Light-Year Converter
Type a distance in any astronomical unit, from metres to megaparsecs, and see it in light-years, parsecs, astronomical units, kilometres, miles and the light-seconds, light-minutes, light-hours and light-days in between. You also get how long light takes to cross that distance and how long the trip would take at Voyager 1's speed, an airliner's or one you choose. Free, no sign-up, and nothing leaves your browser.
Your distance in other units
4.24650
Light-years
1.30198
Parsecs
268553
Astronomical units
4.01750 × 10¹³
Kilometres
2.49636 × 10¹³
Miles
4.24650 years
Light takes
Every unit
| Unit | Symbol | Value |
|---|---|---|
| Metres | m | 4.01750 × 10¹⁶ |
| Kilometres | km | 4.01750 × 10¹³ |
| Miles | mi | 2.49636 × 10¹³ |
| Astronomical units | AU | 268553 |
| Light-seconds | ls | 1.34009 × 10⁸ |
| Light-minutes | lmin | 2.23349 × 10⁶ |
| Light-hours | lh | 37224.8 |
| Light-days | ld | 1551.03 |
| Light-years | ly | 4.24650 |
| Parsecs | pc | 1.30198 |
| Kiloparsecs | kpc | 0.00130198 |
| Megaparsecs | Mpc | 1.30198 × 10⁻⁶ |
How long would it take to travel there?
74888.6 years
Travel time
The light-year is the distance light travels in a Julian year (365.25 days), exactly 9,460,730,472,580,800 m. The astronomical unit is exactly 149,597,870,700 m (IAU 2012) and the parsec exactly 648,000/π AU (IAU 2015).
How to Use the Light-Year Converter
- 1. Enter a distance and its unit. Type a number, including scientific notation such as 9.46e15, and choose the unit from the menu. The Start from an example menu loads the Moon, the Sun, the nearest star or the width of the Milky Way.
- 2. Choose how many digits. Results are written to 4, 6, 8 or 10 significant figures. Very large and very small values switch to scientific notation automatically.
- 3. Read the conversion. The headline boxes give light-years, parsecs, astronomical units, kilometres, miles and the time light takes; the table below lists all twelve units.
- 4. Estimate a journey. Pick a speed, the speed of light, Voyager 1, the Parker Solar Probe, an airliner or a walk, or type your own in km/s, km/h, mph or as a fraction of the speed of light, and read the travel time.
The Units and Their Definitions
| Unit | Exact definition | Size |
|---|---|---|
| Light-second | 299,792,458 m, the distance light travels in 1 s | 299,792 km |
| Astronomical unit (AU) | 149,597,870,700 m (IAU 2012) | 8 min 19 s of light |
| Light-year (ly) | 299,792,458 m/s × 31,557,600 s = 9,460,730,472,580,800 m | 63,241.1 AU |
| Parsec (pc) | 648,000 ÷ π AU (IAU 2015) | 3.26156 ly, 206,265 AU |
| Mile | 1,609.344 m | 1.60934 km |
The light-year is a distance, not a time: how far light goes in a Julian year of exactly 365.25 days. The astronomical unit began as the Earth–Sun distance and has been a fixed number of metres since 2012, so it no longer changes with new measurements. The parsec comes from parallax: a star one parsec away shifts by one arcsecond against the background as Earth moves one AU sideways, and the IAU defined it in 2015 as exactly 648,000/π AU. Professional astronomers mostly use parsecs, kiloparsecs and megaparsecs; popular writing prefers light-years.
Worked Examples
| Distance | Light-years | Parsecs | AU | Kilometres | Light takes |
|---|---|---|---|---|---|
| 1 AU | 1.58125 × 10⁻⁵ | 4.84814 × 10⁻⁶ | 1 | 1.49598 × 10⁸ | 8 min 19 s |
| Earth to the Moon, 384,400 km | 4.06311 × 10⁻⁸ | 1.24576 × 10⁻⁸ | 0.00256956 | 384400 | 1.28222 s |
| 1 light-year | 1 | 0.306601 | 63241.1 | 9.46073 × 10¹² | 1 year |
| Proxima Centauri, 4.2465 ly | 4.2465 | 1.30198 | 268553 | 4.01750 × 10¹³ | 4.2465 years |
| 1 parsec | 3.26156 | 1 | 206265 | 3.08568 × 10¹³ | 3.26156 years |
| 100,000 ly | 100000 | 30660.1 | 6.32411 × 10⁹ | 9.46073 × 10¹⁷ | 100000 years |
The Sun's light takes 8 min 19 s to reach Earth, so the Sun you see is the Sun as it was eight minutes ago. Moonlight takes 1.28 s. Proxima Centauri's light has been travelling for 4.2465 years, and light that crosses the Milky Way takes about a hundred thousand years.
How Long Would the Trip Take?
The speed box turns a distance into a journey. At Voyager 1's speed of about 17 km/s, relative to the Sun as NASA lists it, Proxima Centauri is about 74,900 years away. At the Parker Solar Probe's record speed of about 192 km/s, set in December 2024, it is still about 6,620 years. An airliner at 900 km/h would take over five million years, and walking at 5 km/h to the Moon, 384,400 km away, would take 8.77 years, against 17 days 19 h 6 min 40 s by airliner and 33 min 20 s at the Parker probe's speed.
Nothing outruns light
The speed box refuses speeds above the speed of light. A journey at exactly 1 c takes the light-travel time shown above; any real spacecraft is far slower, and at speeds close to c the traveller's own clock runs slower, an effect this converter does not include.
Which Unit for Which Distance
- Inside the solar system, use the AU, or light-minutes and light-hours for signal delays to spacecraft.
- Between nearby stars, use light-years in everyday speech and parsecs in papers: Proxima Centauri is 4.2465 ly or 1.30198 pc away.
- Across galaxies and the universe, use kiloparsecs and megaparsecs. The Milky Way is tens of kiloparsecs across; the distances in Hubble's law are in megaparsecs.
- On Earth, use metres, kilometres or miles. They are here so the huge numbers have something familiar beside them.
Frequently Asked Questions
- How many kilometres are in a light-year?
- One light-year is exactly 9,460,730,472,580.8 km, about 9.46 trillion kilometres. In metres it is 9,460,730,472,580,800 m, which is the speed of light, 299,792,458 m/s, multiplied by the 31,557,600 seconds in a Julian year.
- How many miles are in a light-year?
- About 5.88 trillion miles: 5,878,625,373,183.6 mi to be exact. Divide the kilometre figure by 1.609344, the number of kilometres in an international mile.
- How many astronomical units are in a light-year?
- About 63,241.1 AU. A light-year of 9,460,730,472,580,800 m divided by an AU of 149,597,870,700 m is 63,241.077. Put the other way round, one AU is 1.58125 × 10⁻⁵ light-years.
- How many light-years are in a parsec?
- One parsec is 3.26156 light-years, or 206,265 AU. It is defined as 648,000/π astronomical units, the distance at which one AU subtends an angle of one arcsecond.
- Is a light-year a unit of time?
- No, it is a unit of distance. It is named for time because it is how far light goes in a year. The year used is the Julian year of exactly 365.25 days, not the calendar or the tropical year.
- How long does light take to get from the Sun to Earth?
- About 8 minutes and 19 seconds, since the AU is 499.005 light-seconds. The time varies by a few seconds over the year because Earth's orbit is slightly elliptical, but the AU itself is a fixed average.