A light-year is the distance light travels in one year — the standard unit astronomers use because cosmic distances are far too large to write in kilometers. This converter turns light-years into kilometers and back, instantly and in both directions, for students, educators and anyone making sense of star and galaxy distances.
How it works
The conversion uses a single exact constant defined by the International Astronomical Union:
1 light-year = 9,460,730,472,580.8 km
That figure is the speed of light (299,792.458 km/s) multiplied by the seconds in a Julian year (365.25 days × 86,400 s). So:
- km = light-years × 9,460,730,472,580.8
- light-years = km ÷ 9,460,730,472,580.8
The tool computes the opposite field as you type, so editing either side updates the other immediately.
Reference distances
| Object | Light-years | Kilometers |
|---|---|---|
| Moon | 0.0000013 | 384,400 |
| Sun | 0.0000158 | 149,600,000 |
| Proxima Centauri | 4.24 | 4.01 × 10¹³ |
| Sirius (brightest star) | 8.6 | 8.13 × 10¹³ |
| Centre of Milky Way | ~26,000 | ~2.46 × 10¹⁷ |
| Andromeda Galaxy | ~2,537,000 | ~2.4 × 10¹⁹ |
These distances illustrate why km breaks down at stellar scales: writing 40 trillion km for the nearest star is unwieldy, while “4.24 light-years” is memorable and immediately relatable.
Why light-years vs parsecs?
Astronomers in professional research usually work in parsecs (1 pc ≈ 3.26 light-years), because the parsec is defined geometrically from stellar parallax — the apparent shift of a nearby star against the background as Earth orbits the Sun. Light-years are more common in popular science and education because the name makes the concept of vast distances feel graspable: if you can visualise how far light travels per second (299,792 km), then a light-year is just that distance multiplied out for a full year.
For example, when you look at Proxima Centauri you are seeing light that left it about 4.24 years ago. That delay — light-travel time — is built into the unit name in a way that “parsec” is not.
Making very large numbers readable
For distances beyond a few hundred light-years, scientific notation is clearest. The converter outputs values in standard decimal form, but for reference:
- 1,000 light-years = 9.46 × 10¹⁵ km
- 1,000,000 light-years = 9.46 × 10¹⁸ km (roughly the distance to the Large Magellanic Cloud)
The calculation runs entirely in your browser with no network calls.