The speed of sound is the distance travelled per unit time by a sound wave propagating through an elastic medium. The SI unit of speed is the metre per second (m/s). In dry air at 20 °C, the speed of sound is 343.2 metres per second (1,126 ft/s). This is 1,236 kilometres per hour (768 mph; 667 kn), or a kilometre in 2.914 s or a mile in 4.689 s.