Speed of Sound Calculator

Enter the air temperature to find the speed of sound in dry air, in meters per second, kilometers per hour, miles per hour, knots and feet per second. Add a time delay to find how far sound travels in that time, which is how you estimate the distance to a lightning strike from the gap before the thunder, or the distance to a cliff from an echo. A chart shows how the speed rises with temperature.

a = √(γ × R × T)

Here γ = 1.4 is the ratio of specific heats for air, R = 287.05 J/(kg·K) is the specific gas constant for dry air, and T is the absolute temperature in kelvins (degrees Celsius plus 273.15). These are the values used in the U.S. Standard Atmosphere and the ISO standard atmosphere, which give 340.29 m/s (761 mph) at sea level and 15 °C.

Light arrives almost instantly, so the delay before the thunder is the time sound takes to reach you. At 20 °C, a 5-second gap means the strike was about 1.7 km (1.1 miles) away. The old rule of counting seconds and dividing by 3 for kilometers or by 5 for miles works because sound covers roughly a kilometer in 3 seconds. The National Weather Service advises that if you can hear thunder at all, you are close enough to be struck, so go indoors.

Water vapor is lighter than the nitrogen and oxygen it replaces, so humid air carries sound very slightly faster, by up to a few tenths of a percent at warm temperatures. This calculator uses dry air, which is accurate enough for most purposes. Other gases have different values of γ and R: sound travels about three times as fast in helium, which is why breathing it raises the pitch of your voice. In water, sound travels at roughly 1,500 m/s, and in steel at around 5,000 to 6,000 m/s.

An echo travels to a surface and back, so the distance to the surface is half the distance sound covers in the measured time. Clap near a cliff at 20 °C and hear the echo after 2 seconds, and the cliff is about 343 meters away. The same principle, using sound or ultrasound, underlies sonar on ships, parking sensors on cars and medical ultrasound imaging, though those work in water or tissue where sound is much faster.

An aircraft's Mach number is its speed divided by the local speed of sound. Because cruising altitudes are much colder than the ground, Mach 1 at 11 km, where the standard temperature is −56.5 °C, is about 295 m/s, well below the sea-level value. For converting speeds to Mach at standard altitudes, use the Mach number calculator, and for how motion shifts the pitch of a sound, the Doppler effect calculator.