Astronomical Units to Kilometers Converter

About

An astronomical unit (AU) is the SI-accepted unit of length used to describe distances inside the Solar System, defined as exactly 149,597,870,700 metres. It is approximately the mean Earth-Sun distance and is the natural scale for planetary orbits, probe trajectories, and observational geometry.

How it works

Until 2012 the AU was tied to an experimentally determined Earth-Sun distance, which made it implicitly mass-dependent through Kepler's third law. IAU Resolution B2 (2012) redefined it as a fixed constant in metres, decoupling it from the Sun's varying mass and from Newtonian/relativistic frame choices.

1 AU         = 149,597,870,700 m (exact, IAU 2012)
1 AU         = 1.495978707 x 10^11 m
1 AU         = 92,955,807.273 mi
1 light-year = 63,241.077 AU
1 parsec     = 648,000/pi AU = 206,264.806 AU = 3.2616 light-years
Light time   = 499.005 s = 8 min 19.005 s

Worked example

A radio command from JPL takes how long to reach the New Horizons spacecraft at 60.5 AU from Earth (early 2026)?

1. Distance: 60.5 AU.
2. Convert to metres: 60.5 x 149,597,870,700 = 9.051 x 10^12 m.
3. Divide by c = 299,792,458 m/s: 9.051 x 10^12 / 2.998 x 10^8 = 30,193 s.
4. Convert to hours: 30,193 / 3600 = 8.39 hours one way.
5. Round-trip light time: 16.78 hours, hence the deliberate, choreographed command sequences.

Reference table

Why astronomers use the AU

Solar-system distances are awkward in kilometres: Neptune sits 4.5 billion km from the Sun, a number that is hard to picture and tedious to write. Expressed in astronomical units, Neptune is a tidy 30 AU, and the whole planetary system fits in a single scale from Mercury at 0.39 AU to Pluto at 39.5 AU. The AU turns the Solar System into countable steps, which is exactly why orbital tables, probe trajectories, and textbook diagrams all use it.

The unit also carries a built-in sense of light-travel time. Because light crosses one AU in about 8 minutes 19 seconds, multiplying a distance in AU by roughly 8.32 minutes gives the one-way communication delay to a spacecraft. Sunlight reaches Earth (1 AU) in 8.3 minutes, Jupiter (5.2 AU) in 43 minutes, and Voyager 1 (around 167 AU) in nearly a full day. That single conversion explains why deep-space commands are scripted in advance rather than flown live: a joystick nudge to a Saturn probe would arrive 80 minutes after you moved it.

For distances beyond the Solar System the AU becomes unwieldy again, since the nearest star is over 270,000 AU away. Astronomers switch to light-years and parsecs there, but the parsec is itself defined through the AU (the distance at which one AU subtends one arcsecond), so the humble Earth-Sun yardstick underpins the entire cosmic distance ladder.

Common pitfalls

• Using 150 million km. Fine for rough estimates but loses 0.27 percent precision. Use the exact 149,597,870.7 km for mission planning or paper-grade work.
• Confusing AU with light-year. 1 light-year is over 63,000 AU. Casual writing sometimes uses them interchangeably; spacecraft trajectories are AU-scale, stellar surveys are light-year or parsec scale.
• Forgetting Pluto's orbit eccentricity. Pluto ranges from 29.7 AU (inside Neptune) to 49.3 AU. The mean 39.5 AU hides large variation.
• Treating AU as a time. AU measures distance, not light-travel time. Use AU x 499.005 s/AU to convert.
• Mixing 2012 and pre-2012 definitions. Pre-2012 AU values vary slightly across ephemerides because they were derived constants. The 2012 redefinition fixed the AU as exact in metres.

Related tools and glossary

Frequently asked questions

Sources

• International Astronomical Union (IAU), Resolution B2 (2012), Re-definition of the astronomical unit of length.
• NIST Reference on Constants, Units, and Uncertainty, astronomical unit value.
• NASA/JPL Solar System Dynamics, planetary mean distances (DE441 ephemeris).
• NASA Voyager Mission Status, current distance from Earth.

Last updated 2026-05-28.