Astronomical Distance Converter
Convert between kilometers, miles, astronomical units, light-years, and parsecs.
How these units are defined
An astronomical unit (AU) is the average distance from Earth to the Sun — a natural yardstick for distances within our solar system. A light-year is the distance light travels in one Julian year, used for distances between stars. A parsec, used mainly by astronomers, is defined from parallax angle measurements and is the largest of the three.
Common conversions
- 1 AU ≈ 149.6 million km — roughly the Earth-Sun distance
- 1 light-year ≈ 9.46 trillion km ≈ 63,241 AU
- 1 parsec ≈ 3.26 light-years ≈ 30.86 trillion km
- The nearest star system to our Sun, Alpha Centauri, is about 4.24 light-years away
The distances behind the factors
Kilometers are the base. One AU is 149,597,870.7 km, one light-year is 9,460,730,472,580.8 km (light-speed times a 365.25-day year), one parsec is 30,856,775,814,913.672 km and one mile is 1.609344 km. Results appear as you type, up to six decimals, with locale-based digit grouping. The default converts 1 light-year to km.
Worked examples
- 1 light-year = 9,460,730,472,580.8 km and 63,241.077084 AU.
- 1 parsec = 3.261564 light-years.
- 4.24 light-years = 268,142.166837 AU.
- 5.2 AU = 777,908,927.64 km.
- 384,400 km = 238,855.086296 miles.
- 100 miles = 160.9344 km.
Rounding and limits
Everything is cut off at six decimals of the target unit, so going from small to huge units loses the result: 1 km in light-years shows as 0, and 93 miles in AU shows 0.000001. Pick a smaller target unit for short distances. Large answers like 1 parsec in km are shown in full, and negative values are converted as typed. There is no scientific notation and no conversion into meters or other small units.
Distances to compare
Ten parsecs is 32.615638 light-years, and one light-year is 63,241.077084 AU, which is why astronomers switch from AU to light-years once they leave the solar system. An object 40 AU away is 5,983,914,828 km from the Sun. The usual figure for the Sun's distance, 93,000,000 miles, is 1.000475 AU, and something 225,000,000 km away is 1.504032 AU.
Light-years and parsecs measure distance, not time, even though the name of one includes a year. Results keep up to six decimal places.
Frequently asked questions
Why do astronomers use parsecs instead of light-years?
A parsec comes directly from how astronomers actually measure stellar distances — via parallax, the apparent shift in a star's position as Earth orbits the Sun — making it a more natural unit for their measurement method, even though light-years are more intuitive for general audiences.
Is a light-year a unit of time or distance?
Distance, despite containing the word 'year'. It's the distance light travels in one year, used as a convenient distance unit precisely because interstellar distances are so vast that kilometers become an unwieldy number of digits.
How many AU are in a light-year?
63,241.077084. Choose light-years as the source and astronomical units as the target.
Why is 1 km shown as 0 light-years?
A kilometer is about 1.06 × 10⁻¹³ light-years, far below the six decimals the tool displays. Convert to a smaller unit such as miles or kilometers instead.
How far is 4.24 light-years in kilometers?
Choose light-years as the source and km as the target. The tool multiplies 4.24 by 9,460,730,472,580.8, and shows the full figure with digit grouping.