Convert light-years to kilometers

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1 ly is exactly 9,460,730,472,580.8 km, or about 9.46 trillion kilometers. Type a distance in light-years and the converter gives it in kilometers, written out in full and in scientific notation. The field also takes words, so 2.5 million works as it is.

ly

The field also accepts scientific notation, for example 6.5e-7 or 6.5 × 10⁻⁷.

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What is a light-year?

A light-year is a unit of distance. The International Astronomical Union defines it as the distance light travels in a vacuum in one Julian year of 365.25 days, and gives it the symbol ly[1].

Its length follows from two numbers. A Julian year has 31,557,600 seconds[1], and the speed of light in a vacuum is exactly 299,792,458 m/s[2]. Multiply them and you get a distance that nobody has to measure, because it comes straight from the definition.

You will mostly meet the light-year in popular science books, articles and museum displays. In research papers astronomers usually give star distances in parsecs, and distances inside the Solar System in astronomical units[1]. You can turn the latter into kilometers with the au to km converter.

The Andromeda Galaxy among stars, with a bright core, brown dust lanes in bluish spiral arms and two smaller patches of light beside it
The Andromeda Galaxy photographed through a telescope against the stars of the Milky Way. Its light takes millions of years to reach us, which is why its distance is easier to give in light-years than in kilometers. Photo: Arnaud Mariat, Unsplash.

How many kilometers are in a light-year?

A light-year is exactly 9,460,730,472,580.8 km[1]. Rounded, that is 9.46 trillion kilometers or about 5.88 trillion miles[3]. To get miles for any other distance, run the kilometers through the km to miles converter.

Older British books can trip you up here. Before 1974, a British billion meant a million million, the number that is called a trillion today[4]. A pre-1974 text that puts a light-year at about 9.5 billion kilometers means the same distance as a modern one that says 9.5 trillion.

In scientific notation, which UK schools call standard form, a light-year is 9.4607304725808 × 10¹² km. The exponent 12 tells you how many places the decimal point moves to the right to give the full number. The converter shows this form under the result field in the Scientific notation line, rounded to six digits: 9.46073 × 10¹² km.

How to convert light-years to kilometers: formula and example

Multiply the number of light-years by the length of one light-year: kilometers = light-years × 9,460,730,472,580.8. To go the other way, divide: light-years = kilometers ÷ 9,460,730,472,580.8.

Proxima Centauri, the nearest known star after the Sun, is about 4.24 light-years away[5]. Multiplying gives 40,113,497,203,742.6 km, a little over 40 trillion kilometers. The light reaching Earth from that star today set off more than four years ago.

The converter shows results to fifteen significant digits, because that is how many digits a browser keeps in a number without rounding error. A longer result is rounded at the fifteenth digit and the rest are shown as zeros: 124 light-years gives 1,173,130,578,600,020 km, although the exact product ends in 019.2. If you start from kilometers, use the km to light years converter.

How many kilometers are in a given number of light-years?

