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.
Example
The distance to Proxima Centauri, 4.24 light-years:
4.24 × 9,460,730,472,580.8 ≈ 4.01 × 10¹³ km (about 40 trillion kilometers)
| Light-years | Kilometers |
|---|---|
| 0.5 | 4.730 × 10¹² |
| 1 | 9.4607 × 10¹² |
| 4.24 (Proxima Centauri) | 4.011 × 10¹³ |
| 100 | 9.4607 × 10¹⁴ |
The calculation is done entirely in your browser with no network calls.