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Light Years to Km Converter

Use this free light years to km converter to turn astronomical distances into kilometres, miles, astronomical units (AU) or parsecs — and back again. One light year is 9,460,730,472,580.8 km, the distance light travels in a year, and this converter handles the huge numbers for you, so you can see at a glance how far away the nearest stars and galaxies really are.

How light years to km conversion works

A light year is defined by the speed of light. Light travels through a vacuum at exactly 299,792,458 metres per second, and a year — the Julian year astronomers use for this definition — is 365.25 days, or 31,557,600 seconds. Multiply the two and you get the distance light covers in a year: 299,792,458 × 31,557,600 = 9,460,730,472,580,800 metres, which is 9,460,730,472,580.8 km. That is 9.46 trillion kilometres, or about 5.88 trillion miles.

The converter uses that exact figure, so every other unit follows from it. One light year equals 63,241.1 astronomical units, because one AU — the average distance from the Earth to the Sun — is 149,597,870.7 km. It equals 0.306601 parsecs, because one parsec is 3.26156 light years, or about 30.86 trillion km. To convert the other way, divide instead of multiplying: kilometres divided by 9,460,730,472,580.8 gives light years, and parsecs multiplied by 3.26156 gives light years.

Despite the name, a light year is a unit of distance, not time. The word "year" describes how long the light has been travelling, which is exactly what makes the unit so useful: a star 4.24 light years away is seen as it was 4.24 years ago, because its light left the star that long ago. Looking at distant galaxies is literally looking into the past, and the light year is the unit that makes that idea concrete.

Distances that show the scale

The Solar System is small in light years. The Sun is 8 light minutes from Earth — 0.0000158 light years — and Neptune, the farthest planet, is about 4 light hours away, or 0.0005 light years. The Voyager 1 spacecraft, the farthest human-made object, is roughly 15 billion miles from the Sun, which is still only about 0.0026 light years. On the scale of the galaxy, our whole Solar System is a speck.

The nearest star, Proxima Centauri, is 4.24 light years away — about 40.1 trillion km. Sirius, the brightest star in the night sky, is 8.6 light years away. The centre of the Milky Way is about 26,000 light years from us, and the galaxy itself is roughly 100,000 light years across. The Andromeda galaxy, the nearest large galaxy, is 2.5 million light years away, and the farthest galaxies telescopes can see are more than 13 billion light years away — their light set out when the universe was young.

Worked example

Convert 1 light year to kilometres, and find the distance to Proxima Centauri in kilometres and parsecs.

Step 1: One light year is the speed of light times one year — 299,792,458 m/s × 31,557,600 s = 9,460,730,472,580,800 metres.
Step 2: Divide by 1,000 to get kilometres: 9,460,730,472,580.8 km.
Step 3: In miles, divide by 1.609344 — 9,460,730,472,580.8 ÷ 1.609344 = 5,878,625,373,183.6 miles.
Step 4: Proxima Centauri is 4.24 light years away, so its distance is 4.24 × 9,460,730,472,580.8 = 40,113,497,203,742 km (about 40.1 trillion km).
Step 5: In parsecs, 4.24 ÷ 3.26156 = 1.30 parsecs — the same distance in the unit professional astronomers use most.

For a sense of how slow even light is on this scale: a spacecraft travelling at 70 km per second, the fastest any human-made object has flown, would take 9,460,730,472,580.8 ÷ 70 = 135 billion seconds to cover one light year — about 4,300 years. That is why interstellar travel is a problem of physics, not just engineering.

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