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.
Speed at common temperatures
At T = 20°C: c = 331.3 × √1.0732 ≈ 343.2 m/s — about 1235 km/h or 767 mph.
| Temperature | Speed (m/s) | km/h | mph |
|---|---|---|---|
| −10°C | 325.4 | 1172 | 728 |
| 0°C | 331.3 | 1193 | 741 |
| 15°C | 340.3 | 1225 | 761 |
| 20°C | 343.2 | 1235 | 767 |
| 30°C | 349.1 | 1257 | 781 |
| 40°C | 354.9 | 1278 | 794 |
The speed rises by roughly 0.6 m/s per degree Celsius — so moving from a cold winter morning at 0°C to a hot summer afternoon at 30°C increases the speed by about 18 m/s.
Why temperature (not pressure) controls the speed
Sound travels by molecular collisions transferring kinetic energy. Warmer air molecules move faster on average, so they collide more frequently and the pressure wave propagates more quickly. Air pressure at constant temperature has almost no direct effect on the speed: doubling pressure also doubles density, and those two changes cancel in the wave equation. This is why the speed of sound at sea level and at altitude (same temperature) is essentially the same — altitude changes pressure but not temperature directly.
Humidity has a small positive effect — moist air is slightly less dense than dry air at the same conditions — but the difference is modest at practical humidity levels and this calculator models dry air.
Practical applications
Estimating lightning distance
When lightning strikes, the flash travels at the speed of light (effectively instant). The thunder travels at the speed of sound. Count the seconds between the flash and the bang:
- Divide by 3 to get the distance in kilometres (at typical outdoor temperatures, sound covers about 343 m/s)
- Divide by 5 to get the distance in miles
For example, a gap of 6 seconds means the lightning was roughly 2 km (1.2 miles) away.
Audio engineering and room acoustics
At 20°C, sound takes about 2.9 ms to travel one metre. Knowing this, audio engineers set speaker delay lines and calculate early reflection arrival times in room acoustics. An open-air stage where the monitor speaker is 5 m from the performer introduces a delay of about 14.6 ms — enough to cause comb filtering if not compensated.
Sonar and ultrasound
Speed-of-sound calculations underpin ranging for sonar, parking sensors, and medical ultrasound — though those operate in liquids (water, tissue) where the speed is much higher and depends on different material properties, not covered by this calculator.
All calculations run in your browser; nothing is uploaded.