What this calculator does
The speed of light in a vacuum is 299,792,458 metres per second, usually written as the constant c. It is not an approximation or a measured average that could later be revised; it is an exact, defined value, which is what makes it such a reliable yardstick for distances that are otherwise hard to picture.
This calculator uses that constant to convert a distance into how long light takes to cross it, or a time into how far light travels in it. Enter a distance in kilometres, million kilometres, astronomical units or light-years, or flip it around and enter a time instead.
The formula
Travel time is distance divided by c. Distance is c multiplied by time. The calculator converts whatever unit you choose into metres and seconds first, runs the one division or multiplication that matters, and converts the result back into the units that make sense at that scale.
| Term | Meaning |
|---|---|
| c | The speed of light in a vacuum: 299,792,458 metres per second, exact by definition. |
| AU | Astronomical unit, the average Earth-Sun distance, about 149.6 million kilometres. |
| Light-year | The distance light travels in one Julian year: about 9.46 trillion kilometres. |
The inputs explained
| Field | What to enter |
|---|---|
| Solve for | Choose whether you know a distance and want the travel time, or know a time and want the distance. |
| Distance (million km) | The distance, in whichever unit is selected below. |
| Distance unit | The unit the distance above is measured in. |
| Time (minutes) | The time, in whichever unit is selected below. |
| Time unit | The unit the time above is measured in. |
When to use it
Picturing an astronomical distance
Raw distances in kilometres or astronomical units are hard to feel. Converting a distance to a light travel time (light minutes to the Sun, light years to the nearest star) is often the more intuitive figure.
Checking a communication delay
Radio signals to spacecraft travel at the speed of light. The one-way delay to a probe at a known distance, or the round trip, comes straight out of the travel-time side of this calculator.
Working through a physics problem
Many introductory relativity and astronomy problems give a distance or a light travel time and ask for the other; this handles either direction without switching formulas by hand.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How light travel time changes with distance in the solar system
A range of distances, from the Moon out to the edge of the solar system, solving for time.
| Distance | Travel time (h:mm:ss) | Travel time in seconds |
|---|---|---|
| 0.384 million km | 0:01 (h:mm:ss) | 1.28088613 s |
| 150 million km | 8:20 (h:mm:ss) | 500.3461428 s |
| 780 million km | 43:22 (h:mm:ss) | 2,601.799943 s |
| 1,430 million km | 1:19:30 (h:mm:ss) | 4,769.966561 s |
| 4,500 million km | 4:10:10 (h:mm:ss) | 15,010.38428 s |
| 5,900 million km | 5:28:00 (h:mm:ss) | 19,680.28162 s |
How far light travels over different times
A range of times, solving for the distance light covers in each.
| Time | Distance travelled | In light-years |
|---|---|---|
| 1 s | 299,792.458 km | 0.00000003 ly |
| 60 s | 17,987,547.48 km | 0.00000190 ly |
| 3,600 s | 1,079,252,849 km | 0.000114 ly |
| 86,400 s | 25,902,068,370 km | 0.002738 ly |
| 31,557,600 s | 9,460,730,473,000 km | 1.0000 ly |
Questions
What is the speed of light?
Exactly 299,792,458 metres per second in a vacuum. It is a defined constant, not a rounded measurement, since 1983 the metre itself has been defined in terms of this value and the second.
How fast is the speed of light in km/h and mph?
About 1.079 billion km/h, or roughly 671 million mph. At that speed, light could circle the Earth’s equator about seven and a half times in one second.
Does light travel slower through air, glass or water?
Yes. The value above is the speed of light in a vacuum. Light slows down when passing through a medium, by a factor called the refractive index; glass and water slow it to somewhere between 60% and 75% of its vacuum speed.
Why use light-years instead of kilometres for astronomical distances?
Kilometre figures for interstellar distances have so many digits that they stop being meaningful at a glance. A light-year packs the same distance into a single number that also tells you how old the light you are seeing actually is.
For sound rather than light, and how temperature changes it, see the speed of sound calculator. For everyday speed, distance and time problems, see the speed, distance and time calculator.