How this calculator works
Light arrives almost immediately on weather-scale paths, while thunder travels at the speed of sound. Multiplying the flash-to-first-thunder delay by the chosen sound speed estimates distance to the audible channel. The light-travel time is neglected. Temperature, wind, echoes and an extended lightning channel affect the estimate. Thunder indicates nearby lightning; this calculation does not establish a safe outdoor distance.
Formula
d approximately equals c_sound * delay; delay = d/c_sound. A representative near-surface sound speed is 343 m/s around 20 C.
Worked example
A 3 s delay at 340 m/s gives 1020 m. For a source 1700 m away at that same sound speed, the delay is 5 s.
Frequently asked questions
Which thunder sound should I time?
Use the first audible thunder associated with the flash. Later rumbles can follow different paths.
Why does the common rule use five seconds per mile?
It is a rounded sound-travel estimate, suitable only for rough distance judgments.
Does a long delay mean it is safe to stay outside?
No. A storm can produce new strikes nearer to you; follow local lightning-safety advice.