Mach 1 is the speed at which an object is traveling at the same speed as the speed of sound in air, which is approximately 761 miles per hour or 1,225 kilometers per hour. The Mach number is a ratio of the speed of an object to the speed of sound in the undisturbed medium through which the object is traveling. Therefore, an object traveling at Mach 2 is flying at twice the speed of sound in air, and so on. It is important to note that the speed of sound varies with altitude and temperature, so the speed of Mach 1 can vary depending on the location.