Astronomical Distance Converter

Convert between astronomical units, light-years, parsecs, kilometres, and miles — for the distances of space, too vast for everyday units.

Astronomical Distance

AU, light-years, parsecs

From

Result

1 km = 6.684587e-9 AU

Popular conversions

What Is an Astronomical Distance Converter?

An astronomical distance converter translates the immense distances of space between units — astronomical units to light-years, parsecs to kilometres. Everyday units like kilometres become unwieldy at cosmic scale, so astronomers use the astronomical unit (AU, the Earth–Sun distance), the light-year (the distance light travels in a year), and the parsec (a parallax-based unit). Each suits a different range, from the Solar System to distant galaxies.

This converter routes through the metre using exact and defined values. One AU is exactly 149,597,870,700 m; a light-year is about 9.46 trillion km; a parsec is about 3.26 light-years. The tool also bridges back to kilometres and miles, so a cosmic distance can be grasped in familiar terms as well as astronomical ones.

This is one category of the full Unit Converter — pair it with our percentage calculator or scientific calculator for related everyday maths.

How Astronomical Distance Conversion Works

Everything routes through the metre

Each unit has a fixed metre value. The converter normalises to metres, then projects to AU, light-years, parsecs, km, and miles.

The AU is Solar-System scale

One astronomical unit is the average Earth–Sun distance (~150 million km). Planet distances are naturally expressed in AU.

The light-year is a distance

Despite 'year' in the name, a light-year is a distance — how far light travels in a year (~9.46 trillion km), not a time.

The parsec ties to parallax

A parsec is the distance at which 1 AU subtends one arcsecond — about 3.26 light-years. It comes straight from how astronomers measure distance.

Core Astronomical Distance Factors

Multiply to reach metres; divide to come back. These units span the Solar System to the cosmos.

AU → km

× 149,597,870.7

One astronomical unit is 149,597,870.7 km — the defined Earth–Sun distance.

Light-year → km

× 9.461 × 10¹²

One light-year is about 9.461 trillion km, the distance light travels in a year.

Parsec → light-year

× 3.26156

One parsec is about 3.262 light-years, derived from arcsecond parallax.

How to Use the Astronomical Distance Converter

1
Enter the distance
Type the cosmic distance you want to convert — a planet's orbit, a star's distance, a galaxy's span.
2
Choose the 'from' unit
Pick AU, light-years, parsecs, kilometres, or miles as your starting unit.
3
Choose the 'to' unit
Select the target unit, or swap the two to reverse direction.
4
Read every unit at once
The all-units table shows the distance in AU, light-years, parsecs, and km together for scale comparisons.

Key Astronomical Distance Concepts

Astronomical unit (AU)

The defined Earth–Sun distance, exactly 149,597,870,700 m. The natural unit for distances within the Solar System.

Light-year

The distance light travels in one year (~9.46 trillion km). A unit of distance, not time — the nearest star is 4.24 light-years away.

Parsec

About 3.26 light-years; the distance at which one AU subtends one arcsecond of parallax. The professional astronomer's distance unit.

Look-back time

Because light takes time to arrive, a distance in light-years also tells you how far into the past you're seeing that object.

Real-World Astronomical Conversions

🌍

Earth to Sun

1 AU = 149.6 million km = 8.3 light-minutes. Sunlight takes about eight minutes to reach us.

🪐

Planet distances

Neptune orbits at ~30 AU (4.5 billion km). Expressing it in AU keeps Solar-System distances manageable.

⭐

Nearest star

Proxima Centauri is 4.24 light-years (1.30 parsecs) away — about 40 trillion km. Stellar distances demand light-years.

🌌

Galaxy scale

The Milky Way is ~100,000 light-years across; Andromeda is 2.5 million light-years away. Galactic distances dwarf the AU.

🔭

Parallax measurement

A star at 10 parsecs (32.6 light-years) shows 0.1 arcsecond of parallax. The parsec comes directly from this method.

