Speed of Sound Calculator
| Air temp | m/s | The day |
|---|---|---|
| -20 C | 319.2 | hard frost, sound drags |
| 0 C | 331.3 | the textbook freezing point |
| 15 C | 340.4 | the ICAO standard atmosphere |
| 20 C | 343.4 | a warm room - the everyday 343 |
| 30 C | 349.5 | hot afternoon, sound sprints |
The speed of sound is not one number - it is the speed of a pressure wave through whatever the air is doing, and air temperature is what sets it: about 331.3 m/s at freezing, 343 m/s at a comfortable 20 C, and 349 m/s on a 30 C day. Type a temperature and the converter gives the sound speed in m/s, km/h, mph and ft/s; type an object speed and it reads the Mach number against that same air.
The dependence is on temperature alone - not pressure, not humidity worth mentioning - because sound speed tracks how fast molecules jiggle, and temperature is the jiggle. That is why the NWS's own calculator needs one input, why jets measure Mach instead of mph (the speed of sound at 35,000 feet is about 295 m/s in the cold thin air, so a plane at the same mph is a very different Mach up there), and why a crack of thunder reaches you later on a cold night than a hot afternoon.
How to use
- Type the air temperature in Celsius or Fahrenheit - the sound speed appears in m/s, km/h, mph and ft/s instantly.
- Type a speed to get its Mach number against the current sound speed - the standard that actually matters for aircraft.
- Count thunder the standard way: three seconds from flash to rumble is about one kilometer, five seconds about a mile.
Frequently asked questions
How fast is the speed of sound?
At 20 C - the standard lab and everyday answer - about 343 m/s, which is 1,235 km/h or 767 mph. But the honest answer always carries a temperature tag: 331.3 m/s at 0 C, 340 m/s at 15 C, and roughly 295 m/s at cruising altitude where the air is fifty degrees colder. There is no single speed of sound any more than there is a single temperature of air.
Why does temperature change the speed of sound?
Sound is molecules bumping their neighbors, and that relay runs at the speed the molecules already move - hotter air, faster molecules, faster relay. The standard formula for ordinary air is v = 331.3 + 0.606 T in meters per second with T in Celsius, which is exactly the relationship the NWS calculator uses. Humidity nudges it by well under one percent; pressure barely at all, because denser air is also stiffer in exactly the ratio that cancels.
What is Mach number?
The ratio of your speed to the local speed of sound - Mach 1 is the sound speed at your altitude and temperature, not a fixed 767 mph. A jet at Mach 0.8 at 35,000 feet is going about 655 km/h of true airspeed in the cold thin air, while the same Mach 0.8 at sea level on a hot day is faster in every unit. That is why aircraft instruments care about Mach and road signs never do.
Does sound travel faster in water or steel?
Water: about 1,480 m/s, four times air, because molecules there are packed close enough to pass the push almost instantly. Steel: around 5,900 m/s along a rail - the reason you hear a rail-tap twice, one arrival through the steel and a later one through the air. The tighter and stiffer the medium, the faster the relay, which is why the table here is air-specific.
Why does sound carry farther on cold nights?
Refraction, not raw speed: when air is coldest near the ground, sound bends upward and fades; when a temperature inversion puts warm air over a cold layer, sound bends back down and carries for miles. The speed difference itself is small - a few percent - but the bending it causes across layers is what lets you hear a distant highway at night and not at noon.