Speed of Light Calculator
How long does light take to travel any distance? Explore the cosmic speed limit at 299,792.458 km/s.
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Cosmic Distances
Speed of Light
299,792.458
kilometers per second
186,282
miles/s
670.6M
mph
The speed of light in a vacuum is exactly 299,792.458 km/s (about 186,282 miles/s).
Nothing with mass can travel at the speed of light — it would require infinite energy.
Einstein's special relativity (1905) established that the speed of light is the same for all observers.
Light takes about 8 minutes and 20 seconds to travel from the Sun to Earth.
A light-year is about 9.461 trillion kilometers (5.879 trillion miles).
In fiber optic cables, light travels at about 2/3 the speed of light in vacuum.
The speed of light defines the meter: it is the distance light travels in 1/299,792,458 of a second.
When you look at the Andromeda Galaxy, you see it as it was 2.5 million years ago.
Special Relativity
In 1905, Albert Einstein published his theory of special relativity, establishing that the speed of light in a vacuum (denoted c) is the universal speed limit.
The famous equation E = mc² shows that energy and mass are interchangeable, and that accelerating an object with mass to the speed of light would require infinite energy.
As an object approaches the speed of light, time dilation occurs: time passes slower for the moving object relative to a stationary observer. At light speed, time would stop entirely.