Speed of Light Calculator
Convert speed of light to km/s, mph, and calculate travel time and distance at light speed.
What Is the Speed of Light?
The speed of light in vacuum is an exact physical constant in SI units: \(c = 299{,}792{,}458\) m/s. It is the maximum speed at which information or energy can travel through space. Astronomers, physicists, and engineers use it to convert distances into light-travel times and to express velocities in convenient units.
Common Conversions
$$c = 299{,}792.458\ \text{km/s}$$ $$c \approx 670{,}616{,}629\ \text{mph}$$ $$c \approx 63.24\ \text{AU per year}$$Travel time for a distance \(d\) is simply \(t = d / c\). For example, light from the Moon (about 384,400 km away) reaches Earth in about 1.28 seconds.
Frequently Asked Questions
Why is the speed of light exact?
The meter is defined from the speed of light since 1983, so \(c\) is fixed by definition and the meter is derived from it.
How do I calculate light travel time?
Divide distance by \(c\). Convert your distance to meters first, then \(t = d / 299{,}792{,}458\) seconds.
What is an astronomical unit per year?
It expresses how many Earth–Sun distances light travels in one Julian year (365.25 days). One light-year is about 63,241 AU.
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