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Light-years to Kilometres Converter

Enter a value in light-years to convert it to kilometres, astronomical units.

How it is calculated

This is a straight multiplication by two fixed factors. Kilometres = ly × 9,460,730,472,580.8, the distance light covers in one Julian year of 365.25 days at 299,792,458 m/s. Astronomical units = ly × 63,241.077, because one AU is about 149.6 million km. The kilometre answer is always printed in scientific notation with four decimals, for example 9.4607 × 10¹² km, where the raised 12 is the power of ten; the AU answer is printed with two decimals and thousands separators. There are no unit options, and a value of 0 gives 0. Fractions of a light-year are allowed, so 0.5 or 4.24 both work. Only the digits shown in the output are rounded; the multiplication itself is not.

Worked example

Convert 4.24 light-years, roughly the distance to the nearest neighbouring star system. Kilometres = 4.24 × 9,460,730,472,580.8 ≈ 40.1 trillion km, which the tool prints as 4.0113 × 10¹³ km. Astronomical units = 4.24 × 63,241.077 = 268,142.17 AU. So that star system lies more than 268,000 times farther from us than the Sun does.

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Common mistakes

  • Reading the output as if it were light-years of time. A light-year is a distance, so the km result is a length, not a number of years of travel at ordinary speeds.
  • Misreading the scientific notation. 4.0113 × 10¹³ is 40,113,000,000,000, not 4.0113 or 4.0113 × 13; the raised number after 10 is the power of ten.
  • Entering parsecs instead of light-years. One parsec is about 3.26 light-years, so a value typed in parsecs gives a result roughly 3 times too small.
  • Rounding the result back to a few digits and then using it for a precise calculation. The output keeps only five significant figures, so keep the original light-year value for further work.

About this calculator

A light-year is a unit of distance, not time: the distance light travels through a vacuum in one Julian year, equal to exactly 9,460,730,472,580.8 km (roughly 9.461 × 10¹² km), based on the defined speed of light (299,792,458 m/s) and a 365.25-day year. Because interstellar distances are unmanageably large in kilometres, astronomers use the light-year (or the closely related parsec) as a more intuitive scale — for example, the nearest star system, Alpha Centauri, is about 4.24 light-years away.

Astronomers, planetarium educators, and science communicators convert light-years to kilometres when a lay audience needs the raw distance made concrete, since "4.24 light-years" is a meaningful comparison to other stars but the roughly 40 trillion km behind it is what conveys the actual scale.

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Comments (1)

  • Editorial team

    To feel the scale, divide one light-year by the speed of Voyager 1, about 17 kilometres a second: the answer comes to roughly 17,600 years, and that is for a single light-year. This is our own back-of-the-envelope figure, so treat it as a ballpark. Which distance are you converting, and would parsecs be a useful extra unit for you?

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