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).