Article — Astronomical Unit (AU) Converter
Astronomical unit (AU) to kilometres: the solar-system yardstick
One astronomical unit is exactly 149,597,870.7 kilometres, or about 93 million miles. This is the standard yardstick for distances inside our solar system. The International Astronomical Union (IAU) fixed this exact value in 2012, replacing the old measurement-based definition.
Astronomers prefer AU over kilometres for two reasons. First, planet distances become readable numbers: Jupiter sits at 5.2 AU instead of 778 million kilometres. Second, Kepler's third law links orbital period and distance in clean form when both are in AU and years: the period squared equals the semi-major axis cubed.
What an astronomical unit is
The astronomical unit is the mean distance from Earth to the Sun. After the 2012 IAU resolution, the AU is no longer a measurement at all; it is a defined constant equal to exactly 149,597,870,700 metres. This locks the unit so that improving Earth-Sun measurements does not retroactively change every published AU figure.
Originally, the AU was derived from transit-of-Venus observations and later from radar bounce-back times to other planets. As measurement precision climbed past one part in a billion, the IAU decided to detach the unit from any ongoing physical measurement.
Light takes 499 seconds (8 minutes 19 seconds) to travel one AU. Multiply any AU distance by 8.317 to get the one-way light-time in minutes. Mars sits 4 to 22 light-minutes from Earth depending on orbital geometry, which is why NASA cannot fly rovers in real time.
History of the astronomical unit
The first numerical estimate of an AU came from the 1761 and 1769 transits of Venus, when international teams measured how Venus crossed the Sun's disk from different points on Earth. Using parallax geometry, astronomers got an AU within about 1% of the modern value. Subsequent transits in 1874 and 1882 refined the number further.
Radar replaced visual observation in the mid-20th century. Bouncing radio signals off Venus and asteroids gave astronomers AU values accurate to a few hundred metres. By 1976, the International Astronomical Union had standardised the AU at 149,597,870 km. In 2012 the value was promoted to a defined constant and pinned to 149,597,870,700 m exactly.
1 AU = 149,597,870.7 km1 AU = 92,955,807.3 miles1 AU = 499 light-seconds1 light-year = 63,241 AU1 parsec = 206,265 AUConverting AU to kilometres
The conversion is a single multiplication. For AU to kilometres, multiply by 149,597,870.7. For kilometres to AU, divide by the same number. Since the factor is exact, the only precision loss is the rounding you choose to apply when displaying the result.
For mental arithmetic, treat 1 AU as 150 million km. The error is about 0.27% — too small to matter for ballpark figures. For mission planning, use the full nine-significant-figure value and let the rounding happen in software.
Planet distances in astronomical units
Every planet in the solar system has a semi-major axis traditionally quoted in AU. Mercury orbits at 0.387 AU, Venus at 0.723 AU, Mars at 1.524 AU. The outer planets stretch the scale: Jupiter at 5.2 AU, Saturn at 9.5 AU, Uranus at 19.2 AU, Neptune at 30.1 AU. Pluto, downgraded to a dwarf planet in 2006, orbits at a mean 39.5 AU.
These numbers feed straight into Kepler's third law. Earth's period of 1 year squared (1) equals its semi-major axis of 1 AU cubed (1). For Mars, period² = 1.881² = 3.54, which equals 1.524³ = 3.54. The match is exact only because the AU is the unit of choice.
- Mercury = 0.387 AU = 57.9 million km
- Venus = 0.723 AU = 108.2 million km
- Earth = 1.000 AU = 149.6 million km
- Mars = 1.524 AU = 227.9 million km
- Jupiter = 5.203 AU = 778.6 million km
- Saturn = 9.537 AU = 1.43 billion km
- Voyager 1 (2024) = ~165 AU = 24.7 billion km
Astronomical unit versus light-year and parsec
AU is small on a cosmic scale. One light-year (the distance light travels in one Julian year) is 63,241 AU. One parsec (the distance at which one AU subtends one arcsecond) is 206,265 AU, or 3.262 light-years. Astronomers use parsecs and kilo-parsecs for stellar work and mega-parsecs for galactic and cosmological distances.
Proxima Centauri, the nearest star to the Sun, is 268,770 AU away — about 4.24 light-years or 1.30 parsecs. Quoting that in AU works numerically but feels unwieldy, which is why astronomers shift units at the boundary between solar-system and stellar work.
Astronomical units in space missions
NASA, ESA, JAXA, and other agencies plan trajectories in AU. The Parker Solar Probe holds the record for closest approach to the Sun: 0.046 AU at perihelion, or about 6.9 million km. Voyager 1 has reached roughly 165 AU and crossed the heliopause (the boundary between solar wind and interstellar plasma) at about 121 AU. Pioneer 10 last reported from about 80 AU.
For mission planning, light-time delay scales linearly with AU. Mars at conjunction is 22 light-minutes one way; at opposition it drops to 4. This determines what operations a rover can perform autonomously versus by ground command.
Common AU conversion mistakes
Three errors recur. First, treating "150 million km" as exact. The precise value is 149,597,870.7 km. The 0.27% error compounds in trajectory calculations and is easily avoided. Second, confusing AU with parsec. Parsec is 206,265 times larger; using AU where parsec is expected gives wildly wrong stellar distances. Third, forgetting that "Earth at 1 AU" is the mean orbit. The instantaneous Earth-Sun distance varies between 0.9833 AU at perihelion (early January) and 1.0167 AU at aphelion (early July). For close-tolerance work, use the actual ephemeris value.
An astronomical unit is the Earth-Sun distance. A parsec is 206,265 times larger and used for stellar parallax. Confusing the two in catalog work yields stellar distance errors of five orders of magnitude. The shorthand "pc" never means AU.