Calculate how far away a lightning strike is by counting the number of seconds from when you initially saw the lightning flash until you heard the lightning strike. A typical thunderstorm is about 15 miles in diameter and people usually begin hearing the sound of thunder when the storm is about 10 miles away (the distance lightning can strike from its parent thunderstorm). According to NOAA, there have been confirmed strikes as much as 50 miles away from the thunderstorm. So the rule of thumb is: if you can hear thunder, stay indoors!
The sound of thunder can travel at about 1200 feet per second (or 350 meters) on a warm day (82 degrees).