Astronomical Unit Calculator

Convert astronomical units into kilometers, miles, light years, and more.

Supports metric, astronomical, and imperial/US length units including parsecs and solar radii.

Updated August 1, 2026

CreatorFrank Zhao

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Introduction / overview

The Astronomical Unit Calculator translates distances between astronomical units (au) and a wide range of everyday and cosmic units — metres and kilometres, light years, parsecs, solar radii, miles, and more. It works in both directions, so you can start from whichever unit you happen to have.

Astronomy is full of awkward numbers. Jupiter sits about 5.2 au from the Sun, Voyager-scale probes drift tens of au out, and an au itself is roughly 150 million kilometres. When you want that distance in miles for an article, or in light years to compare with a star, the conversion used to mean hunting for a table. This calculator does it instantly and keeps every unit consistent.

The au is not an approximation — the IAU fixed its exact value.

By international agreement the astronomical unit is exactly 149,597,870,700 metres, and the converter uses that precise definition.

Who is this for?

Students working through physics or astronomy problems that quote distances in au.
Hobby astronomers and stargazers who meet planet and comet distances in mixed units.
Science writers and educators turning mission updates into numbers people can picture.
Curious readers who want to know exactly how far “5.2 au” really is.

If your distances are mostly between the stars rather than between planets, the Light Year Conversion calculator is a perfect companion — it focuses on light years, parsecs, and the units used beyond the Solar System.

How to use / quick start

1Enter a distance in whichever field you have — astronomical units, kilometres, miles, light years, anything. Type the number and the rest of the fields fill in on their own.
2Read the other fields. Every field now shows the same distance in a different unit, so you can compare scales at a glance.
3Flip the direction any time by typing into the field you want to treat as the input. The other values recalculate automatically — there is no “convert button” to worry about.
4Start over whenever you need a fresh conversion — the reset control returns every field to empty.

Quick example: one astronomical unit in kilometres

Type 1 into the Astronomical units field. The Kilometres field shows 149,597,870.7:

1\ \mathrm{au}

=

149\,597\,870\,700\ \mathrm{m}

\div 1000

=

149\,597\,870.7\ \mathrm{km}

Going the other way is just as easy: type 149,597,870.7 into the Kilometres field and the Astronomical units field returns 1. Whatever you type, the rest of the fields stay perfectly consistent.

How to interpret the results

The fields you did not type into show the auto-calculated equivalents — that is where you read the converted value.
All fields represent the same physical distance, so you can read any of them to get the scale you want.
Distances are non-negative — a physical distance can never be less than zero, so the calculator flags negative entries as invalid.

Real-world examples / use cases

1) Jupiter's distance in kilometres

Background: You are reading that Jupiter orbits at about 5.2 au and want the figure in kilometres to picture the gap.

Input: Enter 5.2 in the Astronomical units field.

5.2\ \mathrm{au}

\times 149\,597\,870.7\ \frac{\mathrm{km}}{\mathrm{au}}

=

777\,908\,927.64\ \mathrm{km}

What it means: Jupiter is roughly 778 million kilometres from the Sun — more than five times Earth's 150 million kilometres. Converting to kilometres makes the scale of the outer Solar System tangible.

2) The nearest star, in astronomical units

Background: Proxima Centauri is about 4.2465 light years away. You want to see what that looks like in au to appreciate the jump from interplanetary to interstellar.

Input: Enter 4.2465 in the Light years field.

4.2465\ \mathrm{ly}

\times 63\,241\ \frac{\mathrm{au}}{\mathrm{ly}}

\approx

268\,552\ \mathrm{au}

What it means: The nearest star is almost 270,000 au away — au numbers explode the moment you leave the Solar System, which is exactly why astronomers switch to light years and parsecs out there. For that scale, the Light Year Conversion calculator is the more natural tool.

3) How many Suns fit across Earth's orbit?

Background: You want a visual for classroom slides: how the size of the Sun compares with the scale of Earth's orbit.

Input: Enter 1 in the Astronomical units field and read the Solar radii field.

\frac{1\ \mathrm{au}}{\mathrm{R_\odot}}

=

\frac{149\,597\,870.7\ \mathrm{km}}{695\,700\ \mathrm{km}}

\approx

215\ \mathrm{R_\odot}

What it means: The Sun would fit about 215 times across Earth's orbital radius. Flipped around, the Sun's own radius is barely $0.00465\ \mathrm{au}$ — a striking reminder of how much empty space dominates the Solar System.

