Speed of light to millimeters per second conversion calculator shown above calculates how many millimeters per second are in 'X' speed of light (where 'X' is the number of speed of light to convert to millimeters per second). In order to convert a value from speed of light to millimeters per second (from c to mm/s) just type the number of c to be converted to mm/s and then click on the 'convert' button.