The table gives the converter's results for fractions of a light-year, the distances of nearby stars and the distances of galaxies. Each result is the number of light-years multiplied by 9,460,730,472,580.8, written in full and in scientific notation.
Light-yearsKilometersScientific notation
0.001 ly9,460,730,472.581 km9.460730473 × 10⁹ km
0.01 ly94,607,304,725.808 km9.460730473 × 10¹⁰ km
0.1 ly946,073,047,258.08 km9.460730473 × 10¹¹ km
0.5 ly4,730,365,236,290.4 km4.730365236 × 10¹² km
1 ly9,460,730,472,580.8 km9.460730473 × 10¹² km
2 ly18,921,460,945,161.6 km1.892146095 × 10¹³ km
3 ly28,382,191,417,742.4 km2.838219142 × 10¹³ km
4 ly37,842,921,890,323.2 km3.784292189 × 10¹³ km
4.2 ly39,735,067,984,839.4 km3.973506798 × 10¹³ km
5 ly47,303,652,362,904 km4.730365236 × 10¹³ km
10 ly94,607,304,725,808 km9.460730473 × 10¹³ km
12.5 ly118,259,130,907,260 km1.182591309 × 10¹⁴ km
20 ly189,214,609,451,616 km1.892146095 × 10¹⁴ km
25 ly236,518,261,814,520 km2.365182618 × 10¹⁴ km
40 ly378,429,218,903,232 km3.784292189 × 10¹⁴ km
50 ly473,036,523,629,040 km4.730365236 × 10¹⁴ km
100 ly946,073,047,258,080 km9.460730473 × 10¹⁴ km
500 ly4,730,365,236,290,400 km4.730365236 × 10¹⁵ km
1,000 ly9,460,730,472,580,800 km9.460730473 × 10¹⁵ km
10,000 ly94,607,304,725,808,000 km9.460730473 × 10¹⁶ km
100,000 ly946,073,047,258,080,000 km9.460730473 × 10¹⁷ km
1,000,000 ly9,460,730,472,580,800,000 km9.460730473 × 10¹⁸ km
50,000,000 ly473,036,523,629,040,000,000 km4.730365236 × 10²⁰ km
1,000,000,000 ly9,460,730,472,580,800,000,000 km9.460730473 × 10²¹ km
Read 9.460730473 × 10¹² km as 9.460730473 multiplied by one trillion.

How far away are well-known stars and galaxies in kilometers?

The distances in light-years come from Wikipedia, and the converter worked out the kilometers. The first three rows are stars that the United States, the United Kingdom and Australia named in the International Astronomical Union's 2019 NameExoWorlds campaign, each with a planet of its own.
ObjectDistanceKilometersScientific notation
Nushagak (HD 17156), named by the United States253 ly2,393,564,809,562,940 km2.39356481 × 10¹⁵ km
Gloas (WASP-13), named by the United Kingdom750 ly7,095,547,854,435,600 km7.095547854 × 10¹⁵ km
Bubup (HD 38283), named by Australia124 ly1,173,130,578,600,020 km1.173130579 × 10¹⁵ km
Sirius8.6 ly81,362,282,064,194.9 km8.136228206 × 10¹³ km
Gaia BH1, the nearest known black hole1,560 ly14,758,739,537,226,100 km1.475873954 × 10¹⁶ km
Centre of the Milky Way26,000 ly245,978,992,287,101,000 km2.459789923 × 10¹⁷ km
Width of the Milky Way100,000 ly946,073,047,258,080,000 km9.460730473 × 10¹⁷ km
Andromeda Galaxy2,500,000 ly23,651,826,181,452,000,000 km2.365182618 × 10¹⁹ km
Triangulum Galaxy3,000,000 ly28,382,191,417,742,400,000 km2.838219142 × 10¹⁹ km
Nushagak takes its name from a river in Alaska[6], Gloas, chosen by school children, is the Manx word for to shine[7], and Bubup is the Boonwurrung word for child[8]. The other distances come from the table of orders of magnitude in the article on the light-year[5].

Why do sources give different kilometer figures for a light-year?

A light-year depends on which year goes into the sum. Today's definition uses the Julian year, while older tables and some modern sources work from the tropical or the Gregorian year. The gap reaches 200 million kilometers, more than the distance between the Earth and the Sun.
Worked out fromLight-yearDifference from the definition
Julian year, 365.25 days (IAU definition)9,460,730,472,580.8 kmexact value
Gregorian year, 365.2425 days9.460536207 × 10¹² km194,265,580.8 km less
Tropical year of epoch 1900 and today's speed of light9.460528405 × 10¹² km202,067,580.8 km less
IAU system of constants from 19649.460530 × 10¹² km200,472,580.8 km less
Rounded to 9.46 trillion km9,460,000,000,000 km730,472,580.8 km less
The IAU's 1964 system of astronomical constants, in use from 1968 to 1983, contained the tropical year and a measured rather than defined speed of light. The Julian year has been used since 1984[5].

Which number formats does this converter handle?

