Convert astronomical distances. This free light year calculator turns light-years into kilometres, miles, astronomical units and parsecs – and back – and shows the light-travel time for any distance.
Enter a distance in light-years to see it in kilometres, miles, astronomical units and parsecs – and the time light takes to cross it.
Enter a distance in your chosen unit to convert it into light-years (and the equivalent light-travel time).
A light-year is the distance light travels in one year – about 9.46 trillion km (5.88 trillion miles). It’s a measure of distance, not time. Light from the Sun takes about 8.3 minutes to reach Earth.
How the Light Year Calculator Works
This tool converts astronomical distances to and from light-years, translating them into kilometres, miles, astronomical units, and parsecs, and showing how long light itself takes to cross that distance.
Formula: 1 light-year = 9,460,730,472,580.8 km (the distance light travels in one Julian year at 299,792,458 m/s). To convert from light-years: distance = light-years × 9,460,730,472,580.8 km, then re-expressed in miles, AU (1 AU = 149,597,870,700 m), or parsecs (1 pc = 3.0856775814913673 × 10^16 m). To convert to light-years, the entered distance is divided by the light-year distance in the same unit.
- Convert — toggle for direction: “From light-years” or “To light-years”
- Light-years — distance in light-years, used when converting from light-years
- Distance + unit — a distance value paired with a unit dropdown (km, miles, AU, parsecs, or metres), used when converting to light-years
Example Scenarios
| Input | Input Unit | Result |
|---|---|---|
| 1 | light-year | 9,460,730,472,581 km |
| 1 | light-year | 63,241.08 AU |
| 1 | light-year | 0.3066 parsecs |
| 4.367 | light-years | ≈41.3 trillion km |
| 10 | parsecs | ≈32.62 light-years |
| 384,400 | km (Moon distance) | ≈4.064 × 10â»â¸ light-years |
Light Year Calculator FAQ
How many kilometres are in one light-year?
This calculator uses 9,460,730,472,580.8 km per light-year, based on light travelling at 299,792,458 m/s for one Julian year (365.25 days) — commonly rounded to about 9.46 trillion km.What’s the difference between a light-year and a parsec?
A light-year is the distance light travels in one year (~9.46 trillion km). A parsec is a separate astronomical unit based on stellar parallax and equals roughly 3.26 light-years — this calculator converts between both.How do I convert astronomical units (AU) to light-years?
Switch the Convert toggle to “To light-years,” enter your distance value, and choose AU from the unit dropdown next to the Distance field. The result appears instantly in light-years and other units.Why does the calculator show a “light travel time”?
Because a light-year is defined by how far light travels in a year, the light travel time for any distance you enter is simply that distance divided by the speed of light — useful for understanding how “old” the light from a star is by the time it reaches us.Is a light-year a measure of time or distance?
Despite the name containing “year,” a light-year is purely a unit of distance — the distance light covers in one year — not a duration.How far is 4.367 light-years in kilometres?
Using this calculator’s conversion factor, 4.367 light-years works out to roughly 41.3 trillion kilometres (4.367 × 9,460,730,472,580.8 km).Related Calculators
Light-year distances are built directly on the speed of light, so it’s worth comparing that to the much slower speed of sound calculator, or exploring the wave properties of light itself with the wavelength & frequency calculator. For handling the huge numbers involved, the scientific notation converter is a handy companion.