Light Year Distance Calculator
Convert astronomical distances between light-years, km, miles, AU, and parsecs.
Includes travel time at various speeds and known star distances for scale.
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.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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