Convert between light-years, parsecs, AU, and km. Calculate light travel times and compare cosmic distances to known objects.
A light-year is the distance that light travels in one year through vacuum — approximately 9.461 trillion kilometers (5.879 trillion miles). Despite containing the word "year," it is a measure of distance, not time. It is one of the most commonly used units in astronomy for expressing the vast distances between stars, galaxies, and other cosmic structures.
Alongside light-years, astronomers use parsecs (1 pc ≈ 3.262 ly), astronomical units (1 AU ≈ Earth-Sun distance), and straightforward metric units depending on the scale of the measurement. Stellar distances within the Milky Way are typically expressed in light-years or parsecs, while solar system distances use AU.
This calculator converts between all major astronomical distance units, computes light travel times, estimates travel durations at custom speeds, and provides an interactive cosmic distance scale. Whether you are calculating how far a spacecraft would need to travel or simply converting between unfamiliar astronomical units, all the math is handled instantly with detailed breakdowns.
Converting between astronomical distance units by hand is error-prone due to the enormous numbers involved. This calculator handles all the conversions instantly, adds context with light travel times and cosmic object comparisons, and makes the scale of the universe tangible. The note above highlights common interpretation risks for this workflow. Use this guidance when comparing outputs across similar calculators. Keep this check aligned with your reporting standard.
1 light-year = 9.461 × 10¹² km = 63,241.1 AU = 0.3066 parsecs. Light travel time = distance / c, where c = 299,792.458 km/s. Travel time = distance / speed.
Result: 4.01 × 10¹³ km = 1.30 parsecs = 268,132 AU
Proxima Centauri at 4.24 light-years is about 40.1 trillion km. Light from there takes 4.24 years to reach Earth.
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One light-year is approximately 9.461 × 10¹² kilometers (about 9.461 trillion km) or 5.879 trillion miles.
Both are distance units. One parsec equals about 3.262 light-years (3.086 × 10¹³ km). Parsecs are preferred in professional astronomy because they relate directly to the parallax measurement method.
At Voyager 1 speed (~17 km/s), it would take about 17,600 years. At 10% the speed of light, it would take 10 years.
Light-years provide intuitive scaling for cosmic distances and directly tell you how old the light you see is — light from a star 100 light-years away left 100 years ago. Use the examples and notes as a quick consistency check before trusting any value.
The average distance from Earth to the Sun: approximately 149.6 million km (93 million miles). It is used primarily for solar system distances.
The observable universe has a radius of about 46.5 billion light-years. The most distant light we see (cosmic microwave background) has traveled about 13.8 billion years but the objects have since moved much farther away.