Convert between light-years and kilometers in either direction.
How it works
A light-year is the distance light travels in one Julian year, giving exactly 1 light-year = 9,460,730,472,580.8 km. Proxima Centauri, the nearest star to the Sun besides the Sun itself, sits about 4.24 light-years away — over 40 trillion km.
What this does not include
This is a pure distance conversion. It doesn’t account for the fact that looking at a distant object also means looking back in time — light from an object 4 light-years away shows how it looked 4 years ago, a separate astronomical concept from the distance conversion itself.
How to use this calculator
- Enter a value in light-years or kilometers.
- The other field updates automatically.
A worked example
4.24 light years (the distance to Proxima Centauri, our nearest star) converts to 40,113,497,203,743 km.
50,000,000,000,000 km converts to 5.285004 light years — the reverse calculation.
What the variables mean
| Variable | Meaning |
|---|---|
| Light years | Distance light travels in one year |
| Kilometers | The equivalent distance in kilometers |
Edge cases worth knowing
A light year measures distance, not time — a common point of confusion, since “year” appears in the name. It’s simply how far light travels in a year, used because interstellar distances in kilometers become unwieldy.
Missing both fields makes the conversion impossible, so the calculator declines to show a result without at least one known value.
Why use light-years instead of kilometers for astronomical distances?
Interstellar distances in kilometers become unwieldy, multi-trillion-digit numbers — light-years give a far more manageable scale for comparing distances between stars and galaxies.
Is a light-year a measure of time or distance?
Distance, despite the word “year” in its name — it’s simply defined as how far light travels in that amount of time, then used as a fixed distance unit.
How does a light-year compare to our solar system’s scale?
Enormously larger — Earth orbits about 150 million km from the Sun, while even the nearest star beyond our Sun is over 40 trillion km away, roughly 270,000 times further.
