Convert Kilometer to Astronomical Unit
Convert kilometers to astronomical units instantly. 1 kilometer = 6.684587e-9 astronomical unit — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Astronomical Unit to Kilometer converter for the reverse conversion.
Units explained
Kilometer
A kilometer is a metric unit of length equal to one thousand meters. It is the standard unit for measuring road distances, geographic distances, and other large-scale measurements in metric countries.
The kilometer was defined alongside the meter in 1795 using the standard SI prefix kilo- (from Greek chilioi, "thousand"), denoting one thousand units.
Kilometers are used worldwide (except the United States and a few others) for road signage, geographic distance, athletic events, and scientific distances at planetary scale. Speed limits in most countries are given in km/h.
Adopted 1795 in France as part of the original metric system; the kilometer became the global standard for road and geographic distance through the 19th and 20th century metric adoption.
Astronomical Unit
The astronomical unit (AU) is a unit of length equal to exactly 149,597,870,700 meters, approximately the average distance from Earth to the Sun. It is the fundamental unit for distance within the solar system.
The AU was originally defined as the semi-major axis of Earth's orbit. In 2012, IAU Resolution B2 redefined the AU as exactly 149,597,870,700 meters, severing its dependence on the actual fluctuating Earth–Sun distance.
AUs are used to express distances within the solar system (Mars: 1.5 AU, Pluto: ~40 AU at perihelion), exoplanet orbital radii, and other planetary-scale distances. Standard in planetary science and astronomy.
Original definition tied to Earth's orbit (Newton, 17th century); fixed at exactly 149,597,870,700 m by IAU Resolution B2 in 2012.
Kilometer to Astronomical Unit conversion formula
The relationship between kilometers and astronomical units:
To convert kilometers to astronomical units, multiply the value in kilometers by 6.684587e-9. To reverse, multiply astronomical units by 1.495979e+8.
How to use this converter
Type a value into the calculator. The result in astronomical units updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Astronomical Unit to Kilometer converter for the reverse direction.
Step-by-step: convert kilometers to astronomical units
- Write down the value in kilometers (km).
- Multiply that value by the factor 6.684587e-9.
- The product is the equivalent value in astronomical units (AU).
- To reverse, multiply the astronomical unit value by 1.495979e+8.
Worked examples
Example 1 — Convert 1 km to AU:
1 × 6.684587e-9 = 6.684587e-9 AU
Example 2 — Convert 100 km to AU:
100 × 6.684587e-9 = 6.684587e-7 AU
Real-world example — Road-sign distances across systems
A 100-kilometer road sign converts cleanly into astronomical units — exactly the conversion drivers planning trips abroad rely on when reading road signs in a different measurement system.
100 km × 6.684587e-9 = 6.684587e-7 AU
Real-world example — Endurance race distances
A 50-kilometer cycling or running race converts to a recognizable distance in astronomical units. Endurance athletes coming from a different measurement system use this conversion to compare training plans across regions.
50 km × 6.684587e-9 = 3.342294e-7 AU
Real-world example — Maritime exclusion zone
A 200-kilometer exclusion zone (a common maritime boundary) converts to a different value in astronomical units that's useful when describing the same zone in everyday land-distance units.
200 km × 6.684587e-9 = 1.336917e-6 AU
Kilometer to Astronomical Unit conversion table
Standard reference values for converting kilometers to astronomical units:
| Kilometer [km] | Astronomical Unit [AU] |
|---|---|
| 0.01 | 6.684587e-11 |
| 0.1 | 6.684587e-10 |
| 1 | 6.684587e-9 |
| 2 | 1.336917e-8 |
| 3 | 2.005376e-8 |
| 4 | 2.673835e-8 |
| 5 | 3.342294e-8 |
| 10 | 6.684587e-8 |
| 20 | 1.336917e-7 |
| 30 | 2.005376e-7 |
| 40 | 2.673835e-7 |
| 50 | 3.342294e-7 |
| 100 | 6.684587e-7 |
| 500 | 3.342294e-6 |
| 1000 | 6.684587e-6 |
Frequently asked questions
How many astronomical units is 1 kilometer?
How do I convert kilometers to astronomical units?
How do I convert astronomical units back to kilometers?
How many astronomical units is 100 kilometers?
Popular length unit conversions
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Metric / SI (17 units)
Imperial / US Customary (27 units)
Nautical (1 units)
Astronomical (9 units)
Atomic / Physics (6 units)
Typographic (3 units)
Sources & references
Conversion factor (1 km = 6.684587e-9 AU) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Astronomical Union — System of Astronomical Constants
The IAU defines astronomical units including the AU (149597870700 m exactly) light-year and parsec used in astronomy and astrophysics.