how to calculate speed of light? 

The speed of light in vacuum, normally denoted by symbol c, is a universal physical constant which is very important in various areas of physics. Its exact value is 299,792,458 metres per second, a figure which is exact due to the length of the metre is considered from this constant and the international standard for time. In imperial units this speed is approx 186,282 miles per second.

As per special relativity, c is the maximum speed at where all matter, energy, and information in the universe can travel. It is exact speed at all massless particles & associated fields that including electromagnetic radiation such as light or travel in vacuum. It is also the speed of gravity for example of gravitational waves is predicted by current theories. Such particles or waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer. Theory of Relativity, c interrelates space & time, and appears in the famous equation of mass–energy equivalence E = mc2.