Convert ly to km light years ↔ kilometres Free Online Secure
Type a value in light years and read it in kilometres at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert ly to km (light years ↔ kilometres)
| ly | km |
|---|---|
| 1 | 9460730472580.8 |
| 2 | 18921460945161.6 |
| 3 | 28382191417742.4 |
| 5 | 47303652362904 |
| 10 | 94607304725808 |
| 20 | 189214609451616 |
| 50 | 473036523629040 |
| 100 | 946073047258080 |
Where these units come from
The metre was first defined in 1793 as a ten-millionth of the distance from the equator to the North Pole — a definition nobody could check without surveying a quarter of the planet. It has been redefined four times since, each time to something steadier: a platinum bar, then a prototype kept in Paris, then a wavelength of krypton light, and since 1983 the distance light travels in vacuum in 1/299,792,458 of a second. The inch stopped being a separate thing in 1959, when six countries agreed to define it as exactly 25.4 mm. That agreement is why every conversion on this page between a metric and an imperial length is exact rather than measured: both sides now descend from the same definition.
Where you meet them
Millimetres are the working unit of engineering drawings and of anything made in a workshop; centimetres belong to tailoring, furniture and school rulers; metres to rooms, fabric and running tracks. Feet and inches survive in aviation altitudes worldwide, in American and British construction, and in the diagonal of every screen you have ever bought. Nautical miles are not a quaint survival — one is a minute of latitude, which is why ships and aircraft still navigate in them.
Questions people ask
Is a light year a unit of time?
No, it is a distance: how far light travels in a vacuum in one year, which is about 9.46 trillion kilometres. The nearest star beyond the Sun is some 4.2 light years away. The name misleads because years usually measure time, but it works well for distances in space because the numbers in kilometres are unmanageable.
Which countries still measure road distances in miles?
The United States and the United Kingdom, where road signs show miles, together with a handful of smaller countries such as Liberia and Myanmar. Almost everywhere else road distances are in kilometres. One kilometre is 1000 metres and about 0.621 of a mile, so a distance in miles is about five-eighths of the same distance in kilometres.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Turning starlight into kilometres: how to convert ly to km
From the sky to the road
Astronomers love the light-year because it tells a story: the light you see from a star left it that many years ago. But a light-year is hard to feel. Ask someone how far four light-years is, and you will usually get a shrug. Express it in kilometres, the unit of road signs and odometers, and the figure becomes both more concrete and more shocking: about 38 trillion kilometres. To get there you have to convert ly to km, and this page does it as you type, displaying the answer in full and in scientific notation.
The calculation is a multiplication by 9.46 trillion, which makes the results extremely large even for small inputs. One light-year already has thirteen digits in kilometres, and a nearby star has fourteen. The sections below on how to convert ly to km present the rule, worked examples with real stars, the history and meaning of both units, a comparison, a tour of the situations in which people need the conversion and a list of mistakes to avoid.
How to convert ly to km: multiply by 9.4607 trillion
The factor in detail
A light-year is 9,460,730,472,580.8 km. To convert ly to km, multiply the number of light-years by this value. 1 ly is about 9.46e12 km, 2 ly about 1.89e13 km, 10 ly about 9.46e13 km and 100 ly about 9.46e14 km. Fractions work in the same way: half a light-year is 4.73e12 km, and 0.1 ly is 9.46e11 km.
Examples with real stars
Proxima Centauri, 4.24 ly, is 4.01e13 km away. Sirius, 8.6 ly, is 8.14e13 km. Vega, 25 ly, is 2.37e14 km. The Pole Star, about 433 ly, is 4.1e15 km. Each time the answer has more digits than the input, but the number of significant digits stays the same.
Rounding the factor
For a quick convert ly to km, use 9.46 trillion or even 10 trillion. Using 10 trillion gives an overestimate of 5.7 per cent, small enough for a sketch. For scientific writing keep 9.4607e12. For exact work, use the full 9,460,730,472,580.8 km.
Orders of magnitude
Count digits: the answer in kilometres has about 13 digits for 1 ly, 14 for 10 ly, 15 for 100 ly and 16 for 1,000 ly. This is a quick check that your conversion makes sense.
The light-year
Distance, defined by light
The light-year is the distance that light travels in a vacuum during one Julian year of 365.25 days. Light moves at exactly 299,792.458 km per second, and a Julian year has 31,557,600 seconds, so the product is 9,460,730,472,580.8 km. The unit is not part of the SI system, but it is accepted in astronomy and in popular science.
What it conveys
Saying that a star is 10 light-years away means more than a distance. It means that we see the star as it was ten years ago. The unit combines space and time in a way that links directly to everyday experience.
The kilometre
A thousand metres
The kilometre is one thousand metres and the common unit for road and flight distances. It is short enough that you can walk it in a quarter of an hour and long enough that a car crosses it in under a minute.
Why use it for stars
Kilometres are useful for translating starry distances into something comparable with familiar journeys. A trip around the Earth is 40,075 km. The distance to the Moon is 384,400 km. A light-year is about 236 million trips around the Earth.
