The spee of light in the vacuum of free space is an important physical constant usually denoted by the letter c. It is the speed of all electromagnetic radiation, including visible light, in free space. It is the speed of anything having zero rest mass. The SI metre is defined such that the speed of light in a vacuum is exactly 299,792,458 metres per second (1,079,252,849 km/h). The speed of light can be assigned a definite numerical value because the fundamental SI unit of length, the metre, has been defined since October 21, 1983, as the distance light travels in a vacuum in 1/299,792,458 of a second; in other words, any increase in the measurement precision of the speed of light would refine the definition of the metre, but not alter the numerical value of c. The approximate value of 3×108 m/s is commonly used in rough estimates (the error is 0.07%). In imperial units, the speed of light is about 670,616,629.4 miles per hour or 983,571,056.4 feet per second (roughly one foot per nanosecond), which is about 186,282.397 miles per second.