For everyday lengths on Earth — a room, a garden, a road trip — au is far too large a unit. The Length Converter handles millimetres to miles with the same instant, two-way conversion.

Common scenarios / when to use

Homework & exam problems

Physics and astronomy problems routinely give distances in au and ask for kilometres, miles, or light years. Converting here keeps every intermediate step consistent and lets you check your manual math.

Reading mission updates

Space-agency updates quote probe distances in au, kilometres, or both. When an article says a probe is “now 30 au from the Sun,” a quick conversion gives you the figure in miles or kilometres that your audience can picture.

Teaching planetary scale

The Solar System is mostly empty space, and au numbers make that visceral — the Sun fits ~215 times across Earth's orbit. Teachers and presenters use the converter to turn planet distances into ratios students can grasp.

This calculator works well when:

You are working with distances inside the Solar System or to nearby objects.
You want to compare the same distance across several units at once.
You need a quick, exact conversion without memorising conversion ratios.

It may not be what you need when:

You are converting ordinary Earth distances — centimetres to miles, say. The Length Converter is the better fit.
Your distances are between stars or galaxies — au becomes impractically huge. Reach for the Light Year Conversion calculator.
You need a planet's current distance — the converter translates whatever number you enter; it does not track moving planets.

Tips & best practices

Start from whichever unit you have
There is no “right” input. If a comet chart gives you kilometres, type kilometres; if a paper quotes au, type au. The converter figures out the direction for you, so you never need to remember whether to multiply or divide.
Think of au as exact, not “93 million miles”
The IAU value is exactly $149\,597\,870\,700\ \mathrm{m}$ , which is about $92\,955\,807\ \mathrm{mi}$ . For anything where numbers must line up — homework, writing, comparing sources — use the exact constant the converter uses.
Match the unit to the scale
Use au for planets and comets, light years for nearby stars, and parsecs or megaparsecs for galaxies. If a result looks absurd — like a star that seems to be a fraction of an au away — check the unit you entered before doubting the math.
Know which values are conventions
The au and the light year are exact definitions, but the solar radius ( $695\,700\ \mathrm{km}$ ) is a nominal reference value astronomy uses for convenience. Solar-radius results are only as precise as that convention.

Pairing the converter with the Light Year Conversion calculator gives you one consistent picture from the Sun out to the galaxies.

Calculation method / formula explanation

Every unit in the converter is defined by how many metres it equals. A distance is converted into metres first, then into the unit you asked for — so any two units can talk to each other without a separate formula for every pair.

The general conversion

L_t = L_s \times \frac{f_s}{f_t}

where $L_s$ is the value in the source unit, $L_t$ is the value in the target unit, $f_s$ is the metres per source unit, and $f_t$ is the metres per target unit.

The astronomical unit itself

1\ \mathrm{au}

=

149\,597\,870\,700\ \mathrm{m}

Fixed exactly by the IAU in 2012, this is the value every conversion in this calculator starts from.

The light year, parsec, and solar radius

1\ \mathrm{ly}

=

c \times 365.25 \times 86\,400

=

9\,460\,730\,472\,580\,800\ \mathrm{m}

1\ \mathrm{pc}

=

\frac{648\,000}{\pi}\ \mathrm{au}

1\ \mathrm{R_\odot}

=

695\,700\,000\ \mathrm{m}

A light year is the distance light travels in one Julian year ( $c = 299\,792\,458\ \mathrm{m/s}$ ). A parsec is the distance at which one au subtends one arcsecond of angle, and the solar radius is the IAU nominal value for the Sun's size.

Mega units for the far cosmos

1\ \mathrm{Mly}

=

10^6\ \mathrm{ly}

1\ \mathrm{Mpc}

=

10^6\ \mathrm{pc}

Mega light years and mega parsecs are the units astronomers use for galaxy surveys and cosmology — a million light years and a million parsecs respectively.