Paste the distance the way you have it: with a point, with commas between the thousands, with a word like million or in scientific notation. The field accepts every one of these formats and converts it by itself, so there is nothing to tidy up before you paste. The table shows what the converter does with each one, and it worked out the results itself.
What you typeWhat the converter showsHow we read it
4.2440113497203742.6 kmThe decimal point is read exactly as English writes it.
4,2440113497203742.6 kmA comma with two digits after it is read as a decimal comma, the way most of Europe writes it.
1,50014191095708871200 kmA comma followed by three digits is a thousands separator, so the field converts one thousand five hundred light-years. A note under the field shows the other possible reading, 1.5.
1.50014191095708871.2 kmA point is the decimal mark in English, so this reads as one and a half light-years. A note under the field shows the other possible reading, 1500.
2,500,00023651826181452000000 kmCommas between the thousands are skipped, so the field converts two and a half million light-years.
2.5 million23651826181452000000 kmThe word million multiplies the number by a million, which is how articles and textbooks give galaxy distances.
2.5e623651826181452000000 kmScientific notation from a calculator or spreadsheet is read as it stands: 2.5e6 is 2,500,000.
2.5 × 10⁶23651826181452000000 kmStandard form from a textbook, with a multiplication sign and a superscript exponent, works the same as 2.5e6.
1/24730365236290.4 kmWe work out a slash fraction ourselves, so half a light-year needs no conversion to 0.5 first.
2½23651826181452 kmThe ½ character copied from a document is added to the number in front of it.
4.24 ly40113497203742.6 kmA unit symbol pasted along with the number is left out and only the number is converted.

How to convert light-years to kilometers in Excel

For a single value the converter is quicker. A whole column of distances takes one formula dragged down the sheet.

  1. The CONVERT function

    Excel knows the light-year as "ly", and the prefix k in front of the meter gives kilometers[9]. For a value in cell A1, type =CONVERT(A1,"ly","km"). A plain multiplication, =A1*9460730472580.8, gives the same result.

  2. Scientific format for long results

    Select the result cells and apply the Scientific format from the Number group on the Home tab[10]. A light-year then shows as 9.46E+12, where E+12 means multiplied by 10¹².

  3. The precision limit

    Excel keeps numbers to a maximum precision of 15 digits[10]. The light-year constant has fourteen, so it fits. A product with a number of several significant digits, such as 1,234 light-years, has more, and Excel turns the last digits of the result into zeros.

Convert other units to Kilometers

miles to kmmeters to kmcm to kmmm to kmdm to kmau to kmyards to kmfeet to km

Sources

  1. Measuring the Universe: The IAU and astronomical units — International Astronomical Union
  2. The International System of Units (SI Brochure) — BIPM
  3. Voyager 1: What Is a Light-Day? — NASA
  4. Long and short scales — Wikipedia
  5. Light-year — Wikipedia
  6. HD 17156 — Wikipedia
  7. WASP-13 — Wikipedia
  8. HD 38283 — Wikipedia
  9. CONVERT function — Microsoft
  10. Display numbers in scientific (exponential) notation — Microsoft

Every figure in the tables was computed from the unit definitions and checked arithmetically before publication. Historical values and the names of local units come from the sources listed above.

Content last reviewed: · written and checked by the Arteon team

Common questions about converting light-years to kilometers

Is a light-year a unit of time?

No. A light-year measures distance, and the word year in the name only tells you how long light takes to cover it. That is why asking how many years are in a light-year has no answer in years, only in kilometers or miles.

How many astronomical units are in a light-year?

About 63,241 astronomical units[1]. An astronomical unit is roughly the average distance between the Earth and the Sun, and the au to km converter gives its exact length in kilometers.

How many light-years are in a parsec?

About 3.26 light-years. The IAU puts a parsec at roughly 30.857 × 10¹² km[1], and dividing that by the length of a light-year gives 3.2616.

How long would it take to drive a light-year?

At a steady 60 mph, about 11.2 million years. Sixty miles an hour is 96.56 km/h, so a car covers about 846,450 km in a Julian year without stopping, and 9,460,730,472,580.8 km divided by that is roughly 11.18 million.

Arteon free tool screenshot: Convert light-years to kilometers

Have a question or suggestion?

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