Speed of Light Calculator

Your details

Choose which quantity to calculate. Enter the other two.
Light slows down in materials. Select a medium to automatically apply its refractive index, or choose custom to enter your own.
Distance to travel. Default is 1 AU (Earth-Sun average distance) in km.
Travel timeVacuum (c)
499.004784s

Time for light to cross the entered distance in the chosen medium.

Travel time (formatted)8 min 19.005 s
Speed in medium299,792,458m/s
Speed in medium299,792.458km/s
Fraction of c100%
Vacuum vs medium delayNo delay (vacuum)
Refractive index (n)1
100% c
Dense medium<50%Moderate medium50%-70%Fast medium70%-95%Near-vacuum / vacuum95%+

Travel time: 8 min 19.005 s

  • Light travels at 2.99792e+8 m/s in this medium (100.0000% of c).
  • Light travels at its maximum possible speed in vacuum.
  • At over a minute of travel time, this distance is comparable to the Earth-Sun gap (~499 s).

Next stepFor optical fibre networks, use n = 1.467. One-way propagation delay (microseconds) = distance (km) / 200. For two-way (round-trip) double the result.

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