Astronomical Unit (AU) Calculator
Enter any distance in astronomical units (AU) and get instant conversions to kilometres, miles, light-years, parsecs, light-minutes, and light-seconds. You can also work in reverse: enter kilometres, miles, or light-years and get the AU equivalent. The "Show your work" panel explains each step, and the solar system reference table lists every planet's average distance from the Sun.
Formula
Worked example
1 AU = 149,597,870.7 km = 92,955,807.3 miles = 1/63241.1 light-years = 1/206264.8 parsecs. Light crosses 1 AU in 499.0 seconds, or about 8 minutes 19 seconds.
What is an astronomical unit?
An astronomical unit (AU or ua) is the standard unit of length used in astronomy for distances within our solar system and nearby interstellar space. Since 2012 the International Astronomical Union has defined it exactly as 149,597,870,700 metres, or roughly 149.6 million kilometres and 93 million miles. It was originally based on the mean Earth-Sun distance but is now a fixed constant, independent of the actual orbital geometry. Because AU is tuned to solar-system scales, it produces convenient numbers: Mars is about 1.52 AU from the Sun, Jupiter is about 5.2 AU, and Neptune is about 30 AU. For larger distances, astronomers switch to light-years (ly) and parsecs (pc), but AU remains the natural unit for anything inside the heliosphere.
How AU, light-years, and parsecs relate
One light-year is the distance light travels in one Julian year (365.25 days) at 299,792,458 m/s, which works out to exactly 9,460,730,472,580.8 km. Dividing that by 1 AU gives the conversion: 1 ly = 63,241.1 AU, so 1 AU = about 0.0000158 ly. A parsec is defined as the distance from which 1 AU subtends a parallax angle of exactly one arcsecond; it equals 206,264.8 AU or about 3.262 ly or 30,856,775,814,913.7 km. To put it in context: the nearest star system, Alpha Centauri, sits at about 268,770 AU (4.37 ly or 1.34 pc) from the Sun. Voyager 1, humanity's most distant spacecraft, crossed about 100 AU from the Sun in 2012 and was past 165 AU by 2024.
Light travel time and why it matters
Nothing travels faster than light (299,792,458 m/s), so the time for light to cross a given distance is a hard physical limit on communication and observation. Light takes about 8 minutes 19 seconds (499 seconds) to reach Earth from the Sun at 1 AU. That means we always see the Sun as it was 8 minutes ago. For Mars at 1.524 AU the one-way travel time is about 12.7 minutes at average distance, so a rover command sent from Earth faces a minimum round-trip delay of about 25 minutes near average separation and over 40 minutes near greatest separation. For Jupiter at 5.2 AU the one-way delay is about 43 minutes. These delays are a fundamental engineering constraint for all interplanetary missions.
How to use this calculator
Choose a conversion direction from the mode selector. The default mode converts from AU into kilometres, miles, light-years, parsecs, light-minutes, and light-seconds simultaneously. Switch to km -> AU, Miles -> AU, Light-years -> AU, or Parsecs -> AU to convert in the opposite direction. All conversions use the IAU 2012 exact definition of the AU and the 2019 SI definition of the metre. The "Show your work" panel reveals each intermediate step with your actual values. The solar system reference table at the bottom gives a quick sense of scale relative to the planets.
Solar system distances from the Sun
| Object | Distance (AU) | Distance (km) | Light travel time |
|---|---|---|---|
| Mercury | 0.387 | 57,910,000 | 3.2 min |
| Venus | 0.723 | 108,200,000 | 6.0 min |
| Earth | 1.000 | 149,598,000 | 8.3 min |
| Mars | 1.524 | 227,940,000 | 12.7 min |
| Jupiter | 5.203 | 778,500,000 | 43.3 min |
| Saturn | 9.537 | 1,432,000,000 | 1 hr 19 min |
| Uranus | 19.191 | 2,870,000,000 | 2 hr 40 min |
| Neptune | 30.069 | 4,495,000,000 | 4 hr 10 min |
| Pluto (avg) | 39.5 | 5,910,000,000 | 5 hr 28 min |
| Voyager 1 (2024) | ~165 | ~24,700,000,000 | ~22.8 hr |
| Alpha Centauri | ~268,770 | ~40,200,000,000,000 | ~4.37 yr |
Average (semi-major axis) distances. Light travel time is from the Sun at average distance.
Frequently asked questions
How many kilometres is 1 astronomical unit?
Exactly 149,597,870.7 kilometres (the IAU 2012 definition gives 149,597,870,700 metres). In miles that is about 92,955,807.3 mi, and in light-time it is about 8 minutes and 19 seconds (499 seconds).
How many astronomical units are in a light-year?
There are approximately 63,241.1 AU in one light-year. Equivalently, 1 AU equals about 1/63,241 of a light-year, or roughly 0.00001581 ly. For stars and galaxies, light-years or parsecs are far more practical.
How many AU is a parsec?
One parsec equals exactly 206,264.806 AU. The parsec is defined as the distance at which 1 AU subtends a parallax angle of exactly one arcsecond as seen from Earth, linking the two units geometrically.
How far is Earth from the Sun in AU?
By definition, the Earth-Sun average distance is 1 AU. However, Earth's orbit is elliptical: at perihelion (closest, early January) Earth is about 0.983 AU from the Sun, and at aphelion (farthest, early July) it is about 1.017 AU. The AU itself is a fixed number, not the live Earth-Sun distance.
What is the farthest human-made object in AU?
Voyager 1, launched in 1977, is the most distant spacecraft. As of 2024 it is more than 165 AU from the Sun, well into interstellar space beyond the heliopause at about 123 AU. Voyager 2 is the second most distant at roughly 138 AU.
Why do astronomers use AU instead of kilometres for solar system distances?
Kilometres produce unwieldy numbers at solar-system scales. Writing "5.2 AU" for Jupiter is far cleaner than "778,500,000 km." AU also makes orbital ratios intuitive: because of Kepler's third law, a planet's orbital period in years squared equals its distance in AU cubed, so knowing the distance in AU lets you infer the period with no conversion factors.
How do you convert AU to metres?
Multiply the number of AU by 149,597,870,700 (the IAU 2012 exact definition). For example, 5 AU = 5 x 149,597,870,700 = 747,989,353,500 m, or about 748 million km.