Light Year Calculator
Turn cosmic distances into numbers you can feel: light-years to kilometers, miles, AU and parsecs, and how long the trip would take.
By Manzoor Ahmad, pharmacist · Updated How far is a light-year?
One light-year is exactly 9,460,730,472,580.8 km (about 9.46 trillion km or 5.88 trillion miles), the distance light travels in a Julian year. It equals 63,241 AU or 0.3066 parsecs. The nearest star, Proxima Centauri, is 4.2465 light-years away: 40.2 trillion km, or about 75,000 years at Voyager 1’s speed.
Light-year conversion table
| 1 light-year equals | Value | Definition |
|---|---|---|
| Kilometers | 9,460,730,472,580.8 km | c × 365.25 days (exact) |
| Miles | 5,878,625,373,184 mi | ÷ 1.609344 |
| Astronomical units | 63,241.08 AU | 1 AU = 149,597,870.7 km (exact) |
| Parsecs | 0.306601 pc | 1 pc = 3.26156 ly |
| Light-seconds | 31,557,600 ls | seconds in a Julian year |
Distances to famous objects
| Object | Distance | Light takes |
|---|
Some stellar distances (Polaris especially) carry real uncertainty. For the speed needed to leave a planet in the first place, use the escape velocity calculator.
Light-year FAQs
A light-year is the distance light travels in a vacuum in one Julian year (365.25 days): exactly 9,460,730,472,580.8 km, about 9.46 trillion km or 5.88 trillion miles. It is a unit of distance, not time.
9.4607 × 10¹² km (9,460,730,472,580.8 km exactly).
About 5.879 × 10¹² miles, or 5.88 trillion miles.
About 63,241 astronomical units. One AU, the average Earth–Sun distance, is defined as exactly 149,597,870.7 km.
A parsec is the distance at which 1 AU subtends an angle of one arcsecond, equal to 3.2616 light-years or 206,265 AU. Astronomers prefer parsecs because they come directly from parallax measurements.
Proxima Centauri is about 4.2465 light-years away, or 40.2 trillion km. Its light takes 4.25 years to reach us.
At Voyager 1's speed of about 17 km/s, roughly 17,600 years. A car at 100 km/h would need about 10.8 million years.
About 8 minutes 19 seconds on average (499 light-seconds). Moonlight takes about 1.28 seconds.
About 2.5 million light-years. The light we see from it today left before modern humans existed.
No, it measures distance. It does, however, tell you the look-back time: an object 100 light-years away is seen as it was 100 years ago.
The Milky Way's stellar disk is roughly 100,000 light-years across, and the Sun sits about 26,000 light-years from the galactic center.
NASA's Parker Solar Probe reached about 192 km/s (around 690,000 km/h) at its closest approach to the Sun, the fastest of any human-made object. Even at that speed, Proxima Centauri would be about 6,600 years away.
Sources
Travel times assume constant speed in a straight line and ignore acceleration, gravity assists and relativity. Distances to stars and galaxies are current best estimates.