Lightning Safety Distance Calculator
Calculate your distance from lightning using the flash-to-bang method.
How many seconds between flash and thunder equals distance in miles or km.
The flash-to-bang method uses the difference in the speed of light and sound to estimate how far away a lightning strike occurred.
Physics: Light travels ~300,000 km/s (essentially instant to human perception). Sound travels ~343 m/s at sea level in typical conditions. The delay between the lightning flash and the thunder you hear represents the time for sound to travel the distance.
Formula:
- Distance (km) = seconds / 3
- Distance (miles) = seconds / 5
The 30-30 Rule (lightning safety):
- Seek shelter if thunder occurs within 30 seconds of the flash (strike is within 10 km / 6 miles)
- Wait 30 minutes after the last thunder before going back outside
Why 30 minutes? Lightning can strike up to 10 miles (16 km) from the storm center — “bolts from the blue” can arrive even under clear skies if a storm is nearby. The storm must fully pass before the risk subsides.
Safe shelters: Substantial buildings with wiring and plumbing (they conduct and ground strikes). Hard-top vehicles with windows closed (the metal shell, not the rubber tires, provides protection).
Unsafe locations: Open fields, hilltops, under isolated trees, near bodies of water, in open structures (tents, picnic shelters, dugouts).
Lightning speed: A lightning strike completes its return stroke in 0.2–0.3 milliseconds. The main channel reaches temperatures of 30,000 K (5× the surface of the Sun).
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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