Light-year and kilometre compared
One light-year is 9.46 trillion kilometres. The ratio is so big that a kilometre looks like a rounding error next to it. In plain terms: if you drove at 100 km/h without stopping, one light-year would take you about 10.8 million years.
For practical communication, use light-years for objects beyond the Solar System, astronomical units for the planets, and kilometres for anything on or near Earth. The conversion to kilometres is mostly done for illustration, to give a feel for the numbers.
Where you will need it
Popular science articles
An article says that a newly discovered planet orbits a star 40 light-years away. The editor wants a figure in kilometres for the sidebar: 3.78e14 km. The number is hard to read, so the sidebar adds the comparison that a car at 100 km/h would need 430 million years.
School projects
Students building a model of the Solar System can convert the distance to the nearest star to kilometres, then scale it. At 1:1,000,000,000 the Sun is 1.4 m across and Proxima Centauri is 40,000 km away, more than the circumference of the Earth.
Journalism and quizzes
Quiz writers love the question that needs a “convert ly to km” calculation: how far is a light-year in kilometres? The answer, 9.46 trillion, is a classic. When you add that this number has 13 digits, readers realise how enormous it is.
Science fiction
Authors who want to describe a ship's range convert a jump of five light-years into kilometres to give an impression of scale: 4.7e13 km. Most readers will not remember the digits but will remember that they were many.
The nearest stars in kilometres
A short list
Proxima Centauri: 4.24 ly, 4.01e13 km. Alpha Centauri A and B: 4.37 ly, 4.13e13 km. Barnard's Star: 5.96 ly, 5.64e13 km. Wolf 359: 7.86 ly, 7.44e13 km. Sirius: 8.6 ly, 8.14e13 km. Each is several thousand times farther than the farthest probe.
How long would a probe take?
At Voyager 1's 17 km/s, a probe reaches Proxima in about 75,000 years. At the speed of the fastest solar probe, 190 km/s, it would take around 6,700 years. Reaching the nearest star within a human lifetime needs a speed of several per cent of the speed of light.
History of the unit
Bessel and the first stellar distance
In 1838 Friedrich Bessel measured the distance to 61 Cygni by parallax. He found about 10 ly, 9.5e13 km, and thus gave the first direct measurement of the distance to a star. The result confirmed that the stars are suns at enormous distances.
Public use
The light-year became popular in the 20th century through newspapers and books. It carries a charm that the parsec, preferred by professionals, lacks.
Light-years in culture
Songs, films and games
Titles and plots refer to light-years as units of dramatic distance: ships jump across 20 light-years in a scene, and characters mourn someone who is a hundred light-years away. Converting these numbers to kilometres shows how enormous the claims are.
Checking fiction
When a character says that a ship travelled 3 light-years in a day, you can compute the speed: 3 times 9.46e12 km in 86,400 s is 3.3e8 km/s, which is more than a thousand times the speed of light.
Putting a trillion in context
A trillion kilometres
A trillion kilometres is about a tenth of a light-year, and light needs 38.6 days to cross it. A light-year is about 9.46 of those. If you divide the kilometres by 1e12, you get an easy reading in trillions.
Comparing with money
If each kilometre were a euro, a light-year would cost roughly a tenth of what the whole world produces in a year. Analogies of this kind help readers sense the size of the number.
Planning a conversion table
A table for your wall
1 ly is 9.46e12 km; 2 ly 1.89e13; 5 ly 4.73e13; 10 ly 9.46e13; 25 ly 2.37e14; 50 ly 4.73e14; 100 ly 9.46e14; 1,000 ly 9.46e15. Print the list and use it for quick reference. For anything in between, multiply the nearest entry.
Interpolating
To find 7 ly, take the 5 ly entry and add twice the 1 ly entry: 4.73e13 plus 1.89e13 is 6.62e13 km. The method works for any value and avoids a calculator.
A walk through one worked problem
The question
Suppose a news item says that a rocky planet has been found 12.5 light-years away and asks how far that is in kilometres. Write down 12.5 ly, then multiply by 9.4607e12. The product is 1.1826e14 km. If you only want two significant figures, give 1.2e14 km, which is 120 trillion kilometres. Dividing by the speed of a car, 100 km/h, gives 1.2e12 hours, or about 137 million years of driving.
Checking the answer
Start from the other end: divide 1.18e14 by 9.46e12 and you should get 12.5. Count digits too: the answer has 15 digits, which fits a distance between 10 and 100 light-years. A tidy way to convert ly to km is to make the digit check part of every calculation.
How the page helps with long numbers
Full number or scientific notation
Our result is shown in both forms so that you can pick the one you need. The long form, with thousands separators, is useful when you must copy the exact value into a database. The scientific form is the one to put in a text, because it is shorter and shows the order of magnitude at a glance. If you convert ly to km for a report, quote the scientific form in the text and the full number in an appendix.
Rounding sensibly
No distance to a star is known to thirteen digits, so printing the full value of a conversion is a courtesy to the reader, not a claim of accuracy. State clearly how many digits are significant and round your published figure to that number. Doing so is part of honest communication.
