Feet per second to millimeters per second converter on this page calculates how many millimeters per second are in 'X' feet per second (where 'X' is the number of feet per second to convert to millimeters per second). In order to convert a value from feet per second to millimeters per second (from ft/s to mm/s) type the number of ft/s to be converted to mm/s and then click on the 'convert' button.