How the light speed distance formula works
Light moves at a constant speed in a vacuum, so the distance it covers is simply its speed multiplied by the travel time. This is the same idea as distance = speed × time you learned in school, just with a very big speed.
distance = speed of light × time
c = 299,792.458 km/s = 186,282 miles/s
Worked example
How far does light travel in one hour? Multiply 299,792 km/s by 3,600 seconds: 299,792 × 3,600 = 1.079 billion km (about 671 million miles), also known as one light-hour.
Years of travel and light-years
A light-year is the distance light covers in one Julian year of 365.25 days, about 9.4607 trillion km (5.879 trillion miles, or 63,241 AU). So if light travels for a number of years, the distance in light-years is the same number: choose years as the time unit above and the light-year figure equals what you typed. This is also why a star's distance in light-years tells you how old the light you see from it is.
Worked example: Proxima Centauri
Light from Proxima Centauri, the nearest star to the Sun, travels for about 4.24 years to reach us, so it is 4.24 light-years away: 4.24 × 9.4607 trillion ≈ 40.1 trillion km. The light you see tonight left the star in 2022.
Frequently asked questions
How far does light travel in one second?
About 299,792 kilometers (186,282 miles) in a vacuum, fast enough to circle Earth about 7.5 times in a single second.
How far does light travel in one year?
About 9.46 trillion kilometers (5.88 trillion miles). This distance is called one light-year.
What is the speed of light?
Exactly 299,792,458 meters per second, or about 186,282 miles per second (roughly 671 million miles per hour).
How far does light travel in one minute?
About 17.99 million kilometers (11.18 million miles), known as one light-minute.
How is light-travel distance calculated?
Multiply the speed of light by the time: distance = c × time. One hour equals 299,792 × 3,600 ≈ 1.079 billion km.
How far is a light-year in miles?
About 5.88 trillion miles, the distance light covers in a year at 186,282 miles per second.
Is a light-year a measure of time or distance?
Distance. It is how far light travels in one year, about 9.46 trillion km, even though the name contains a unit of time.
How long does light from the nearest star take to reach Earth?
About 4.24 years. Proxima Centauri is 4.24 light-years away, so its light has been travelling that long when it reaches us.