Light Year Conversion Calculator
How far is a light year? Exactly 9,460,730,472,580.8 km. This light year calculator converts light-years to kilometres, miles, metres, astronomical units (AU), and parsecs, live and both ways. A light-year is a unit of distance: how far light travels in one Julian year.
Two-way converter
Result
All units at once
Type in any field; the other five update instantly.
Quick table: light-years to km, miles, AU, parsecs
Tap a light-year value to load it into the converter.
| Light-years | Kilometres | Miles | AU | Parsecs |
|---|---|---|---|---|
| 4,730,365,236,290.4 | 2,939,312,686,591.8 | 31,620.54 | 0.153301 | |
| 9,460,730,472,580.8 | 5,878,625,373,183.6 | 63,241.08 | 0.306601 | |
| 18,921,460,945,161.6 | 11,757,250,746,367.2 | 126,482.15 | 0.613203 | |
| 40,174,991,951,814.4 | 24,963,582,647,224.2 | 268,553.23 | 1.301983 | |
| 94,607,304,725,808 | 58,786,253,731,836 | 632,410.77 | 3.066014 | |
| 946,073,047,258,080 | 587,862,537,318,361 | 6,324,107.71 | 30.660139 | |
| 9,460,730,472,580,800 | 5,878,625,373,183,608 | 63,241,077.08 | 306.601394 |
Light-years in the real universe
Nearest star: Proxima Centauri
4.2465 ly = 40,174,991,951,814.4 km
Milky Way diameter
~105,700 ly ≈ 999,999,210,951,790,560 km
Sunlight travel time
8 minutes 20 seconds to cross 1 AU; 1 ly = 63,241.07708427 AU
Light-year vs parsec
1 ly = 0.3066 pc · 1 pc = 3.2616 ly
Worked examples
1 light-year to km
1 ly × 9,460,730,472,580.8 = 9,460,730,472,580.8 km
In every unit: 9,460,730,472,580,800 m · 5,878,625,373,183.6 mi · 63,241.07708427 AU · 0.3066 pc
4 light-years to km
4 ly × 9,460,730,472,580.8 = 37,842,921,890,323 km
4 × 9,460,730,472,580.8 km per light-year
Proxima Centauri distance to km
4.2465 ly × 9,460,730,472,580.8 = 40,174,991,951,814.4 km
Also: 24,963,582,647,224.2 mi · 268,553.23 AU
1 parsec to light-years
1 pc × 3.261563777167434 = 3.261563777167434 ly
Inverse: 1 ly = 0.30660139378555057 pc
Light year questions
What is a light-year?
A light-year is the distance that light travels in empty space in one Julian year. It is a unit of distance, not time: 1 light-year equals exactly 9,460,730,472,580,800 metres, because the speed of light is defined as exactly 299,792,458 metres per second and a Julian year is exactly 365.25 days.
How far is a light year in km?
One light-year is 9,460,730,472,580.8 km, about 9.461 trillion kilometres. Multiply any number of light-years by 9,460,730,472,580.8 to convert to kilometres.
How far is a light year in miles?
One light-year is 5,878,625,373,183.6 miles, about 5.879 trillion miles. That is the distance light covers in one Julian year, converted with the exact mile of 1,609.344 metres.
How many metres are in a light-year?
There are exactly 9,460,730,472,580,800 metres in one light-year. It is exact because it comes from two exact definitions: the speed of light (299,792,458 m/s) and the Julian year (365.25 days).
How do you convert 4 light years to km?
Multiply by 9,460,730,472,580.8 km per light-year: 4 x 9,460,730,472,580.8 = 37,842,921,890,323 km. Four light-years is about 37.84 trillion kilometres.
How many AU are in a light-year?
One light-year equals 63,241.07708426628 astronomical units (AU), usually rounded to 63,241 AU. An AU is the mean Earth-Sun distance, defined as exactly 149,597,870,700 metres.
How many parsecs are in a light-year?
One light-year equals 0.30660139378555057 parsecs, about 0.3066 pc. The parsec is defined from the astronomical unit and the angle of one arcsecond, which makes it about 3.26156 light-years long.
How many light-years are in a parsec?
One parsec equals 3.261563777167434 light-years, about 3.2616 ly. Astronomers prefer parsecs for professional work, but the light-year is more familiar to the public; this converter handles both.
What is the formula to convert light-years to km?
km = light-years x 9,460,730,472,580.8. To go the other way, light-years = km x 1.05700083402e-13, which is 1 divided by 9,460,730,472,580.8.
Is a light-year a unit of time or distance?
Distance. The name is misleading: a light-year measures how far light travels in a year, not how long something takes. Use years, days, or seconds for time, and light-years, kilometres, or miles for the distance.
Why is the light-year based on the Julian year?
The International Astronomical Union defines the light-year using the Julian year of exactly 365.25 days (31,557,600 seconds). A calendar year varies between 365 and 366 days, so the fixed Julian year keeps the light-year an exact, unambiguous constant.
How long does light take to travel one light-year?
Exactly one Julian year, 365.25 days, by definition. That is the whole idea of the unit: it turns a travel time for light into a distance. Light from the Sun, by comparison, takes only about 8 minutes 20 seconds to reach Earth.
What is the nearest star to Earth in light-years?
Proxima Centauri, at 4.2465 light-years away. That is 40,174,991,951,814.4 km, or 24,963,582,647,224.2 miles, or 268,553.23 AU. It is part of the Alpha Centauri triple-star system.
How many light-years across is the Milky Way?
About 105,700 light-years in diameter. That is roughly 999,999,210,951,790,560 km, essentially 1.0 x 10^18 km (one quintillion kilometres). Light from one edge of the galaxy takes over a hundred thousand years to reach the other edge.
What is the difference between a light-year and a parsec?
Both measure interstellar distance. A light-year is defined by the speed of light times one Julian year (9,460,730,472,580,800 m). A parsec is defined by parallax geometry (about 3.08567758149137e16 m), which works out to 3.261563777167434 light-years. They are simply different yardsticks for the same distances.
Can this tool convert miles and AU at the same time?
Yes. The multi-unit panel converts light-years, metres, kilometres, miles, astronomical units, and parsecs simultaneously: type in any field and the other five update instantly. The two-way converter above adds a swap button and adjustable decimal precision.
See also
- Length Converter — convert between metres, kilometres, miles, feet, inches, and more everyday length units.
- Miles to Kilometers — the classic imperial to metric distance conversion, both directions.
- Wavelength to Frequency Calculator — converts with the same speed of light (299,792,458 m/s) that defines the light-year.
- Unit Converter Table — the broader hub: quick reference tables for length, weight, temperature, and more.
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