OneTaskBy Tim King · Updated
Speed of Light Calculator
Light Travel Time and Distance in Any Medium
OneTask is free, with no sign-up, and built by one person.
If this tool saved you time, a coffee helps keep new ones coming.
How the Speed of Light Calculator works
This speed of light calculator tells you how long light takes to travel a distance, or how far it goes in a given time. Enter metres, kilometres, miles, astronomical units or light-years to get a travel time and round trip, or enter a time from nanoseconds to years to get a distance. It also handles light slowed down in air, water, optical fibre, glass and diamond.
Light is fast but not instant, and at large scales or high precision the delay matters. Sunlight is already eight minutes old when it reaches you, a signal to Mars takes several minutes each way, and in computing a nanosecond is only 30 cm of travel, which limits how fast networks and chips can talk. In glass fibre light runs about a third slower, which sets the floor on internet latency.
How the speed of light calculator works
The formula is time = distance ÷ speed, where the speed is c ÷ n. Here c = 299,792,458 m/s exactly and n is the refractive index of the medium. For a time input it runs the other way, distance = speed × time. A light-year is 9,460,730,472,580,800 m and an astronomical unit is 149,597,870,700 m, both defined values.
First example. One AU, the mean Sun–Earth distance, is 149,597,870,700 ÷ 299,792,458 ≈ 499 s, which is 8 min 19.0 s. The round trip is 16 min 38.0 s.
Second example. In optical fibre (n = 1.468) light covers 299,792,458 ÷ 1.468 × 0.001 ≈ 204.2 km in a millisecond, 68.12% of its vacuum speed.
How many seconds does light take to reach the Moon? 384,400 km ÷ 299,792.458 km/s ≈ 1.28 s.
How long light takes to cross everyday, planetary and galactic distances
| From and to | Distance | Light travel time |
|---|---|---|
| 1 nanosecond of travel | 29.9792 cm | 1 ns |
| Across a data centre (100 m) | 100 m | 333.564 ns |
| London to New York (5,570 km) | 5,570 km (3,461.04 miles) | 18.5795 ms |
| London to Sydney (16,990 km) | 16,990 km (10,557.1 miles) | 56.6725 ms |
| Once around the equator | 40,075 km (24,901.5 miles) | 133.676 ms |
| Earth to a geostationary satellite | 35,786 km (22,236.4 miles) | 119.369 ms |
| Earth to the Moon (average) | 384,400 km (238,855 miles) | 1.28222 s |
| Sun to Mercury | 0.387 AU (57,894,400 km) | 3 min 13.1 s |
| Sun to Venus | 0.723 AU (108,159,000 km) | 6 min 0.8 s |
| Sun to Earth | 1 AU (149,598,000 km) | 8 min 19.0 s |
| Sun to Mars | 1.524 AU (227,987,000 km) | 12 min 40.5 s |
| Sun to Jupiter | 5.203 AU (778,358,000 km) | 43 min 16.3 s |
| Sun to Saturn | 9.537 AU (1,426,710,000 km) | 1 h 19 min |
| Sun to Uranus | 19.19 AU (2,870,780,000 km) | 2 h 39 min |
| Sun to Neptune | 30.07 AU (4,498,410,000 km) | 4 h 10 min |
| Sun to Pluto | 39.48 AU (5,906,120,000 km) | 5 h 28 min |
| Proxima Centauri | 4.2465 light-years | 4.2465 years |
| Alpha Centauri A and B | 4.37 light-years | 4.37 years |
| Sirius | 8.6 light-years | 8.6 years |
| Vega | 25 light-years | 25 years |
| Centre of the Milky Way | 26,000 light-years | 26,000 years |
| Andromeda Galaxy | 2,500,000 light-years | 2,500,000 years |
Tips and pitfalls
- A light-year is a distance. It measures how far light goes in a year, not a length of time.
- Fibre is about a third slower. Allow roughly 5 µs per kilometre of cable, not 3.3 µs.
- Ping is a round trip. Network latency counts there and back, so double the one-way time.
- Planet distances change. Mars ranges from about 3 to 22 light-minutes away depending on the orbits.
FAQ
How fast is the speed of light?
In a vacuum light travels at exactly 299,792,458 metres per second, about 300,000 km/s or 186,282 miles per second. The figure is exact because the metre has been defined from it since 1983. In any material it is slower, divided by that material's refractive index.
How long does light from the Sun take to reach Earth?
On average about 8 minutes 19 seconds, since the Earth is one astronomical unit or about 149.6 million km away. The time varies from about 8 min 10 s in early January, when Earth is nearest the Sun, to about 8 min 27 s in early July.
How far is a light-year?
A light-year is the distance light travels in a Julian year of 365.25 days, which is 9,460,730,472,580,800 m, about 9.46 trillion km or 5.88 trillion miles. The nearest star beyond the Sun, Proxima Centauri, is about 4.24 light-years away.
How far does light travel in one second?
In a vacuum light travels 299,792.458 km in one second, enough to circle the Earth about 7.5 times. In a nanosecond it covers about 30 cm, a handy rule for electronics, and in a millisecond about 300 km, which is useful for estimating network delays.
Why is light slower in glass and water?
Light interacts with the electrons in a material, which delays it as it passes through. The refractive index n tells you how much: water has n ≈ 1.333, so light moves at about 225,000 km/s, and diamond at n ≈ 2.417 slows it to about 124,000 km/s, which is part of why diamonds sparkle.
See also: all Physics tools, Chemistry, Electronics.
Related tools
Frequency To Wavelength Calculator
Convert frequency and wavelength for radio, light and sound.
Energy Calculator
Kinetic, potential, electrical and mass energy in joules and kWh.
Map Distance Measurer
Click points on a map to measure a route in km or miles.
Time Zone Converter
Convert a date and time between any two cities.
"As Seen In" Strip
Build a muted "as seen in" row of press and media logos.