About the Light Year Converter
This light year converter translates astronomical distances between light-years, parsecs, astronomical units (AU), kilometers and miles. Enter a distance in any of those units and every other unit is shown at once, with scientific notation for the very large numbers that astronomy produces.
It is handy for students and amateur astronomers reading star catalogs (which often list parsecs), news stories quoting light-years, or space-mission material that measures the solar system in AU or kilometers. A light-year is a distance, not a time: it is how far light travels in one Julian year of 365.25 days.
The constants follow the International Astronomical Union: 1 AU is exactly 149,597,870.7 km (IAU 2012) and 1 parsec is 648,000/π AU (IAU 2015), about 3.2616 light-years.
With the default inputs, the light-years is 4.2465 ly. Change any value above to recalculate instantly.
How to use the light year converter
- 1Enter the distance.
- 2Select the unit it is in: light-years, parsecs, AU, kilometers or miles.
- 3Read the equivalent in every other unit.
- 4Use the scientific-notation line for very large values.
Formula and method
A light-year is the distance light travels in a vacuum in one Julian year (365.25 days × 86,400 s = 31,557,600 s) at exactly 299,792.458 km/s, giving 9.4607 × 10^12 km. The astronomical unit is fixed by IAU 2012 Resolution B2 at exactly 149,597,870.7 km, so 1 ly ≈ 63,241.08 AU.
A parsec is the distance at which 1 AU subtends one arcsecond, defined by IAU 2015 Resolution B2 as exactly 648,000/π AU ≈ 206,264.8 AU ≈ 3.26156 ly. The converter turns any input into kilometers and divides by each unit's size in km. One mile is exactly 1.609344 km.
- ly
- Light-year
- pc
- Parsec
- AU
- Astronomical unit (mean Earth–Sun distance)
Worked examples
Distance to Proxima Centauri (4.2465 ly)
4.2465 × 9.4607 × 10^12 km = 4.0175 × 10^13 km (about 40.17 trillion km). Dividing by 3.26156 ly per parsec gives about 1.302 pc, or roughly 268,553 AU.
One parsec
A parsec is 648,000/π = 206,264.806 AU. Multiplying by 149,597,870.7 km and dividing by 9.4607 × 10^12 km gives 3.2616 light-years.
Neptune at 30 AU
30 × 149,597,870.7 km = 4.488 billion km, which is only about 0.00047 light-years — light covers it in about 4.2 hours.
Frequently asked questions
How many kilometers are in a light-year?+
One light-year is 9,460,730,472,580.8 km, about 9.46 trillion kilometers, or roughly 5.88 trillion miles.
How many light-years are in a parsec?+
One parsec is about 3.26156 light-years. Astronomers prefer parsecs because they come directly from parallax measurements: a star with a parallax of 1 arcsecond is 1 parsec away.
Is a light-year a unit of time?+
No. Despite the name, a light-year is a distance — how far light travels in one year. To find how long light takes to cross a distance, the number of light-years equals the number of years.
How long would it take to travel one light-year?+
At the speed of the Parker Solar Probe's record of roughly 690,000 km/h, one light-year would take about 1,560 years. At highway speed of 100 km/h it would take about 10.8 million years.
What is an astronomical unit?+
An AU is roughly the average Earth–Sun distance, fixed by the IAU in 2012 at exactly 149,597,870.7 km. It is the standard unit for distances within the solar system.