Light Years Calculator
A light year is a distance, not a time: it is how far light travels through vacuum in one year. That makes it a convenient yardstick for the gap between stars, and it converts cleanly into parsecs and parallax angles.
Trip time in years
21.2
- Distance in parsecs
- 1.299991415
- Parallax angle in arcseconds
- 0.7692358491
- Distance in light minutes
- 2,230,070.4
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
The formula this calculator uses
d = c t
- d
- Distance, in light years or in metres
- c
- Speed of light in vacuum, 299792.458 kilometres per second
- t
- Time for light to cover the distance
- pc
- Parsecs, where one parsec is 3.26156 light years
How to check the result by hand
- 1
Be clear that the unit measures distance
A light year is a length, not a duration. The phrase tells you how long light would take to cross the gap, but the quantity itself is measured in kilometres, and treating it as a time produces nonsense.
- 2
Use the light year constant
One light year is 9460730472580.8 kilometres, or 9.4607304725808 trillion kilometres. Every conversion from light years to kilometres multiplies by that figure.
- 3
Divide by 3.26156 for parsecs
Astronomers usually quote distances in parsecs. A 4.24 light year gap to Proxima Centauri is 1.2999914 parsecs, and dividing one by that gives the parallax in arcseconds.
- 4
Take the reciprocal for the parallax angle
A distance of 1.2999914 parsecs corresponds to a parallax of 0.7692358 arcseconds. This reciprocal relationship is what makes the parallax method so direct.
- 5
Divide by the speed fraction for travel time
At 20 percent of light speed the trip to Proxima Centauri takes 4.24 divided by 0.2, which is 21.2 years. The result is in years because a light year is exactly one year of light travel.
For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Light Years page.