Astronomical Unit Conversion Calculator and Formula

An astronomical unit (AU) conversion calculator converts a distance in AU into kilometres, metres, miles, or other distance units. Enter a nonnegative distance in AU; the calculator returns the equivalent distance in kilometres and miles, using the defined value of one AU.

Formula

The conversion multiplies the number of astronomical units by the equivalent distance represented by one AU:

• Distance in kilometres = distance in AU × 149,597,870.7 km/AU.
• Distance in miles = distance in AU × 92,955,807.3 mi/AU.
• Distance in metres = distance in AU × 149,597,870,700 m/AU.

Here, au is the entered distance, measured in astronomical units. The constants express one AU in each output unit. The astronomical unit is defined exactly as 149,597,870,700 metres; the kilometre and mile factors shown are rounded representations of that exact definition. The calculator displays results to two decimal places, so the underlying conversion is more precise than the displayed digits suggest.

Worked Example

Using the default input of 1 AU:

1. Kilometres: 1 × 149,597,870.7 = 149,597,870.70 km.
2. Miles: 1 × 92,955,807.3 = 92,955,807.30 mi.
3. Metres: 1 × 149,597,870,700 = 149,597,870,700.00 m.

These are the calculator’s outputs rounded to two decimal places. In everyday descriptions, one AU is often rounded to about 149.6 million kilometres, or about 93 million miles.

What an Astronomical Unit Measures

An astronomical unit is a unit of length used especially for distances within the Solar System. It is not a measure of time, brightness, or orbital speed. The International Astronomical Union adopted the exact metre definition in 2012, replacing an earlier definition tied to the dynamics of Earth’s orbit. This fixed value makes conversions consistent regardless of the precise shape or changing details of a planet’s orbit.

The scale is useful because many Solar System distances are large in kilometres but manageable in AU. Earth’s average distance from the Sun is close to 1 AU, while Jupiter orbits at roughly 5.2 AU on average. These rounded orbital figures give a quick sense of scale, not an exact distance at every point in an orbit.

AU is also used in astronomy beyond the Solar System, but it becomes inconvenient for distances between stars. A light-year or parsec is usually more suitable at those scales. One light-year is about 63,241 AU, so even the nearest stars are hundreds of thousands of AU away.

How to Use the Calculator

• Enter the distance in AU. Use a plain number, including a decimal if needed; for example, 0.5 or 2.3.
• Read the result in kilometres, miles, or metres, depending on the unit that suits your comparison.
• For a reverse conversion, divide the known distance by the corresponding conversion factor. For example, AU = kilometres ÷ 149,597,870.7.

The input represents a distance, so zero is valid and negative values are not ordinarily meaningful for this conversion. If your source gives a range, convert each endpoint separately. Keep the source precision in mind: a calculator cannot make an approximate input into an exact measurement.

Choosing the Right Unit and Reading Precision

Kilometres are convenient for comparing Solar System distances in metric contexts. Miles may be more familiar to readers using US customary units, while metres are useful in equations that use SI base units. The same distance has different numerical values in each unit, but the physical length is unchanged.

Rounding depends on why you are converting. Two decimal places are usually more than enough for a broad comparison, but they do not imply that an astronomical observation is accurate to a fraction of a kilometre. If a distance was reported as 1.2 AU, its limited input precision matters more than the many digits in a converted result. Preserve the source’s significant figures when presenting results.

For a quick mental estimate, multiply AU by 150 million to get kilometres, or by 93 million to get miles. Those approximations are handy for scale, but use the calculator’s factors for a more precise conversion. Avoid mixing units in the same calculation: multiplying an AU value by a kilometres-per-AU factor gives kilometres, not miles.

Common Misconceptions and Limits

One AU is often described as the Earth–Sun distance. That is a useful picture, but it should not be read as the exact Earth–Sun distance at every moment. Earth’s orbit is slightly elliptical, so its distance from the Sun changes during the year. The modern AU is an exact unit definition, not a claim that Earth remains exactly that far from the Sun.

Conversion also does not determine what a reported astronomical distance means. A value might refer to an average orbital radius, a current separation, a closest approach, or a modelled distance at a specified time. The calculator changes the unit only; it cannot identify the measurement convention or correct uncertainties in the original data. For orbital comparisons, check whether a source reports a mean distance, a range, or a distance at a particular date.

Frequently Asked Questions

How many kilometres are in one astronomical unit?
One AU is exactly 149,597,870,700 metres, which is 149,597,870.7 kilometres. In rounded everyday terms, that is about 149.6 million kilometres.

How many miles is one AU?
One AU is approximately 92,955,807.3 miles, commonly rounded to about 93 million miles.

How do I convert kilometres to AU?
Divide the distance in kilometres by 149,597,870.7. For example, 299,195,741.4 km is 2 AU.

Is one AU exactly the distance from Earth to the Sun?
One AU is an exact unit of length, but Earth’s actual Sun distance varies through its orbit. The AU is close to Earth’s average orbital distance, rather than a fixed measurement of the separation at every moment.

When should I use AU instead of light-years?
AU is convenient for distances within the Solar System. Light-years or parsecs are generally easier for describing distances between stars and across the wider galaxy.

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in au | Distance | AU | 1 | 0
out km | Distance | km | au * 149597870.7 | 2
out mi | Distance | mi | au * 92955807.3 | 2
out m | Distance | m | au * 149597870700 | 2