How Far Away Is Lightning? The Flash-to-Bang Method, Explained
Count the seconds between the flash and the thunder to estimate how far away lightning is in miles — and why distance matters more than a storm count.
Count the seconds: flash-to-bang
Light reaches you almost instantly, but sound travels at roughly one mile every five seconds. So the simplest way to estimate how far away lightning struck is to start counting the moment you see a flash and stop when you hear the thunder.
Divide the number of seconds by five to get the distance in miles. A 10-second gap means the strike was about two miles away. A 5-second gap means roughly one mile — close enough that you should already be indoors.
This is the flash-to-bang method, and the National Weather Service teaches it as a quick field estimate. It is not precise to the foot, but it reliably tells you whether a storm is closing in or pulling away.
Why distance beats a city-wide count
Most weather apps tell you a storm is somewhere over your city, or give you a count of strikes in a wide region. That does not answer the question that actually keeps you safe: how close is the nearest strike to me, right now, and is it getting closer?
Distance is the signal that maps to a decision. Two miles and approaching means take cover now. Eight miles and moving away means the danger is passing. A raw count can spike while the storm is still 20 miles out, or stay low as a single cell bears down on you.
That is why StrikeCast puts the nearest-strike distance in miles front and center, with an approaching or moving-away trend, instead of burying it under a count. The number you see is the number you act on.
How a tracker measures it for you
Counting works for one storm you can see and hear. But lightning is dangerous up to about 10 miles away — often before you hear any thunder at all. That is where a tracker helps.
StrikeCast uses NOAA GOES-GLM satellite data, which detects lightning across the entire United States in near real time. The app continuously compares each new strike to your location and shows the distance to the closest one, rounded to a realistic figure given satellite accuracy.
The trend arrow does the math you cannot do by ear: it compares recent strikes to tell you whether the storm is approaching or moving away, so you get a heads-up before the first close strike rather than after.
See the live distance to the nearest strike and get free alerts when lightning approaches — powered by NOAA satellite data. Never shared, never tracked.
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Frequently asked
How do you calculate how far away lightning is?
Count the seconds between the flash and the thunder, then divide by five for the distance in miles. Sound travels about one mile every five seconds, so a 10-second gap means the strike was roughly two miles away.
Is lightning 10 miles away dangerous?
Yes. Lightning can strike up to about 10 miles from the parent storm, sometimes from a part of the sky that looks clear. The National Weather Service advises seeking shelter as soon as you can hear thunder.
How does StrikeCast measure distance to lightning?
StrikeCast uses NOAA GOES-GLM satellite data to detect strikes across the US, then shows the distance in miles to the nearest one plus whether it is approaching or moving away.
More guides on the StrikeCast lightning safety hub.