About
Speed of light c = 299,792,458 m/s exactly. Distances: 1 light-second = ~300,000 km. 1 light-year = 9.46 × 10¹² km. Sun to Earth: 8.3 light-min. Alpha Centauri: 4.3 ly. Milky Way diameter: ~100,000 ly.
Formula
Frequently asked questions
What is the exact speed of light?
The speed of light in a vacuum is exactly 299,792,458 metres per second, often rounded to 3 x 10^8 m/s. This is not a measured value with uncertainty but a defined constant: since 1983 the metre itself has been defined as the distance light travels in 1/299,792,458 of a second. In everyday units that is about 300,000 km per second or roughly 186,282 miles per second.
How is the distance light travels calculated?
Multiply the speed of light by the elapsed time: distance = c x time. The only catch is units. Convert the time to seconds first, then multiply by 299,792,458 m/s to get metres. The calculator does this for any time unit from nanoseconds to years and then converts the result into kilometres, miles, astronomical units, and light-years.
What is a light-year?
A light-year is the distance light travels in one year, about 9.461 x 10^15 metres or 9.46 trillion kilometres. It is a unit of distance, not time, despite the word year. Astronomers use it because interstellar distances are enormous: the nearest star system, Alpha Centauri, is about 4.3 light-years away, meaning its light takes 4.3 years to reach us.
How long does light take to reach Earth from the Sun?
About 8 minutes and 20 seconds, roughly 8.3 light-minutes, since the Sun is on average 1 astronomical unit (about 150 million km) away. This means you always see the Sun as it was eight minutes ago. Light from the Moon, only about 384,000 km away, reaches Earth in about 1.3 seconds.
Does light travel slower than c in glass or water?
Yes. The 299,792,458 m/s figure is the speed in a vacuum, which is the universe's maximum speed. In a medium light slows by the refractive index: about 0.75c in water and 0.67c in typical glass. This calculator uses the vacuum speed, which is the right value for space travel, astronomy, and radio signals through air or space.
About the speed of light
The speed of light in a vacuum, written c, is the fastest anything can travel in the universe: exactly 299,792,458 metres per second. Because that exact figure now defines the metre, it carries no measurement uncertainty. This calculator uses it to answer two everyday questions: how far does light travel in a given time, and conversely how long does light take to cross a given distance. Enter a time from nanoseconds to years and it returns the distance in metres, kilometres, miles, astronomical units, and light-years.
The numbers quickly become astronomical, which is exactly why scientists measure cosmic distances in light-travel time. Light crosses the roughly 384,000 km to the Moon in about 1.3 seconds, takes 8.3 minutes to reach Earth from the Sun, and needs 4.3 years to arrive from the nearest star system. A light-year, the distance light covers in a year, is about 9.46 trillion kilometres.
How the calculation works
distance = c x time c = 299,792,458 m/s (exact, by definition) 1 light-second = ~299,792 km 1 light-minute = ~1.799 x 10^7 km 1 light-year = 9.461 x 10^15 m = ~9.46 trillion km 1 AU = ~1.496 x 10^11 m (Sun to Earth)
- Time is converted to seconds first, then multiplied by c.
- c is the vacuum speed; light slows in glass or water but this tool uses the vacuum value.
- Light-year and AU are units of distance, used because metres become unwieldy at cosmic scale.
- Outputs are given in several units so you can pick whichever fits the context.
Worked example
How far does light travel in one minute?
- Convert time to seconds: 1 minute = 60 seconds.
- Multiply by c: 299,792,458 m/s x 60 s = 17,987,547,480 metres.
- Convert to kilometres: divide by 1,000 to get about 17,987,547 km, roughly 18 million km.
- Sense check: that is about 0.12 astronomical units, so light covers about one eighth of the Earth-Sun distance in a minute.
Light-travel times across space
| Distance | Light travel time | Approx distance |
|---|---|---|
| Earth to Moon | about 1.3 seconds | 384,000 km |
| Sun to Earth | about 8.3 minutes | 150 million km (1 AU) |
| Sun to Neptune | about 4.2 hours | 4.5 billion km |
| Sun to Alpha Centauri | about 4.3 years | 4.3 light-years |
| Across the Milky Way | about 100,000 years | 100,000 light-years |
Common pitfalls
- Treating a light-year as time. It is a distance, the distance light covers in a year, not a duration.
- Forgetting to convert time to seconds. Multiplying c by minutes or hours directly gives the wrong answer.
- Using the vacuum speed inside a medium. Light slows in glass (about 0.67c) and water (about 0.75c); this tool assumes a vacuum.
- Confusing the AU with the light-year. One AU is the Earth-Sun distance; a light-year is roughly 63,000 times larger.
- Expecting instant signals. Even light has a finite speed, so radio commands to distant spacecraft take minutes or hours to arrive.