The Astronomical Unit Converter (AU) converts between astronomical units and meters, kilometers, light-years, and parsecs for astronomy and astrophysics calculations. Use it to scale distances in the solar system, galaxy, and beyond — one AU equals exactly 149,597,870,700 meters by IAU definition.
149,597,871
km
92,955,807
mi
8.32
min
149,597,871
km
92,955,807
mi
8.32
min
The calculator for astronomical unit conversion translates distances between AU and the full range of length units used in astronomy — from meters and kilometers for near-space engineering to light-years and parsecs for stellar and galactic distances. Whether you are calculating a spacecraft's position, comparing exoplanet orbital radii, or scaling a model of the solar system, this converter provides instant precision across all relevant astronomical distance scales.
Astronomy uses three distance scales for different regimes of the universe:
These three units differ by factors of ~63,000 and ~206,000 respectively — understanding which scale applies to which physical context prevents order-of-magnitude errors in astronomical calculations. Use this online calculator to convert any distance in AU to the appropriate scale unit for your application.
Space mission engineering uses metric units (km) rather than AU. Converting mission parameters:
The angular size calculator uses these distances to compute apparent angular sizes of solar system objects at any distance.
Exoplanet orbital parameters are routinely expressed in AU because it immediately contextualizes habitability. The habitable zone — where liquid water could exist on a planet's surface — for a Sun-like star spans roughly 0.95–1.37 AU. A planet at 0.7 AU from its star is too hot (Venus analog); at 1.5 AU, potentially too cold (Mars analog). The Kepler Space Telescope discoveries reported orbital periods in days; astronomers convert these to AU using Kepler's third law (a³ = T² for solar-mass stars, with a in AU and T in years) to immediately assess habitability context. The astronomy and astrophysics calculators provide the complete toolkit for astronomical calculations.
The solar system's scale is notoriously difficult to grasp intuitively. At 1 AU = 1 meter scale, Earth is a 12.7 mm marble; the Sun is a 1.39 m sphere; Neptune is 30 m away; the nearest star (Proxima Centauri) would be 272 km away. Most scale model solar systems in museums are not to scale — they cannot be, without making the planets invisible or the model cover an area larger than a city. Understanding AU values makes these scale comparisons immediate and intuitive.
1 AU = 149,597,870.7 km exactly (IAU 2012 definition). 1 AU = 499.005 light-seconds = 8.317 light-minutes. 1 AU = 1/63,241.1 light-years = 1/206,265 parsecs. All values converted to AU first, then to other units. Light travel time in light-minutes = AU x 499.005/60.
Distances below 0.01 AU are within the inner Solar System near the Sun (dangerous zone for spacecraft without special shielding). The habitable zone of the Sun spans roughly 0.95 to 1.37 AU. The asteroid belt spans 2.2 to 3.2 AU. The Kuiper Belt spans 30 to 50 AU. The heliopause (boundary of the heliosphere) is about 100-120 AU.
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Neptune orbits at about 30 AU. Light takes over 4 hours to travel from Earth to Neptune — explaining why commands sent to the Voyager 2 flyby in 1989 had a round-trip delay of over 8 hours.
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One AU is 149.6 million km. Light from the Sun takes 8.317 minutes (about 8 minutes 19 seconds) to reach Earth — the definition anchor of the AU.
Since 2012, one AU is defined exactly as 149,597,870.7 km. It was originally defined as the mean Earth-Sun distance, but because Earth's orbit is elliptical, the actual Earth-Sun distance varies between 147.1 million km (perihelion in January) and 152.1 million km (aphelion in July). The defined value is close to the average of these extremes.
The AU is convenient because it puts Solar System distances in the range of single digits to thousands, rather than hundreds of millions of kilometers. It also makes Kepler's third law simple: P^2 = a^3 when P is in years and a is in AU, for planets orbiting the Sun.
Early estimates used the parallax of Mars or observations of the distance to the Moon combined with the lunar parallax. The most accurate pre-radar measurements used transits of Venus in 1761 and 1769 to triangulate the Earth-Sun distance from different points on Earth. Modern laser ranging to corner reflectors on the Moon and radar ranging of Venus established the current precise value.
Perihelion is the point in Earth's elliptical orbit where it is closest to the Sun (about 147.1 million km, reached in early January). Aphelion is the farthest point (about 152.1 million km, in early July). Earth's orbital eccentricity is only 0.0167 — very close to circular. Contrary to intuition, Earth is actually closest to the Sun during northern hemisphere winter.
Pluto's orbit is highly elliptical with an eccentricity of 0.25. Its perihelion (closest approach to Sun) is 29.7 AU and its aphelion is 49.3 AU. The mean distance is about 39.5 AU. From 1979 to 1999, Pluto was actually closer to the Sun than Neptune, which has a more circular orbit at 30.07 AU.
As of 2026, Voyager 1 is approximately 165 AU from the Sun — the farthest any human-made object has ever traveled. Launched in 1977, it crossed the heliopause (the boundary of the solar wind) in 2012 and is now in interstellar space. Radio signals from Voyager take about 23 hours to reach Earth at the speed of light.
At their average distances: Mercury 3.2 min, Venus 6.0 min, Mars 12.7 min (varies from 3.1 to 22.4 min as orbits change), Jupiter 43 min, Saturn 79 min, Uranus 159 min, Neptune 251 min. This makes real-time control of interplanetary spacecraft impossible — commands and responses always involve significant time delays.
The Hill sphere of a planet is the region within which the planet's gravity dominates over the Sun's tidal forces, allowing it to retain satellites. Earth's Hill sphere extends to about 1.5 million km (0.01 AU), well beyond the Moon's orbit (0.0026 AU). Jupiter's Hill sphere is enormous at about 0.35 AU, explaining why it has captured so many moons.
The Sun's gravitational sphere of influence extends to about 120,000-150,000 AU, beyond which the gravity of other stars begins to dominate. The Oort Cloud, the reservoir of long-period comets, is thought to extend from about 2,000 to 100,000 AU. The nearest star (Proxima Centauri) is about 268,000 AU away.
At about 550 AU from the Sun, the Sun's gravity acts as a gravitational lens, focusing light from stars directly behind it to a point. This is known as the solar gravitational focus. Some have proposed sending a telescope to this distance to use the Sun as a lens for imaging exoplanets or other distant objects with extraordinary resolution — a mission concept called the Solar Gravitational Lens mission.
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