A light-year is the distance light travels in a Julian year.
Light-years to Kilometers Converter — ly to km
Convert Light-year (ly) to Kilometer (km) using the exact conversion factor (1 light-year = 9.46073e+12 kilometer). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Light-years to Kilometers converter
Converter
Astronomy Converter
Convert astronomical length, mass, time, and distance units by compatible family.
Context values
Some unit families need one extra value for a physically accurate result.
Kilometer is 1000 meter units.
Result = input x 9.46073047e+15 / 1000.
Distance units are compatible.
Reserved above the conversion cards and below the converter.
About Converting Light-years to Kilometers
Light-year and Kilometer both describe astronomical distances at very different scales — one for measuring within our solar system, the other for the distances between stars — and converting between them helps put the scale of space into perspective. Light-year is typically associated with used internationally in astronomy and popular science for interstellar distances., while Kilometer is typically associated with standard road, distance, and mapping unit in nearly every country except the United States.
Formula
kilometers = light-years × 9.46073e+12
This factor comes from each unit's defined relationship to the category's base unit: 1 light-year equals 9.46073e+15 base units, and 1 kilometer equals 1000 base units, so dividing one by the other gives the direct light-year-to-kilometer factor of 9.46073e+12.
Simple example
1 ly × 9.46073e+12 = 9.46073e+12 km
1 light-year = 9.46073e+12 kilometers.
Real-world example
4.24 ly × 9.46073e+12 = 4.01135e+13 km
4.24 light-years = 4.01135e+13 kilometers.
Conversion table
| Light-year (ly) | Kilometer (km) |
|---|---|
| 1 ly | 9.46073e+12 km |
| 4.24 ly | 4.01135e+13 km |
| 10 ly | 9.46073e+13 km |
| 100 ly | 9.46073e+14 km |
| 1,000 ly | 9.46073e+15 km |
| 1,000,000 ly | 9.46073e+18 km |
Reverse conversion: Kilometer to Light-year
9.46073e+12 km × 1.057001e-13 = 0.99999995 ly
9.46073e+12 kilometers = 0.99999995 light-years.
light-years = kilometers × 1.057001e-13
For a page dedicated to this direction, see Kilometer to Light-year.
Light-year vs. Kilometer
| Aspect | Light-year | Kilometer |
|---|---|---|
| Family | Non-SI unit (astronomical) | SI derived (metric prefix) unit |
| Typical use | Measuring distances to stars and galaxies | Road signage, speed limits, and vehicle odometers outside the US |
| Where it's used | Used internationally in astronomy and popular science for interstellar distances. | Standard road, distance, and mapping unit in nearly every country except the United States |
Understanding the Light-year
Light-year (ly) measures distance. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. It is classified as a Non-SI unit (astronomical).
Where it's used: Used internationally in astronomy and popular science for interstellar distances.
Uses of the Light-year today
- Measuring distances to stars and galaxies
- Popular science communication about the scale of the universe
- Astrophysics research
History of the Light-year
The light-year emerged in 19th-century astronomy as scientists began measuring stellar distances via parallax (Friedrich Bessel's 1838 measurement of 61 Cygni is often cited as the first reliable stellar distance). Expressing these vast distances in light-travel-time became a practical convention, and the unit's value follows directly from the defined speed of light and the length of a Julian year.
Historical origin. Light-year was not the invention of a single named person; it developed through the process described above.
Understanding the Kilometer
Kilometer (km) measures distance. A kilometer is 1,000 meters — the meter is the SI base unit of length. It is classified as a SI derived (metric prefix) unit.
Where it's used: Standard road, distance, and mapping unit in nearly every country except the United States
Uses of the Kilometer today
- Road signage, speed limits, and vehicle odometers outside the US
- Geographic and mapping distances
- Endurance sports (marathons are measured in kilometers internationally, though the marathon distance itself is a fixed value in both systems)
History of the Kilometer
The meter was defined during the French Revolutionary metric reforms as one ten-millionth of the distance from the North Pole to the Equator along a meridian through Paris. 'Kilo-' is simply the SI prefix for 1,000, making the kilometer a direct decimal multiple.
The meter itself has been redefined several times as measurement precision improved — from a physical platinum-iridium bar (1889) to a krypton-86 wavelength (1960) to its current definition (1983) as the distance light travels in a fixed fraction of a second.
Historical origin. Kilometer was not the invention of a single named person; it developed through the process described above.
Light-year — sources
- IAU — Light year definition — Defines the light-year via the Julian year and the speed of light.
Kilometer — sources
- BIPM — The metre (SI base unit) — Current definition and history of the meter.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many kilometers are in 1 light-year?
1 light-year equals 9.46073e+12 kilometers, using the exact defined conversion factor rather than an estimate.
How do I convert light-year to kilometer?
Multiply the light-year value by 9.46073e+12. The converter above does this instantly to whatever precision you set.
How do I convert kilometer back to light-year?
Use the reverse factor: 1 kilometer equals 1.057001e-13 light-years. You can also use the swap control in the converter above to flip the direction instantly.
What is a light-year?
Light-year (ly) measures distance. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. It is classified as a Non-SI unit (astronomical).
What is a kilometer?
Kilometer (km) measures distance. A kilometer is 1,000 meters — the meter is the SI base unit of length. It is classified as a SI derived (metric prefix) unit.
Is the light-year to kilometer conversion exact?
Yes. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
Where is the light-year still used today?
Used internationally in astronomy and popular science for interstellar distances.