What is a light year?

A light year is the distance light travels in one year, about 9.46 trillion kilometers or 5.88 trillion miles. Despite its name, it measures distance, not time. Distances in space are far too large for kilometers, so astronomers use three units of their own:

  • The light year (ly) is exactly 9,460,730,472,580.8 km: the distance light travels at 299,792,458 meters per second in a Julian year of 365.25 days.
  • The astronomical unit (au) is exactly 149,597,870.7 km, about the average distance from the Earth to the Sun. It suits distances within the Solar System.
  • The parsec (pc) is about 3.26 light years, or 206,265 au: the distance at which one astronomical unit spans an angle of one arcsecond. The distances of stars are often given in parsecs, as they're measured from the stars' parallax.

Tool description

This tool converts distances between light years, kilometers, miles, astronomical units and parsecs. Every light year converter here works the same way, from the general Light Year Distance Calculator to the ones for a single pair, such as light years to kilometers or parsecs to light years: it starts on the units in its name, and the Source and Target menus switch to any of the five units. Type a number and the result appears as you type, ready to copy. The swap button turns the conversion around.

Examples

Distance Converted
1 ly 9460730472580.8 km
1 ly 5878625373183.61 mi
1 ly 63241.0770842663 au
1 pc 3.26156377716743 ly
1 au 149597870.7 km
4.2465 ly, to Proxima Centauri, the nearest star 40174991951814.4 km
2500000 ly, to the Andromeda Galaxy 766503.484463876 pc

How it works

Each unit converts through meters, by its exact definition: the astronomical unit as the International Astronomical Union (IAU) fixed it in 2012, the light year from the speed of light and the Julian year, and the parsec as 648,000/π astronomical units, as the IAU defined it in 2015. Results keep 15 significant digits. From 10²¹ on they're written in exponent notation, such as 9.4607304725808e+22 km for 10 billion light years, and so are results below 0.000001, such as 1.05700083402462e-13 ly for 1 km. Everything runs in your browser.

Tips

  • Sunlight takes about 8 minutes and 19 seconds to reach the Earth, so 1 au is about 8.3 light minutes.