About this tool
Lightning Distance Calculator is a free, in-browser tool that tells you how far away a storm is from the delay between seeing a lightning flash and hearing its thunder. Count the seconds from flash to bang, enter that number (and optionally the air temperature), and it returns the distance in kilometers, miles, meters and feet. Everything is computed on your device; nothing is uploaded.
Light reaches you almost instantly, but sound travels at roughly 343 m/s, so the delay reveals the distance. The well-known field rules are distance in kilometers ≈ seconds ÷ 3 and distance in miles ≈ seconds ÷ 5. For a precise figure the tool uses the actual speed of sound, 331.3 + 0.606 × temperature in °C meters per second, then multiplies by the seconds you counted, since colder air slows sound slightly.
This is a safety tool as much as a curiosity. Under the 30-30 rule, if you hear thunder within 30 seconds of the flash — under about 10 km or 6 miles away — you are within striking range and should seek shelter immediately, and wait 30 minutes after the last thunder before heading back out. When thunder roars, go indoors.