Speed of Sound Calculator
Work out distance based on how long sound takes to travel — for example distance to a lightning strike, or distance from an echo in air. Uses a fixed approximation of 343 m/s at about 20°C; not for underwater sonar.
How to use the speed of sound calculator
- Enter the number of seconds you measured — for example the time from when you saw the lightning to when you heard the thunder.
- Choose the correct measurement type: one-way (lightning/thunder) or round trip (echo in air).
- The calculator works out the distance based on the speed of sound in air.
How the distance is calculated
A well-known rule of thumb is to divide the number of seconds between lightning and thunder by 3 to get the distance in kilometers. That holds up well because sound travels at about 343 m/s, and 343 m/s × 3 seconds is nearly exactly 1 kilometer.
With an echo in air, the sound must travel a round trip — out to the object and back again — before it's registered. So we divide the result by 2 for round-trip measurements, giving the distance to the object rather than the total distance the sound traveled.
Example and measurement limits
At 343 m/s, a 6-second one-way delay corresponds to 2,058 m. For a 6-second echo the distance to the reflecting object is 1,029 m, because the sound travels both ways. A timing error of 0.1 seconds changes the one-way estimate by 34.3 m, or the echo estimate by 17.15 m.
This estimates distance; it does not measure the speed of sound or adjust it for weather. The fixed air value cannot be used for water or other materials. NASA explains how sound speed depends on the gas and temperature.
Frequently asked questions
Why do you divide the number of seconds by 3 to find the distance to lightning in km?
Because sound travels at about 343 m/s, and 343 m/s × 3 seconds gives nearly exactly 1,000 meters. Dividing the number of seconds by 3 is therefore a quick and good approximation for finding the distance to the lightning strike in kilometers.
Why is there a difference between one-way and round-trip measurement?
With a one-way measurement (like lightning and thunder), the sound only travels one way, from the source to you. With an echo in air, you send out the sound yourself, and it has to travel out to the object and back again before you hear it — double the distance — so the time has to be divided by 2 to find the correct distance.
How does temperature affect the speed of sound?
The speed of sound in air increases with temperature, because warmer air molecules move faster and transfer vibrations between each other more quickly. At low temperatures, the speed of sound drops accordingly, which makes calculations using a fixed value of 343 m/s less precise in winter.