Convert kilometers to astronomical units

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Enter a distance in kilometers and the converter gives it in astronomical units. One astronomical unit is 149,597,870.7 km, so 150,000,000 km comes to 1.002688068 au. The field shows very small results with ten significant digits and, below it, in scientific notation.

km

The field also accepts scientific notation, for example 6.5e-7 or 6.5 × 10⁻⁷.

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What is an astronomical unit used for?

Distances in the Solar System are measured in hundreds of millions and billions of kilometers. In astronomical units the same distances become short numbers that are easy to compare. Between stars, distances are given in parsecs and light-years, because the astronomical unit is too small for them[1].

In 2012 the International Astronomical Union defined the astronomical unit as a conventional length of exactly 149,597,870,700 meters. The resolution also gives the reason: astronomy needs a unit of length that approximates the Sun–Earth distance[2].

The unit is useful in computing as well. In a numerical model of the Solar System it provides a scale that keeps floating-point errors small, including errors from numbers that are too large or too small for the format, or cut short[1].

Yellow disc of the Sun against an orange background
The yellow disc of the Sun on an orange background that darkens to brown toward the edges of the photo. The average distance between Earth and the Sun is roughly one astronomical unit, about 150 million km. Photo: Marek Piwnicki, Unsplash.

How many astronomical units are in a kilometer?

One kilometer is 0.000000006684587122 astronomical units. The result is so small because one astronomical unit is 149,597,870.7 kilometers.

With larger distances the results are easier to read. One million kilometers is 0.006684587122 au. One billion kilometers is 6.684587122 au. The rounded figure of 150 million kilometers[3] comes to 1.002688068 au.

Two thresholds follow from these numbers. A distance below one million kilometers gives a result with at least two zeros after the decimal point. A distance above 150 billion kilometers gives more than one thousand astronomical units.

In scientific notation one kilometer is 6.684587122 × 10⁻⁹ au. The negative exponent means the decimal point moves nine places to the left.

The converter above calculates as you type and also accepts scientific notation such as 1.5e8. For results below 0.01 au, the same result also appears in scientific notation under the result field, which saves counting zeros for small distances.

How do you convert kilometers to astronomical units? Formula and examples

To convert kilometers to astronomical units, divide by 149,597,870.7. That is the number of kilometers in one astronomical unit under the 2012 definition.

The formula: astronomical units = kilometers ÷ 149,597,870.7. The other way, multiply: kilometers = astronomical units × 149,597,870.7.

Two examples show different orders of magnitude. The average distance between Earth and the Moon is approximately 385,000 km[4]. Divided by the constant, that gives 0.002573566042 au. A distance of 4.5 billion kilometers comes to 30.08064205 au.

You can convert a result in astronomical units back into kilometers with the au to km converter, and kilometers into miles with the km to miles converter.

How many astronomical units are in a given number of kilometers?

The table covers distances from one kilometer to one trillion kilometers. Each result is the number of kilometers divided by 149,597,870.7, given with ten significant digits and in scientific notation.
KilometersAstronomical unitsScientific notation
1 km0.000000006684587122 au6.684587122 × 10⁻⁹ au
10 km0.00000006684587122 au6.684587122 × 10⁻⁸ au
100 km0.0000006684587122 au6.684587122 × 10⁻⁷ au
1,000 km0.000006684587122 au6.684587122 × 10⁻⁶ au
10,000 km0.00006684587122 au6.684587122 × 10⁻⁵ au
100,000 km0.0006684587122 au6.684587122 × 10⁻⁴ au
384,399 km0.002569548605 au2.569548605 × 10⁻³ au
1,000,000 km0.006684587122 au6.684587122 × 10⁻³ au
10,000,000 km0.06684587122 au6.684587122 × 10⁻² au
50,000,000 km0.3342293561 au3.342293561 × 10⁻¹ au
57,910,000 km0.3871044403 au3.871044403 × 10⁻¹ au
70,000,000 km0.4679210986 au4.679210986 × 10⁻¹ au
100,000,000 km0.6684587122 au6.684587122 × 10⁻¹ au
122,000,000 km0.8155196289 au8.155196289 × 10⁻¹ au
149,597,870.7 km1 au1 × 10⁰ au
150,000,000 km1.002688068 au1.002688068 × 10⁰ au
200,000,000 km1.336917424 au1.336917424 × 10⁰ au
300,000,000 km2.005376137 au2.005376137 × 10⁰ au
421,000,000 km2.814211178 au2.814211178 × 10⁰ au
500,000,000 km3.342293561 au3.342293561 × 10⁰ au
778,000,000 km5.200608781 au5.200608781 × 10⁰ au
1,000,000,000 km6.684587122 au6.684587122 × 10⁰ au
1,400,000,000 km9.358421971 au9.358421971 × 10⁰ au
1,500,000,000 km10.02688068 au1.002688068 × 10¹ au
3,000,000,000 km20.05376137 au2.005376137 × 10¹ au
4,500,000,000 km30.08064205 au3.008064205 × 10¹ au
5,000,000,000 km33.42293561 au3.342293561 × 10¹ au
5,900,000,000 km39.43906402 au3.943906402 × 10¹ au
6,000,000,000 km40.10752273 au4.010752273 × 10¹ au
10,000,000,000 km66.84587122 au6.684587122 × 10¹ au
100,000,000,000 km668.4587122 au6.684587122 × 10² au
1,000,000,000,000 km6,684.587122 au6.684587122 × 10³ au
When the distance grows a thousand times, the decimal point in the result moves three places to the right. Distances in meters can be converted first with the meters to km converter.

