The Correct Way to Estimate Lightning Strike Distance

The 30/30 Rule

Many of us have heard that if you determine the distance lightning is from you by counting the number of seconds between hearing thunder and seeing a lightning strike, that is how far away in miles the lightning is (not true). This notion is actually called “flash-to-bang” method because of the time between seeing and hearing lightning.  Flash-to-bang is real, but not in the way we have traditionally believed.

Statistics show that the majority of people who are struck by lightning are struck before or after a thunderstorm, not during it. Given that lightning injuries often last a lifetime, it is wise to know with scientific accuracy just how far away lightning really is from where you are when a thunderstorm is clearly present.

Lightning comes first and subsequently, thunder.  Seeing and hearing lightning and thunder can be used to protect yourself and others if you understand the 30/30 Rule.  The first 30 is 30 seconds.  If it takes less than 30 seconds to hear thunder after seeing a lightning flash, the probability of being struck by lighting is great because the lightning is within 6 miles of your location. At this time one should seek proper shelter from the storm and remain in that shelter for 30 minutes (the second 30) after hearing the last thunder.

The following table is the correct Flash-to-Bang estimations calculated based on the speed of sound at sea level:


If thunder is heard The lightning is . . .
5 seconds after a flash 1 mile away
10 seconds after a flash 2 miles away
15 seconds after a flash 3 miles away
20 seconds after a flash 4 miles away
25 seconds after a flash 5 miles away
30 seconds after a flash 6 miles away

NOAA and the National Weather Service recommend seeking immediate shelter when lightning is 6 miles or less of your location.  To download a free copy of our wallet-sized lightning safety card click HERE.

Author: Michael