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Light Speed calculator
Explore Einstein's mass-energy relation and the speed of light: convert mass to energy or find speed, distance and time.
Light Speed
E = mc² · c = distance/time · Relativity
E = m × c² | c = distance / time | c = 299,792,458 m/s
Light's speed (about 299,792 kilometres per second in vacuum) is the universe's speed limit and astronomers' measuring tape. This calculator converts between light's travel time and distance, the two quantities every space headline quietly uses.
How to use this calculator
- Enter a time to see how far light travels in it.
- Or enter a distance to see how long light takes to cross it.
- Press calculate to see the result, in everyday and astronomical units.
Why astronomers measure in time
Distances in space defeat ordinary units, so they are stated as light-travel times: the Moon is about 1.3 light-seconds away, the Sun about 8 light-minutes, the nearest star over 4 light-years. That is also why we always see space objects as they were, never as they are.
Frequently asked questions
What is the exact speed of light?
299,792,458 metres per second in vacuum, exact by definition since the metre is now defined from it. In air it is a fraction slower.
How long does sunlight take to reach Earth?
About 8 minutes 20 seconds on average. Looking at the Sun is looking 8 minutes into its past (never look at it directly).
Can anything travel faster than light?
Nothing carrying information or matter is known to, and relativity's tested framework forbids it. Apparent exceptions (shadows, light spots) carry no information and break nothing.