What this calculator does
A light year is a unit of distance, not time, despite the name: it is how far light travels in one Julian year, at exactly 299,792,458 metres per second. That works out to 9,460,730,472,580.8 km, a distance so large it only becomes a practical unit once you are talking about stars rather than anything on Earth or even in the solar system.
This calculator converts a light year to kilometres, miles or astronomical units, in either direction, using the exact value of the speed of light rather than a rounded approximation. Because a light year is fixed by definition, the conversion factor never changes and carries no uncertainty.
The formula
A light year is defined as the distance light travels in one Julian year (exactly 365.25 days), at the speed of light, c = 299,792,458 m/s exactly. Multiplying those together gives 9,460,730,472,580.8 km per light year, the fixed factor this calculator uses for every conversion.
| Term | Meaning |
|---|---|
| Light year | The distance light travels in one Julian year: 9,460,730,472,580.8 km. |
| Speed of light (c) | Exactly 299,792,458 metres per second in a vacuum, a defined physical constant. |
| Astronomical unit (AU) | The average Earth-Sun distance, about 149,597,870.7 km, a smaller unit used within the solar system. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The value to convert, in light-years or in the target unit chosen below. |
| Convert | Choose the conversion direction and target unit: light-years to km, km to light-years, light-years to miles, or light-years to AU. |
When to use it
Putting a stellar distance in context
A star quoted as "4.2 light-years away" converts to kilometres or miles to show just how enormous that distance is compared with anything on a human or planetary scale.
Comparing a light-year figure with an AU-based one
Distances within the solar system are usually given in astronomical units, while distances to other stars are given in light-years; converting between the two puts both scales on the same footing.
Working out how long a light year actually is, in time
A light year is a distance, but people often really want to know how long light takes to cross a given distance; the speed of light calculator answers that travel-time question directly.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
A light year converted to other distance units
A range of light-year values, each converted to kilometres.
| Light-years | Kilometres | In miles |
|---|---|---|
| 1 | 9,460,730,473,000 km | 5,878,623,554,000 mi |
| 2 | 18,921,460,950,000 km | 11,757,247,110,000 mi |
| 4.246 | 40,170,261,590,000 km | 24,960,635,610,000 mi |
| 10 | 94,607,304,730,000 km | 58,786,235,540,000 mi |
| 100 | 946,073,047,300,000 km | 587,862,355,400,000 mi |
| 1000 | 9,460,730,473,000,000 km | 5,878,623,554,000,000 mi |
Questions
How long is a light year?
A light year is a distance, not a duration: 9,460,730,472,580.8 km, the distance light travels in one Julian year (365.25 days) at 299,792,458 m/s.
How much is a light year in km?
About 9.46 trillion km (9,460,730,472,580.8 km exactly), a fixed figure since both the speed of light and the length of a Julian year are defined constants.
What is a light year in time?
The phrase is a common mix-up: a light year measures distance, not time. The related question, how long light takes to cross a given distance, is answered by dividing that distance by the speed of light.
How many miles is a light year?
About 5.88 trillion miles, from converting the 9.46 trillion km figure using the standard km-to-mile factor.
For the wider ladder of astronomical distances including parsecs and astronomical units, see the astronomical distance converter. For how long light actually takes to cross a given distance, see the speed of light calculator.