Light-Year Conversion Calculator
Enter any distance and choose your starting unit. The calculator converts instantly to all other major astronomical and metric units, shows you the step-by-step math, and gives you real-world context so the number actually means something.
What is a light-year?
A light-year (ly) is the distance light travels through a vacuum in exactly one Julian year (365.25 days). Because the speed of light in a vacuum is exactly 299,792,458 meters per second by definition, one light-year works out to precisely 9,460,730,472,580,800 meters, or about 9.46 trillion kilometers (5.88 trillion miles). The unit is defined by the International Astronomical Union (IAU) and is the standard way astronomers describe distances beyond our solar system, where meters and kilometers become hopelessly unwieldy numbers.
How the conversion works
Every unit conversion here follows the same two-step process: first multiply your input by the number of meters per input-unit to get the distance in meters (the SI base unit), then divide by the number of meters per output-unit. For example, converting 1 light-year to kilometers: 1 x 9.4607304725808e+15 m/ly = 9.4607304725808e+15 m, then 9.4607304725808e+15 m / 1000 m/km = 9,460,730,472,580.8 km. All the conversion factors in this calculator use the IAU 2012 resolution values for the light-year and Astronomical Unit, and the IAU 2015 nominal solar values.
Light-years vs. parsecs vs. Astronomical Units
Three units dominate astronomical measurement. The Astronomical Unit (AU) is the average Earth-Sun distance, about 149,597,870.7 km. It is most useful for describing distances within our solar system. The light-year is the distance light travels in a year, mainly used for nearby stars and galaxies in popular science writing. The parsec (pc) is the professional standard for stellar and galactic distances: 1 parsec equals about 3.26 light-years, or 206,265 AU. Parsecs arise naturally from the parallax method of measuring stellar distances, which is why professional catalogs use them. One megaparsec (Mpc) is a million parsecs, common for measuring intergalactic distances.
Real-world scale: putting light-years in context
Because light-years represent enormous distances, it helps to build intuition with comparisons. The Sun is only 8.3 light-minutes away (not even a fraction of a light-year). Proxima Centauri, the nearest star, is 4.24 light-years out. The entire Milky Way disk is roughly 100,000 light-years across. The nearest large galaxy, Andromeda, sits 2.537 million light-years away. The most distant objects ever observed are about 13.4 billion light-years away, so close to the Big Bang that we see them as they were when the universe was less than 400 million years old.
Famous Astronomical Distances
| Object | Distance (ly) | Distance (km) | Distance (parsecs) | Notes |
|---|---|---|---|---|
| Moon (average) | 0.00000125 ly | 384,400 km | 3.84e-11 pc | Closest large natural body |
| Sun | 0.0000158 ly | 149,598,000 km | 4.85e-9 pc | ~8.3 light-minutes |
| Mars (avg) | 0.000024 ly | 225,000,000 km | 7.3e-9 pc | Varies with orbit |
| Pluto (avg) | 0.00063 ly | 5,900,000,000 km | 1.9e-7 pc | ~5.5 light-hours |
| Proxima Centauri | 4.24 ly | 4.01e+13 km | 1.30 pc | Nearest star to the Sun |
| Alpha Centauri | 4.37 ly | 4.13e+13 km | 1.34 pc | Triple star system |
| Sirius | 8.60 ly | 8.13e+13 km | 2.64 pc | Brightest star in night sky |
| Vega | 25 ly | 2.36e+14 km | 7.67 pc | Reference star for magnitude scale |
| Fomalhaut | 25 ly | 2.36e+14 km | 7.67 pc | Has a confirmed debris disk |
| Orion Nebula (M42) | 1,344 ly | 1.27e+16 km | 412 pc | Stellar nursery in Orion |
| Center of Milky Way | 26,000 ly | 2.46e+17 km | 7,970 pc | Sagittarius A* black hole |
| Milky Way diameter | 100,000 ly | 9.46e+17 km | 30,675 pc | Estimated disk diameter |
| Large Magellanic Cloud | 160,000 ly | 1.51e+18 km | 49,000 pc | Satellite galaxy of Milky Way |
| Andromeda Galaxy (M31) | 2,537,000 ly | 2.40e+19 km | 0.778 Mpc | Nearest major galaxy |
| Observable universe radius | 46.5e+9 ly | 4.40e+23 km | 14.25 Gpc | Cosmic light horizon |
Reference distances for well-known objects in the universe, measured in light-years, AU, parsecs, and km.
Frequently asked questions
How many kilometers is one light-year?
One light-year is exactly 9,460,730,472,580.8 kilometers (about 9.46 trillion km). This follows from the definition of the speed of light (299,792,458 m/s) multiplied by the number of seconds in a Julian year (365.25 x 24 x 60 x 60 = 31,557,600 s).
How many miles is one light-year?
One light-year is approximately 5,878,625,373,183.6 miles (about 5.88 trillion miles). This is derived by dividing the kilometer value (9,460,730,472,580.8 km) by 1.609344 km/mile.
What is the difference between a light-year and a parsec?
A light-year is the distance light travels in one year (about 9.46 trillion km). A parsec (pc) is defined geometrically as the distance at which 1 AU subtends an angle of exactly 1 arcsecond, and equals 3.085677581491 x 10^16 meters, or about 3.2616 light-years. Parsecs are the standard unit for professional stellar and galactic distances because they arise directly from the parallax measurement method.
How many Astronomical Units (AU) are in one light-year?
One light-year equals about 63,241 Astronomical Units (AU). The IAU defines 1 AU as exactly 149,597,870,700 meters (the average Earth-Sun distance). Dividing the meters-per-light-year (9.4607304725808e+15) by 149,597,870,700 gives approximately 63,241 AU.
Is a light-year a unit of time or distance?
A light-year is a unit of distance, not time, despite having "year" in its name. The name reflects how it is defined: the distance light travels in one year. When astronomers say an object is 100 light-years away, they mean the object is 9.46e+14 kilometers distant, not that it takes 100 years to reach it at any ordinary speed.
How far is Proxima Centauri in light-years and kilometers?
Proxima Centauri, the nearest known star to the Sun, is about 4.2441 light-years away. In other units that is roughly 40.1 trillion kilometers (4.01e+13 km), 1.30 parsecs, or about 268,500 Astronomical Units.