Convert km to ly kilometres ↔ light years Free Online Secure
Type a value in kilometres and read it in light years 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 km to ly (kilometres ↔ light years)
| km | ly |
|---|---|
| 1 | 0.000000000000105700083402 |
| 2 | 0.000000000000211400166805 |
| 3 | 0.000000000000317100250207 |
| 5 | 0.000000000000528500417012 |
| 10 | 0.00000000000105700083402 |
| 20 | 0.00000000000211400166805 |
| 50 | 0.00000000000528500417012 |
| 100 | 0.0000000000105700083402 |
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
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 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.
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.
How far is a kilometre in light-years? A tiny number, and how to convert km to ly
The light-year is the yardstick of the night sky, and the kilometre is the yardstick of the road. Putting them side by side is the astronomer's way of showing how small a human distance is on a cosmic scale. A journey of 384,400 km to the Moon, a flight of 10,000 km across an ocean, a spacecraft that has travelled 25 billion kilometres: each is a minute fraction of a single light-year. To express such figures properly you must convert km to ly, and this page does it as you type, using scientific notation when the numbers are too small to print in the ordinary way.
The conversion is a division by about 9.46 trillion, so results are tiny and are best read in powers of ten. One kilometre is roughly 1.057 times ten to the minus thirteen light-years. That figure is correct but hard to feel, so the sections below also give comparisons and examples from the Solar System. You will find the rule, worked cases, a profile of each unit, a comparison, a tour of places where the conversion matters and a list of common mistakes.
How to convert km to ly: divide by 9.4607 trillion
The factor
A light-year is the distance light travels in vacuum in one Julian year of 365.25 days, which is 9,460,730,472,580.8 km. To convert km to ly, divide the number of kilometres by 9.4607 times ten to the twelfth. Equivalently, multiply by 1.057 times ten to the minus thirteen. So 1 km is 1.057e-13 ly, 1,000 km is 1.057e-10 ly and one million kilometres is 1.057e-7 ly.
Examples
The Moon, at 384,400 km, is 4.06e-8 ly away, which is 1.28 light-seconds. The Sun, at 149.6 million km, is 1.58e-5 ly, or 8.3 light-minutes. Neptune, at 4.5 billion km, is 4.76e-4 ly, about 4.2 light-hours. A spacecraft that has travelled 25 billion km has covered 2.64e-3 ly.
Reading scientific notation
The notation 1.057e-13 means 1.057 times ten to the minus thirteen, a decimal point followed by twelve zeros and then 1057. You can read it as a position of the decimal point: the exponent minus thirteen says move the point thirteen places to the left. Compare exponents to compare sizes: e-8 is a thousand times bigger than e-11.
A shortcut
For a mental convert km to ly, divide by ten trillion and add six per cent. The sum is close enough for a quick estimate: 1 billion km divided by 10 trillion is 1e-4 ly, plus 6 per cent is 1.06e-4, which compares to the exact 1.057e-4.
The kilometre
Distance on Earth
The kilometre is a thousand metres and the standard distance unit for roads, flights and geography in most of the world. The Earth has a circumference of 40,075 km at the equator; the Moon is about 384,000 km away; the Sun is 150 million km from us, a distance called one astronomical unit.
Limits of the kilometre
Beyond the Solar System, kilometres become unwieldy. The nearest star is about 40 trillion km away, a number of fourteen digits that tells you almost nothing without a scale. Astronomers therefore change unit: first to astronomical units, then to light-years and parsecs.
The light-year
Distance and not time
Despite its name, the light-year measures distance, not time. It is the distance light covers in one year at 299,792.458 km per second. Astronomers define it using the Julian year of exactly 365.25 days of 86,400 seconds, which gives 9,460,730,472,580.8 km.
Where it is used
The light-year is common in popular astronomy and science journalism. Professional astronomers prefer the parsec, about 3.26 light-years, and the astronomical unit. For stars, galaxies and the size of the Universe, the light-year gives a figure ordinary readers can picture as the travel time of light.
Kilometre and light-year compared
A light-year is 9.46 trillion times a kilometre. If a kilometre were a single step, a light-year would be 9.46 trillion of them, a walk that no one could ever finish. This difference of thirteen orders of magnitude is why the conversion from kilometres produces numbers with many zeros after the decimal point.
Use kilometres for anything on or near Earth and light-years for stars. In between, astronomical units are convenient for planets. For comparison, you can describe a distance in light-time: the Moon is 1.3 light-seconds away and the Sun is 8.3 light-minutes away.
Where you will need it
Space missions
Voyager 1, launched in 1977, is the most distant human-made object, more than 24 billion km from Earth, about 2.6e-3 ly, or roughly 22 light-hours. Even after almost half a century at 17 km per second, it has covered less than three thousandths of a light-year.
