Light Year Calculator
Enter a distance in light years (or in kilometres, miles, AU or parsecs) and this calculator instantly converts it to all other units, shows the step-by-step arithmetic, and puts the result in context with a reference table of famous cosmic distances. Reverse conversion is fully supported: type in kilometres or AU and get back the equivalent in light years.
What is a light year?
A light year is the distance that light travels through a vacuum in exactly one Julian year (365.25 days). Because light moves at 299,792,458 metres per second, that works out to roughly 9.461 trillion kilometres, or about 5.879 trillion miles. Despite having "year" in its name, a light year is a unit of distance, not time. Astronomers use it because the distances between stars and galaxies are so enormous that kilometres and miles produce unwieldy numbers with dozens of digits. Saying "Proxima Centauri is 4.244 light years away" is far more useful than writing out 40.14 trillion kilometres.
How to use this calculator
Select the unit you are starting from in the "Input unit" drop-down, then enter the distance. Choose the unit you want highlighted in the "Primary output unit" drop-down - this drives the main result card at the top. All other units (kilometres, miles, astronomical units, parsecs and metres in scientific notation) appear in the secondary outputs below. The calculator also works in reverse: set the input unit to kilometres, miles, AU or parsecs to convert those into light years. The "travel time" mode shows how many years, days and seconds light takes to cover the entered distance - handy for understanding that when you look at a star 4.244 light years away, you are seeing it as it was 4.244 years ago.
Conversion factors and the formula
All conversions in this tool derive from two physical constants: the speed of light (c = 299,792,458 m/s, exact by SI definition) and the Julian year (365.25 days = 31,557,600 s). One light year = c x 31,557,600 s = 9.46073047 x 10^15 metres. Dividing by 1,000 gives kilometres; dividing by 1,609.344 gives miles. One astronomical unit (AU) is defined as exactly 149,597,870,700 metres, so 1 ly = 63,241.1 AU. One parsec is 1 AU / tan(1 arcsecond) = 3.08568 x 10^16 m, giving 1 ly = 0.30660 parsecs, or equivalently 1 pc = 3.26156 ly. These factors are used exactly throughout the calculator.
Light travel time and what it means to look into the past
Light travels at a finite speed, so every observation in astronomy is also a look back in time. When you observe the Sun, you see it as it was 8.32 minutes ago. Sirius appears as it was 8.6 years ago. The Andromeda Galaxy looks as it did 2.537 million years ago. This is why the travel time mode can be useful: it reminds you that a distance of N light years is identical to a time delay of N years. For objects at cosmological distances (billions of light years), the relationship becomes more complex due to the expansion of the universe, but for stars and nearby galaxies the approximation is exact.
Famous cosmic distances
| Object | Distance (ly) | Notes |
|---|---|---|
| Moon | 0.0000013 | Average Earth-Moon distance |
| Sun | 0.0000158 | Average Earth-Sun distance (1 AU) |
| Pluto (average) | 0.00059 | Average distance from Earth |
| Voyager 1 (2026) | ~0.0025 | Most distant human-made object |
| Proxima Centauri | 4.244 | Nearest star to the Sun |
| Alpha Centauri A/B | 4.37 | Nearest star system with Sun-like stars |
| Sirius | 8.60 | Brightest star in the night sky |
| Vega | 25.0 | Once the North Star; bright summer star |
| Betelgeuse | 700 | A red supergiant nearing end of life |
| Galactic Centre | 26,000 | Centre of the Milky Way |
| Milky Way diameter | ~100,000 | Disk of our galaxy |
| Large Magellanic Cloud | 160,000 | Largest satellite galaxy of Milky Way |
| Andromeda Galaxy (M31) | 2,537,000 | Nearest large spiral galaxy |
| Virgo Cluster | 53,500,000 | Nearest large galaxy cluster |
| Observable Universe radius | 46,500,000,000 | Edge of the observable universe |
Approximate distances used as benchmarks. Values are mid-point estimates from the International Astronomical Union and current literature.
Frequently asked questions
How many kilometres are in a light year?
Exactly 9,460,730,472,580.8 kilometres - commonly rounded to 9.461 trillion km. This comes from multiplying the speed of light (299,792,458 m/s) by the number of seconds in a Julian year (31,557,600 s) and dividing by 1,000.
Is a light year a unit of time or distance?
Distance. Despite the word "year" in its name, a light year measures how far light travels in one year - roughly 9.46 trillion kilometres. The confusion arises because the word "year" normally measures time, but here it is used as a yardstick for an enormous distance.
How many light years is it to the nearest star?
Proxima Centauri, the closest known star to the Sun, is approximately 4.244 light years away. At the speed of light that would take 4.244 years; at current spacecraft speeds it would take tens of thousands of years. The Alpha Centauri A and B binary pair is slightly farther at about 4.37 light years.
What is the difference between a light year, an astronomical unit and a parsec?
An astronomical unit (AU) is the average Earth-Sun distance (about 150 million km) and is used for distances within the solar system. A light year (ly) is about 63,241 AU and is used for interstellar distances. A parsec (pc) is about 3.26 light years and is based on the angle of parallax; professional astronomers often prefer parsecs because they arise directly from parallax measurements. All three units measure distance, just at different scales.
How long does it take light to travel one light year?
By definition, exactly one Julian year - 365.25 days, or 31,557,600 seconds. That is also roughly 8,766 hours. The light year is defined as this exact travel time, so the answer is always precisely one year.
Can I use this calculator to convert parsecs?
Yes. Select "Parsecs (pc)" in the "Input unit" drop-down, enter your value, and the calculator converts to light years, kilometres, miles, AU and metres. One parsec equals approximately 3.26156 light years or about 30.857 trillion kilometres.
How far is the Milky Way in light years?
The Milky Way disk is approximately 100,000 light years in diameter and about 1,000 light years thick. Our Sun sits roughly 26,000 light years from the galactic centre. The nearest large galaxy, Andromeda (M31), is about 2.537 million light years away.