The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its precise value is 299792458 metres per second (approximately 3.00×10⁸ m/s), since the length of the metre is defined from this constant and the international standard for time.

Read more here ---> https://gizmodo.com/how-the-speed-of-light-was-first-measured-1138348467

Here's an experiment to measure the speed of light --https://www.youtube.com/watch?time_continue=1&v=H9kZTm4Xm-8