Enter the time delay between lightning flash and thunder to find out how far away the storm is. Choose your preferred unit — kilometers or miles — and the calculator returns the storm distance based on the speed of sound. You can also input air temperature for a more precise result, since sound travels faster in warmer air.
Storm Distance
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Speed of Sound Used
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Safety Recommendation
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Distance in Miles
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Distance in Kilometers
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Divide the number of seconds between the lightning flash and thunder by 3. For example, if you count 9 seconds, the storm is approximately 3 km away. This works because sound travels roughly 343 meters (about 1/3 of a kilometer) per second at 20°C.
Divide the number of seconds between the lightning flash and thunder by 5. Sound travels approximately one mile every 5 seconds at average temperatures. So a 10-second delay means the storm is about 2 miles away.
The 30-30 rule states: if the time between lightning and thunder is 30 seconds or less (storm within ~10 km or 6 miles), seek shelter immediately. After the last thunder, wait 30 minutes before going back outside. This is the standard safety guideline recommended by meteorologists.
The speed of sound increases with air temperature — warmer air molecules vibrate faster, allowing sound to travel more quickly. At 0°C sound travels about 331 m/s, while at 35°C it reaches roughly 352 m/s. Using the actual temperature gives a more precise distance estimate.
Lightning itself travels at roughly one-third the speed of light — about 100,000 km/s (62,000 miles/s). This is why you see the flash almost instantaneously regardless of distance. Thunder, produced by the rapid heating of air, travels at the much slower speed of sound — around 340 m/s.
If you see lightning but cannot hear thunder, the storm is likely more than 20 km (12 miles) away — thunder typically cannot be heard beyond that distance. However, lightning can still travel horizontally up to 16 km (10 miles) from its parent storm, so you should take precautions even for distant storms.
Wait at least 30 minutes after the last clap of thunder before resuming outdoor activities. Lightning can strike several miles ahead of or behind a storm, so staying sheltered until well after the last thunder is heard is the safest approach.
Light travels at approximately 300,000 km/s, while sound travels at only about 0.343 km/s. This enormous speed difference means light from a lightning bolt reaches your eyes almost instantly, while the sound wave (thunder) takes roughly 3 seconds per kilometer (5 seconds per mile) to arrive.