Light Year Distance Calculator
Convert between light years, kilometers, miles, and astronomical units (AU).
Works in both directions.
A light-year is the distance light travels through a vacuum in exactly one Julian year (365.25 days). It is a unit of distance, not time — a common source of confusion.
The calculation: Speed of light: c = 299,792,458 meters per second (exactly) Seconds in a year: 365.25 × 24 × 60 × 60 = 31,557,600 seconds
1 light-year = 299,792,458 × 31,557,600 = 9.461 × 10¹⁵ meters = approximately 9.461 trillion kilometers (5.879 trillion miles)
Converting light-years to other units:
- 1 light-year = 9.461 × 10¹² km
- 1 light-year = 63,241 AU (astronomical units)
- 1 light-year = 0.3066 parsecs
- 1 parsec = 3.262 light-years
Light-travel time formula: Time = Distance (light-years) × 1 year
This means when you observe a star 100 light-years away, you are seeing it as it was 100 years ago. You are looking into the past.
Cosmic distance reference:
| Object | Distance |
|---|---|
| Moon | 1.28 light-seconds |
| Sun | 8.3 light-minutes |
| Nearest star (Proxima Centauri) | 4.24 light-years |
| Galactic center (Milky Way) | ~26,000 light-years |
| Andromeda Galaxy | ~2.537 million light-years |
| Edge of observable universe | ~46.5 billion light-years |
Why not use kilometers for cosmic distances? The nearest star is about 40,000,000,000,000 km away. Light-years (4.24 ly) are simply more practical — the same reason we use miles instead of inches for road distances.