Science teaching
Teachers often ask pupils to convert km to ly for the distance to the Moon or the Sun in light-years. The result, tiny numbers in scientific notation, reveals how cosmic distances dwarf terrestrial ones.
Science fiction
Writers use light-years to describe trips between stars. Compared to a journey of one light-year, 9.46 trillion km, a trip of a million km is a hop. Readers who want to check how far a starship travels in a single jump can use the “convert km to ly” calculation to see.
Data work
Astronomical catalogues store distances in many units. A researcher may need to bring a column in kilometres to light-years before plotting it against stellar distances. Scientific notation avoids losing digits.
Light-travel time
Distance as delay
Since light covers 299,792 km every second, any distance in kilometres divided by that speed gives the light-time in seconds. A signal from the Moon takes 1.28 seconds; from Mars 3 to 22 minutes depending on the orbits; from Jupiter 35 to 52 minutes; from Neptune 4 hours. A light-year is the distance that corresponds to a delay of one year.
Why this helps
Talking in light-time gives a feel for the delays in space communication. A command to a probe at 1e-3 ly takes about nine hours to arrive, which affects how missions are controlled.
History of the light-year
Measuring the first star
In 1838 the astronomer Friedrich Bessel measured the parallax of the star 61 Cygni and found a distance of about 10 light-years. It was the first direct measurement of a stellar distance, and it showed that stars are unimaginably far away.
The unit's birth
The term light-year appeared in the 19th century and became popular in the 20th century. Its convenience is that it links distance to a concept everyone knows, the speed of light.
Common misconceptions
Light-year as time
Many people think the light-year measures time. It measures distance. A star 10 light-years away does not need 10 years for its light to cross a gap; its light does take 10 years to arrive, but the distance is the quantity that is stated.
Faster than light
Science fiction often speaks of travelling many light-years in a day. This is not possible with known physics. Any real probe takes more than a year per light-year.
How big is a trillion kilometres?
Making sense of 10 to the 12
A trillion km is 6,685 AU, or 0.106 ly. Light takes 38.6 days to cover that distance. The number of kilometres in a light-year is thus about 9.46 of those trillions. Think of the light-year as nine and a half bundles of a trillion kilometres.
From a trillion down
A billion km is 1/1000 of that and takes light 55.6 minutes. A million km is 3.3 seconds of light. Such ladders help you place any number of kilometres on the cosmic scale.
Worked examples from start to finish
Distance to Mars at closest approach
At its closest, Mars is about 54.6 million km away. To convert km to ly, divide by 9.4607e12: 5.77e-6 ly, light-time 3 minutes. At its farthest, 401 million km, it is 4.24e-5 ly, 22 minutes.
Distance to Pluto
Pluto averages 5.9 billion km from the Sun, about 6.2e-4 ly, 5.5 light-hours. The New Horizons probe took nine and a half years to reach it in 2015.
Why astronomers do not stop at kilometres
The case for bigger units
Any unit has a range in which it is natural. The metre suits a room, the kilometre a country, the astronomical unit a planetary system and the light-year a neighbourhood of stars. When you convert km to ly, you are stepping across several of these ranges at once, which explains why the result is so small. It is not a sign of error; it is a sign that the kilometre is the wrong yardstick for the question.
Choosing a unit for your own text
If you are writing an article, pick the unit in which the key numbers fall between roughly 0.1 and 1,000. The distance to the Moon is easiest as 384,000 km or 1.3 light-seconds; the distance to Proxima as 4.2 light-years. Use the “convert km to ly” calculation on this page to move between them whenever readers might want both.
Practice problems with answers
Three problems
First, a comet is 3 billion km from the Sun. Dividing by 9.4607e12 gives 3.17e-4 ly, about 2.8 light-hours. Second, a satellite orbits at 36,000 km. In light-years that is 3.8e-9, and its signal needs 0.12 seconds one way. Third, a rocket flies 1 million km in three days; its average speed is 3.86 km/s, which is 1.3e-5 of the speed of light.
How to use them
Work each problem on paper first, then check against the page. If your exponent differs by one, you probably misplaced a decimal point. If the mantissa differs, check the division. Repeat with your own numbers until the pattern is natural.
The size of the Solar System
Planets in light-time
Mercury is 3 light-minutes from the Sun, Earth 8.3, Jupiter 43, Saturn 80, Uranus 2.7 hours, Neptune 4.2 hours. In light-years, Neptune is 4.76e-4. The heliopause, the edge of the Sun's influence, lies around 120 AU, about 1.9e-3 ly.
The Oort cloud
The hypothesised Oort cloud stretches out to perhaps 100,000 AU, or 1.6 ly, 15 trillion km. This region is where long-period comets are thought to originate. It shows how a single light-year already encloses the Sun's gravitational domain.
