Astronomical Units to km Converter

How to use the astronomical units to kilometers converter

1. Enter the distance in astronomical units into the input field.
2. The result in kilometers updates automatically as you type.
3. Use the examples below the calculator for quick reference to common planetary distances.

How the conversion works

The International Astronomical Union defined one astronomical unit as exactly 149,597,870.7 kilometers in 2012. To convert astronomical units to km, multiply the AU value by this constant.

For example, Mars orbits at an average distance of 1.524 AU from the Sun. The calculation is: 1.524 x 149,597,870.7 = 227,987,154.9468 km. Because the constant is exact, the only source of imprecision is the number of decimal places in your starting AU figure. When you need to convert astronomical units to km for scientific work, always use the full IAU constant rather than a rounded approximation like 150 million km.

Planetary distances reference table

The table below lists the semi-major axes of the eight planets in both AU and kilometers, computed with the exact IAU constant and rounded to one decimal place for readability.

These values represent average orbital distances. Actual distances vary throughout each orbit due to eccentricity.

Why astronomers use AU instead of kilometers

Kilometers are practical on Earth, but solar system distances produce unwieldy numbers. Neptune sits about 4.5 billion km from the Sun, which is harder to read, compare, and remember than 30.07 AU. The astronomical unit scales naturally with planetary orbits, making ratios obvious at a glance: Jupiter is roughly five times farther from the Sun than Earth because 5.2 AU is about five times 1 AU.

For interstellar distances, even AU becomes cumbersome, so astronomers switch to parsecs or light-years. One light-year equals approximately 63,241 AU. For maritime or aviation navigation on Earth, units like those handled by the nautical miles to km converter are more appropriate.

Historical background of the astronomical unit

Before 2012, the AU was derived from the Gaussian gravitational constant and depended on the mass of the Sun, which introduced tiny uncertainties. Radar ranging of Venus and spacecraft telemetry eventually pinned down the Earth-Sun distance with extreme precision. In 2012, the IAU adopted Resolution B2, fixing the AU at exactly 149,597,870.7 km. This removed any dependence on solar mass or gravitational measurements, turning the AU into a simple conventional unit of length.

Prior to this resolution, converting astronomical units to km required propagating measurement uncertainties from the gravitational constant. Now the relationship is exact by definition, meaning any remaining uncertainty lies solely in the measured AU value of a given celestial body, not in the conversion factor itself.

Common mistakes when converting AU to km

A frequent error is using outdated textbook values. Older sources sometimes list the AU as 149,597,870 km (without the .7) or even 150 million km as a rough approximation. While 150 million km is fine for back-of-the-envelope estimates, precise orbital mechanics requires the exact IAU figure.

Another mistake is confusing the semi-major axis (the average orbital distance) with the instantaneous distance. Planetary orbits are elliptical, so the actual distance varies throughout the year. Earth, for instance, ranges from about 0.983 AU at perihelion to 1.017 AU at aphelion. Always check whether your source value represents an average or a specific epoch.

Floating-point arithmetic can also introduce subtle errors in software. Multiplying large constants by fractional AU values may lose precision beyond 15 significant digits. This converter rounds results to 12 significant digits before formatting to eliminate such noise while preserving meaningful accuracy for planetary science.

Worked examples

Each result below was produced by the same code the tool runs, so you can check your own numbers against them.