How this calculator works
A light year is the distance light travels in a vacuum in one Julian year of 365.25 days. The speed of light is exactly 299,792,458 m/s, so one light year is exactly 9,460,730,472,580,800 m. The astronomical unit is exactly 149,597,870,700 m by IAU definition, and a parsec is 648,000 / pi astronomical units. The mile shown here is the international mile of exactly 1,609.344 m.
Formula
metres = value x F(unit). 1 ly = 299,792,458 x 365.25 x 86,400 = 9,460,730,472,580,800 m. 1 pc = 648,000 / pi AU = 3.0856775814913673e16 m. kilometres = metres / 1,000. miles = metres / 1,609.344. AU = metres / 149,597,870,700.
Worked example
1 light year = 9,460,730,472,580.8 km = 5,878,625,373,183.61 miles = 63,241.077 AU = 0.306601 pc. Proxima Centauri is about 4.2465 light years away, or about 1.3019 pc.
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Frequently asked questions
Is a light year a unit of time?
No. Despite the name, it is a distance: how far light travels in one year. A light year is about 9.46 trillion kilometres.
Why is a Julian year used?
The light year is defined with the Julian year of exactly 365.25 days so the value does not change with calendar reforms. This keeps the unit exact and reproducible.
How does a parsec compare with a light year?
One parsec is about 3.2616 light years. The parsec comes from parallax measurements, while the light year comes from the distance light travels, so they differ by the factor 648,000 / pi AU divided by one light year.