🚀

Spacecraft range

Voyager 1 is over 160 AU from the Sun — about 0.0025 light-years. Probe distances are tracked in AU.

Best Practices for Astronomical Distance Conversion

✓Use AU within the Solar System. Planet and probe distances are cleanest in astronomical units. Reserve light-years and parsecs for stars and beyond.
✓Remember a light-year is a distance. It measures how far, not how long. Treating it as a time is the most common astronomical-unit misconception.
✓Use parsecs for professional work. Astronomers prefer parsecs because they come directly from parallax measurement. Popular science tends to use light-years.
✓Tie distance to look-back time. An object N light-years away is seen as it was N years ago — a useful intuition the conversion makes available.
✓Keep scientific notation handy. Cosmic distances in km are enormous. Expressing them in AU, light-years, or parsecs keeps numbers readable.

Common Astronomical Distance Mistakes

Treating a light-year as time

A light-year is a distance — how far light travels in a year. Using it as a duration is the classic confusion.

Confusing parsecs and light-years

A parsec is ~3.26 light-years, not equal to one. Swapping them misjudges stellar distances by that factor.

Using km for stellar distances

Kilometres become trillions and quadrillions for stars. AU, light-years, and parsecs exist precisely to tame those numbers.

Forgetting the AU is Earth–Sun

The AU is specifically the Earth–Sun distance, not a generic unit. It anchors Solar-System scale and nothing larger.

Why Astronomical Distance Conversion Matters

Space is so vast that everyday units fail: the distance to the nearest star in kilometres is a 14-digit number. Astronomers built the AU, light-year, and parsec to keep cosmic distances comprehensible, each matched to a scale — the Solar System, nearby stars, or the galaxy and beyond. Converting between them, and back to kilometres, is how those scales connect.

The light-year also carries a profound idea: because light travels at a finite speed, a distance in light-years is also a window into the past. A converter that routes through the metre with exact AU and light-year values lets students, educators, and enthusiasts move fluidly between human and cosmic scales — and appreciate just how far away everything really is.

Built for astronomy students, educators, science writers, and enthusiasts converting between AU, light-years, parsecs, and kilometres.

Linear unit factors follow the BIPM SI brochure, the NIST Guide to the SI, and ISO 80000. Currency rates load live from open.er-api.com; crypto prices from CoinGecko. See our methodology and editorial policy. Educational only — not certified for regulated trading, settlement, medical, or aerospace use.

Astronomical Distance Converter FAQs

Distance. A light-year is how far light travels in one year in a vacuum — about 9.46 trillion kilometres (9.461 × 10¹² km). The 'year' refers to the travel time of light, but the unit itself measures distance. The nearest star, Proxima Centauri, is 4.24 light-years away.

One astronomical unit (AU) is exactly 149,597,870,700 metres — about 149.6 million kilometres or 93 million miles — defined as the average distance between the Earth and the Sun. Light covers this distance in about 8.3 minutes, which is why we say sunlight is roughly eight minutes old.

A parsec is about 3.26 light-years (3.086 × 10¹³ km), defined as the distance at which one astronomical unit subtends an angle of one arcsecond. It comes directly from the parallax method astronomers use to measure stellar distances, which is why professionals favour parsecs over light-years.

Divide light-years by 3.26156 to get parsecs, or multiply parsecs by 3.26156 to get light-years. So 10 light-years is about 3.07 parsecs, and the Andromeda galaxy at 2.5 million light-years is about 767,000 parsecs (767 kiloparsecs) away.

Because the numbers become unmanageable — the nearest star is about 40 trillion kilometres away, a 14-digit figure. Units like the light-year and parsec compress these distances into readable values and match the scale of the objects, just as we use kilometres rather than millimetres for road trips.

It uses the exact defined astronomical unit (149,597,870,700 m) and standard values for the light-year (9.4607 × 10¹⁵ m) and parsec (3.0857 × 10¹⁶ m), routing every conversion through the metre at full precision, so results are exact to your input precision.