Spacecraft distances
How far we have gone
Voyager 1 is more than 24 billion km from Earth, 2.5e-3 ly. To cover one full light-year at its speed of 17 km/s it would need about 17,600 years. The numbers show how tiny our reach is.
Light time
A signal from Voyager takes about 22 hours to reach us. Mission controllers must plan around the delay, sending commands a day in advance and waiting two days for a reply.
Galaxies and the observable Universe
Beyond the neighbourhood
The Milky Way is about 100,000 ly wide, 9.5e17 km. The Andromeda galaxy, 2.5 million ly away, is 2.4e19 km. The observable Universe has a radius of about 46 billion ly, 4.4e23 km. Beyond that you reach numbers with 24 digits, which no one can picture.
Writing huge numbers
Use powers of ten and keep the exponent in view. Scientists use prefixes: a petametre is 1e15 m and an exametre 1e18 m. One light-year is 9.46 petametres.
The prefix ladder for kilometres
Metric prefixes
1 ly is 9.46e15 m, which is 9.46 Pm. In kilometres it is 9.46e12 km, or 9.46 terakilometres. Naming the units this way tames the zeros: 4.24 ly is 40.1 Tkm.
When to switch
If your answer exceeds a trillion kilometres, consider giving it in light-years instead. A result such as 4e13 km can be restated as 4.2 ly, which is more natural to a general reader.
Time as a distance
Light-travel times
One light-second is 299,792 km, one light-minute 17.99 million km, one light-hour 1.079 billion km, one light-day 25.9 billion km, one light-year 9.46 trillion km. The ladder connects the units.
Examples
Multiplying the number of light-days by 25.9 billion gives kilometres. A signal travelling 3.5 light-days has covered 9.1e10 km. Light-years from light-days: divide by 365.25.
Spreadsheet formulas
Formula
To convert ly to km for a whole column, put light-years in column A and enter =A2*9.4607304725808E12 in column B. Format in scientific notation with three decimals. For the reverse, divide.
Checking
Enter 1 and make sure the result matches 9.4607304725808E12. For 4.24 you should read about 4.01E13.
Significant figures
Match the input
If the distance is 4.24 ly, give the result to three significant figures: 4.01e13 km. Writing 40,113,..., with every digit suggests more precision than the star distance has. Parallax measurements carry errors of a few thousandths of a light-year.
Stating uncertainty
Scientists write 4.246 plus or minus 0.003 ly. The corresponding kilometre figure is 4.017e13 km plus or minus 3e10 km. Convert the uncertainty with the same factor.
Common mistakes with exponents
Dropping a digit
When copying 9.46e12, people write 9.46e11 and lose a factor of ten. Always read the exponent aloud: nine point four six times ten to the twelve.
Mixing e and E
In spreadsheets, E means ten to the power. In maths, e is Euler's number. Do not mix the two when typing numbers.
Teaching distance and time
A class demonstration
Mark a point and tell students it is the Sun. Place a pea 8.3 light-minutes away, represented by 1.5 m in the demonstration, for the Earth. Then ask where the nearest star would be at the same scale: about 400 km. Students quickly see why the light-year is needed.
Conversion drills
Pairs of students pick a star, convert its distance to kilometres on the page, and divide by 100 km/h to compute how long a car trip would take. The answers range from millions to billions of years.
More values for convert ly to km
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 ly = 946,073,047,258 km
- 0.5 ly = 4,730,365,236,290 km
- 3 ly = 28,382,191,417,742 km
- 4 ly = 37,842,921,890,323 km
- 7 ly = 66,225,113,308,066 km
- 15 ly = 141,910,957,088,712 km
- 20 ly = 189,214,609,451,616 km
- 75 ly = 709,554,785,443,560 km
- 200 ly = 1,892,146,094,516,160 km
- 500 ly = 4,730,365,236,290,400 km
- 2,000 ly = 18,921,460,945,161,600 km
- 10,000 ly = 94,607,304,725,808,000 km
Mistakes when you convert ly to km
Dividing instead of multiplying
The light-year is the bigger unit, so the answer in kilometres is a larger number. If it is smaller than the input, you divided instead of making a “convert ly to km” calculation.
Confusing with time
A light-year is not a year. Do not add it to dates or divide it by speeds without converting.
Wrong exponent
Check that a 13-digit answer belongs to 1 ly. If you get 12 or 14 digits, an exponent slipped.
Quick answers
How many kilometres in a light-year?
About 9.46 trillion.
How do I convert ly to km?
Multiply the light-years by 9.4607e12.
What is 4.24 ly in kilometres?
About 4.01e13 km.
Is the conversion exact?
The factor is exact to the digits given, because the light-year is defined from the speed of light and the Julian year.
Does my number leave my device?
No. The calculation runs in your browser.
Related pages and a last word
If you work with lengths, these conversions are close at hand: Convert nmi to km, Convert dm to km, Convert km to mi, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L, Convert tbsp to tsp, Convert cm² to in². The reverse of this page is Convert km to ly.
In short, to convert ly to km multiply by 9.46 trillion, expect a long number with an exponent, and keep only the digits your data supports. Your number stays on your device: nothing is uploaded and no account is needed.