Lightning Distance Calculator
Calculate how far away lightning struck by counting seconds between the flash and thunder.
Stay safe during storms.
Lightning Distance
Lightning distance is calculated using the time delay between seeing lightning and hearing thunder.
The science:
- Light travels at ~186,000 miles/sec (300,000 km/sec) — essentially instant
- Sound travels at ~1,125 ft/sec (343 m/sec) at sea level
- This means sound takes about 5 seconds per mile (3 seconds per kilometer)
The formula:
Distance (miles) = Seconds / 5
Distance (km) = Seconds / 3
The 30-30 rule for safety:
- If the delay is 30 seconds or less (6 miles / 10 km), seek shelter immediately
- Wait 30 minutes after the last thunder before going back outside
Lightning safety tips:
- No place outdoors is safe during a thunderstorm
- Avoid open fields, hilltops, and isolated trees
- Stay away from water, metal fences, and tall structures
- If indoors, avoid plumbing and wired electronics
- A car with a metal roof is relatively safe (not convertibles)
Temperature affects sound speed: Sound moves ~1.1 ft/sec faster per 1°F above freezing. The difference is small enough to ignore for safety purposes.