Variable guide

$L_s$ — numeric distance in the source unit
$L_t$ — numeric distance in the target unit
$f_s$ — metres per source unit
$f_t$ — metres per target unit
$c$ — speed of light, exactly $299\,792\,458\ \mathrm{m/s}$
$\mathrm{R_\odot}$ — solar radius, $695\,700\,000\ \mathrm{m}$ (nominal)

Related concepts / background info

Why the au was invented

The au grew out of the distance between Earth and the Sun, which for centuries was measured through solar parallax and refined with radar and spacecraft telemetry. Because Earth's orbit is slightly elliptical, the exact Earth–Sun distance changes through the year — so in 2012 the IAU stopped chasing the measurement and simply fixed the unit at $149\,597\,870\,700\ \mathrm{m}$ , giving every astronomer one stable number.

The scale ladder: au → ly → pc

Inside the Solar System, au keeps numbers friendly. Beyond it, astronomers switch to light years (one ly is about $63\,241\ \mathrm{au}$ ), and for stars measured by parallax they use parsecs (one pc is about $206\,265\ \mathrm{au}$ ). This converter includes all of them so you can move up and down the ladder in one place.

The au is also a clock

Because light travels at a fixed speed, an au is also a unit of light travel time: sunlight takes about $8\ \mathrm{min}\ 20\ \mathrm{s}$ to cover one au. When you see a spacecraft's distance quoted in au, that same number quietly tells you how many minutes its signals take to reach Earth.

For a closer look at the star-distance side of the ladder, see the Light Year Conversion calculator. For any everyday length, the Length Converter has you covered.

Frequently asked questions (FAQs)

How many au is Earth from the Sun?

Roughly one. Because Earth's orbit is a slight ellipse, the exact value swings a little through the year: about $0.983\ \mathrm{au}$ at perihelion (closest) and about $1.017\ \mathrm{au}$ at aphelion (farthest). On average, that is very close to 1 au — which is the whole point of the unit.

How do I convert au to kilometres by hand?

Multiply the number of au by $149\,597\,870.7$ . For example, $2\ \mathrm{au} \times 149\,597\,870.7 = 299\,195\,741.4\ \mathrm{km}$ . To go the other way, divide kilometres by that same factor.

Is the astronomical unit an SI unit?

No. The SI unit of length is the metre. The au is an astronomy-specific unit, now defined by the IAU as exactly $149\,597\,870\,700\ \mathrm{m}$ — precise, but still separate from the SI system. The converter keeps both, so you can move between them freely.

How far is Jupiter from the Sun in au?

Roughly $5.2\ \mathrm{au}$ — about 778 million kilometres. Typing 5.2 into the Astronomical units field shows the kilometres, miles, and light years equivalents all at once.

What is the difference between au, light years, and parsecs?

They are the same idea at different scales. An au suits the Solar System; a light year ( $\approx 63\,241\ \mathrm{au}$ ) suits stars; a parsec ( $\approx 206\,265\ \mathrm{au}$ , defined by parallax) suits distances measured geometrically. All three are included here, so you never need a second tool.

Can this calculator convert light years to au?

Yes. Type a value in the Light years field and read the Astronomical units field — the converter works in both directions. As a shortcut, remember $1\ \mathrm{ly} \approx 63\,241\ \mathrm{au}$ .

Why does 1 au show as such a tiny number in light years?

Because a light year is enormous compared with an au: $1\ \mathrm{au} \approx 1.58\times 10^{-5}\ \mathrm{ly}$ . Tiny or enormous results like this are shown in a compact form so they stay readable — it is a display choice, not a rounding error. Use the Light years field directly when you want to work at that scale.

Limitations & disclaimer

This is a unit conversion tool, not a source of measurement data. It converts the number you enter using exact IAU constants; it does not know where planets, comets, or spacecraft are at any given moment.
The au and light year are exact definitions, but the solar radius is a nominal reference value ( $695\,700\ \mathrm{km}$ ), so solar-radius results carry that convention's precision.
Distances must be non-negative. Some long decimal results are rounded for readability in the display; the underlying conversion stays consistent.
For academic publications or anything mission-critical, cross-check against authoritative sources such as the IAU, NIST, or NASA/JPL before relying on the numbers.

External references / sources

Astronomical unit — Wikipedia(IAU 2012 exact definition of the au)
NASA/JPL Solar System Dynamics — approximate planetary positions(for typical planet distances)

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