How many astronomical units separate Earth and Parker Solar Probe from the Sun?

Earth moves along an ellipse, so its distance from the Sun changes over the year. Perihelion is the point of the orbit closest to the Sun, and aphelion is the farthest point.
DistanceKilometersAstronomical units
Earth at perihelion147,098,074 km0.9832898912 au
Earth at aphelion152,097,701 km1.0167103335 au
Parker Solar Probe on December 24, 2024, from the Sun's surfaceabout 6,100,000 kmabout 0.041 au
The distances for Earth in astronomical units come from the Wikipedia article on the astronomical unit, and the kilometers are those values multiplied by the constant[1]. The Johns Hopkins University Applied Physics Laboratory gives the probe's distance from the Sun's surface as 3.8 million miles, around 6.1 million kilometers[5]. To convert miles to kilometers, use the miles to km converter.

How do you convert kilometers to astronomical units in a spreadsheet?

A spreadsheet helps when you have a whole column of distances, for example from an orbit table or mission data.

  1. Excel: divide by the constant

    The CONVERT function lists light-years and parsecs among its distance units[6]. To get astronomical units, divide: with a distance in cell A1, type =A1/149597870.7.

  2. Distances in millions of kilometers

    If your data is in millions of kilometers, divide by 149.5978707. For the value 778 in cell A1, the formula =A1/149.5978707 returns about 5.2006 au.

  3. Small results in scientific format

    The result for a single kilometer has eight zeros after the decimal point before the first non-zero digit. Select the cells, go to Home, in the Number group click the small More button next to Number, and in the Category list click Scientific. Excel then shows 6.68E-09[7].

Convert Kilometers to other units

km to mileskm to meterskm to cmkm to mmkm to dmkm to yards

Sources

  1. Astronomical unit — Wikipedia
  2. Resolution B2 on the re-definition of the astronomical unit of length — International Astronomical Union
  3. Light-year — Wikipedia
  4. Lunar distance — Wikipedia
  5. Parker Solar Probe Makes History With Closest Pass to the Sun — Johns Hopkins University Applied Physics Laboratory
  6. CONVERT function — Microsoft
  7. Display numbers in scientific (exponential) notation — Microsoft
  8. Parsec — Wikipedia

Every figure in the tables was computed from the unit definitions and checked arithmetically before publication. Historical values and the names of local units come from the sources listed above.

Content last reviewed: · written and checked by the Arteon team

Common questions about converting kilometers to astronomical units

How many astronomical units are in a light-year?

A light-year is about 63,241.077 au. That result comes from dividing 9,460,730,472,580.8 km, the distance light travels in a Julian year[3], by 149,597,870.7 km.

How many astronomical units are in a parsec?

A parsec is 648,000/π astronomical units, about 206,264.806 au. A 2015 resolution of the International Astronomical Union refers to this definition of the parsec[8].

How do I enter a distance given in millions of kilometers?

Type the full number of kilometers, for example 778000000, or the scientific notation 7.78e8. Both give the same result: 5.200608781 au.

How does the converter show very small results?

It gives them with at least ten significant digits. One hundred kilometers appear in the result field as 0.0000006684587122 au and in the Scientific notation line below as 6.68459 × 10⁻⁷ au.

Are au, AU and ua the same unit?

Yes. The International Astronomical Union recommends au, AU is common in astronomical literature, and ua comes from the French unité astronomique, the symbol the International Bureau of Weights and Measures recommended in 2006[1]. Each of them means 149,597,870.7 km.

Arteon free tool screenshot: Convert kilometers to astronomical units

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