Convert astronomical distances between light-years, light-seconds, astronomical units, parsecs, kilometers and miles.
Astronomy • 7 units
All 7 units on the Light Year Converter are defined against the Meter (m), so each result is one conversion factor away from a single reference rather than the end of a chain of roundings.
The conversion this page is most often opened for is ready before you type anything: 1 ly = 9,460,730,470,000 km. Change either side and every row in the table recalculates with it.
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Astronomical distances are too vast for everyday units, so astronomers use the light-year (the distance light travels in one year, ~9.46 trillion km), the astronomical unit (AU, the average Earth–Sun distance) and the parsec (~3.26 light-years). This converter moves between them and familiar kilometers and miles.
It is handy for students, educators and space enthusiasts making sense of cosmic scales — for example, the nearest star, Proxima Centauri, is 4.24 light-years (1.30 parsecs) away.
The smaller light units on this page are the ones with everyday reference points. Light covers just under 300,000 km in a second, so the Moon is about 1.28 light-seconds away and the Sun about 8.32 light-minutes — meaning sunlight leaving now arrives after you have finished reading this paragraph. Those figures explain why round-trip radio delay, not bandwidth, dominates the design of deep-space communication.
The astronomical unit stopped being a measurement in 2012, when it was fixed by definition at exactly 149,597,870,700 metres rather than being tied to Earth's orbit. That matters because the parsec is derived from it: one parsec is the distance at which one astronomical unit subtends one arcsecond, which works out to 206,265 AU and about 3.086 × 10¹⁶ metres. Both units are therefore exact by construction, and a light-year is roughly 63,241 AU.
Distances quoted in light-years carry a second meaning that no conversion can express. Because light takes time to arrive, a galaxy reported at ten million light-years is also being seen as it was ten million years ago. For nearby objects the two readings are interchangeable; at cosmological scales they part company entirely, since the universe has expanded while the light was in transit and the object's present distance exceeds its light-travel time. That is why professional catalogues quote redshift rather than a distance in these units.
Quick reference — 1 Light-year (ly) is equal to:
| Meter | m | 9.460730e+15 |
| Kilometer | km | 9,460,730,470,000 |
| Mile | mi | 5,878,625,370,000 |
| Astronomical unit | AU | 63,241.077100 |
| Light-second | ls | 31,557,600 |
| Parsec | pc | 0.306601394 |
7 units of light year, each a fixed multiple of the m. The table spans 30,856,775,814,913,672:1, from m (1 m) to pc (3.0857e+16 m). Conversion is one multiplication into the base unit and one division out of it, with no lookup table and no approximation.
value_to = value_from × (factor_from ÷ factor_to)1 m = 1 m1 km = 1000 m1 mi = 1609.34 m1 AU = 149598000000 m1 ls = 299792000 m1 ly = 9.4607e+15 m1 pc = 3.0857e+16 mwhere:
Assumptions: Factors are exact definitions. Full double precision is carried internally and rounding is applied only for display.
Result25 ly = 236518000000000 km
A light-year is the distance light travels in one Julian year: 299,792,458 m/s × 365.25 days, which works out to about 9.4607 × 10¹⁵ metres. The name misleads often enough that it is worth stating plainly — it is a unit of length, and a large one, at roughly 63,241 astronomical units.
Professional astronomy actually prefers the parsec, about 3.26 light-years, because it falls straight out of parallax measurement: an object one parsec away shifts by one arcsecond as Earth crosses its orbit. The light-year survives in public writing because it carries an intuition the parsec does not — the distance to a star is also how far into the past you are looking when you see it.