The speed of sound is not a fixed number — in air it depends mainly on temperature. This calculator gives the speed in dry air for any temperature you enter, in m/s, km/h, and mph, and then works out how long sound takes to cross a distance you choose.
How it works
The accurate formula for dry air is:
c = 331.3 × √(1 + T / 273.15) m/s
where T is the temperature in °C and 273.15 converts to kelvin. The base value 331.3 m/s is the speed at 0°C. A widely used linear approximation, c ≈ 331.3 + 0.606·T, is also shown and stays close to the exact value across everyday temperatures.
Travel time over a distance d is simply time = d ÷ c.
Example
At T = 20°C:
c = 331.3 × √(1 + 20/273.15) = 331.3 × √1.0732 ≈ 343.2 m/s- That is about 1235 km/h or 767 mph.
- Sound crosses 343 m in 343 ÷ 343.2 ≈ 1.0 second.
| Temperature | Speed (m/s) | km/h | mph |
|---|---|---|---|
| 0°C | 331.3 | 1193 | 741 |
| 20°C | 343.2 | 1235 | 767 |
| 30°C | 349.1 | 1257 | 781 |
This is why counting the gap between a lightning flash and the thunder, then dividing by about 3, gives the distance in kilometres. All calculations run in your browser; nothing is uploaded.