Nearest stars
Distances in kilometres and light-years
Proxima Centauri is 4.24 ly away, about 4.0e13 km. Alpha Centauri A and B are 4.37 ly, 4.1e13 km. Sirius, the brightest star in the night sky, is 8.6 ly, 8.1e13 km. Going the other way from kilometres, 4e13 km is 4.2 ly.
A probe to Proxima
At Voyager's speed of 17 km/s, covering 4e13 km takes about 75,000 years. Projects such as Breakthrough Starshot study tiny sails pushed by lasers to cut the trip to a few decades.
The Milky Way and beyond
Galactic distances
The Milky Way is roughly 100,000 ly across, which is 9.5e17 km. The Sun is about 26,000 ly from the centre, 2.5e17 km. The Andromeda galaxy is 2.5 million ly away, 2.4e19 km.
Why no one converts back
At such scales, kilometres are meaningless: the numbers have eighteen or more digits and cannot be grasped. The light-year is the natural unit, and the kilometre figure is only useful as a check.
Units related to the light-year
Astronomical unit and parsec
One astronomical unit is 149,597,870.7 km, so a light-year is 63,241 AU. One parsec is 3.0857e13 km, 3.26 ly. Converting kilometres to parsecs requires dividing by 3.0857e13. Learning the relations helps you move between textbooks that use different units.
Light-second, minute and hour
A light-second is 299,792 km, a light-minute 17.99 million km, a light-hour 1.079 billion km, a light-day 25.9 billion km. These smaller steps make the Solar System easier to describe than kilometres do.
Writing very small numbers
Scientific notation versus decimals
Printing 0.0000000000001057 is error-prone. Scientific notation, 1.057e-13 or 1.057 times 10 to the minus 13, is compact and precise. Spreadsheets and calculators accept the form with an E.
Significant figures
Round to the precision of your data. A distance known to three figures, 384,000 km, gives 4.06e-8 ly, three figures again. Extra digits suggest a precision the Moon does not have.
Floating-point and precision
What calculators do
Calculators and computers store numbers with about 15 significant digits. The conversion from kilometres to light-years is safe within that precision. The risk comes when you subtract nearly equal numbers or chain many steps, because rounding accumulates.
Good practice
Keep the full factor 9.4607304725808e12 in a named cell, not a rounded copy. When you share results, state the factor you used so that others can reproduce them.
Spreadsheet formulas
Formulas
To convert km to ly for a column, place kilometres in column A and enter =A2/9.4607304725808E12 in column B. Format the cell in scientific notation with three decimals. For the reverse, multiply by the same factor.
Checking
Test with 9.4607304725808E12 km: the result must be exactly 1. If it is not, the formula or the format is wrong.
Teaching astronomical scale
A scale model
Represent the Sun as a ball of 10 cm. On that scale the Earth is a speck 10 m away, Neptune is 300 m away and Proxima Centauri is about 2,900 km away. The model makes the gap between planets and stars concrete.
Classroom exercise
Ask students to compute the light-year equivalent of the distances in the model, then compare them with the kilometres. The pattern of exponents shows the jump of scale.
More values for convert km to ly
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 km = 0.00000000000001057 ly
- 0.5 km = 0.00000000000005285 ly
- 3 km = 0.0000000000003171 ly
- 4 km = 0.0000000000004228 ly
- 7 km = 0.000000000000739901 ly
- 15 km = 0.0000000000015855 ly
- 20 km = 0.000000000002114 ly
- 75 km = 0.00000000000792751 ly
- 200 km = 0.00000000002114 ly
- 500 km = 0.00000000005285 ly
- 2,000 km = 0.0000000002114 ly
- 10,000 km = 0.000000001057 ly
Mistakes when you convert km to ly
Multiplying instead of dividing
The light-year is the bigger unit, so the answer is a smaller number. If the result is huge, you multiplied.
Losing the exponent
1.057e-13 is not 1.057. Always carry the exponent through every step.
Mixing units
Do not combine kilometres with miles or astronomical units in one formula without converting first.
Quick answers
How many kilometres in a light-year?
About 9.46 trillion.
How do I convert km to ly?
Divide the kilometres by 9.4607e12.
What is 1 million km in light-years?
About 1.057e-7 ly.
Is the conversion exact?
The light-year is defined from the Julian year and the speed of light, so the factor is exact to the digits given.
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 km to m, Convert km to mi, Convert °C to °F, Convert °F to °C, Convert L to gal, Convert gal to L, Convert m² to mm², Convert °R to K, Convert Pa to bar. The reverse of this page is Convert ly to km.
In short, to convert km to ly divide by 9.46 trillion, read the answer in scientific notation, and keep only the digits your data supports. Your number stays on your device: nothing is uploaded and no account is needed.