satisfy your curiosity about the true scale of space. The constants used are the internationally agreed values, so the conversions are exact to the precision shown.

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AU

1 AU = 149,597,870.7 km, the average Earth-Sun distance. Enter a whole number or a decimal.

149,597,870.7 km
kilometres in that distance
In miles92,955,807.3 miles
In metres149,597,870,700 m
Light travel time8 min 19 s

A distance of 1 AU is 149,597,870.7 km, which light crosses in about 8 min 19 s. That equals 0.00001581 light-years or 0.00000485 parsecs.

Constants used: 1 AU = 149,597,870.7 km; speed of light = 299,792.458 km/s; 1 light-year = 9,460,730,472,580.8 km; 1 parsec = 206,264.806 AU.

How it works

Every result comes from multiplying your AU value by a fixed constant. Kilometres are AU times 149,597,870.7; metres are that figure times 1,000; miles use the exact factor 1 km = 0.62137119 miles. Light travel time divides the distance in kilometres by the speed of light, 299,792.458 km per second, and is shown as minutes and seconds. Light-years divide the distance in kilometres by 9,460,730,472,580.8, the exact length of a light-year, and parsecs divide the AU value by 206,264.806, the number of AU in a parsec. Because these constants are defined values, the conversions are exact to the precision displayed.

Worked example

Take 1 AU, the average Earth-Sun distance. In kilometres that is 1 x 149,597,870.7 = 149,597,870.7 km, and in metres 149,597,870,700 m. In miles it is about 92,955,807.3. Dividing 149,597,870.7 km by 299,792.458 km/s gives roughly 499 seconds, which is 8 minutes and 19 seconds, the well-known figure for how long sunlight takes to reach us. The same distance is only about 0.00001581 light-years or 0.00000485 parsecs, a reminder that even the width of Earth's orbit is minuscule next to the distances between stars.

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Frequently asked questions

What is an astronomical unit?

An astronomical unit (AU) is the average distance between the Earth and the Sun. It is defined as exactly 149,597,870,700 metres, or 149,597,870.7 km, and is used to describe distances within the Solar System without resorting to very large numbers.

How long does light take to cross one AU?

Light travels at 299,792.458 km per second, so it covers one AU in about 499 seconds, which is roughly 8 minutes and 19 seconds. That is why sunlight is often said to take just over 8 minutes to reach Earth.

How many AU are in a light-year?

One light-year is about 63,241 AU, and one parsec is about 206,265 AU. So one AU is a tiny fraction of a light-year, roughly 0.0000158 light-years, which shows how much larger interstellar distances are than distances inside the Solar System.