Lightning Strike Distance Calculator
When you see a lightning flash, start counting seconds until you hear the corresponding thunderclap. This is the flash-to-bang interval. Because sound travels at approximately 1,125 feet per second (343 m/s) at typical temperatures, you can estimate the lightning strike distance by dividing the time in seconds by 5 to get miles, or by 3 for kilometres. This is the standard flash-to-bang method recommended by NOAA and the National Lightning Safety Council. The calculator also shows you whether the storm is within the NOAA 30-second danger threshold.
Flash-to-bang formula
Distance (miles) = t / 5
Distance (km) = t / 2.94 (exact: t × 0.343)
Where t is the flash-to-bang time in seconds. The speed of sound at 20 deg C is 343 m/s = 1.125 ft/s. 1 mile = 1.609 km; 1,609 / 343 = 4.69 seconds per km (exact flash-to-bang for km: divide by 2.94). The rule-of-thumb divides by 3 for km as a convenient approximation.
NOAA 30-30 lightning safety rule
- 30 seconds or less (6 miles / 10 km or closer): Seek shelter immediately in a substantial building or hard-topped metal vehicle. Lightning can strike even in areas where it is not raining.
- 31 to 60 seconds: Storm is 6 to 12 miles away. Conditions are changing rapidly. Move toward shelter now.
- Over 60 seconds: Storm is more than 12 miles away. Monitor conditions; move to shelter if the interval decreases.
- Wait 30 minutes: After the last thunder, wait at least 30 minutes before resuming outdoor activities.
Lightning distance calculator: frequently asked questions
How does the flash-to-bang method work?
The flash-to-bang method uses the time difference between seeing a lightning flash and hearing the corresponding thunderclap. Sound travels at approximately 1,125 feet per second (343 m/s) at sea level at 20 deg C. Dividing the elapsed time in seconds by 5 gives the distance in miles; dividing by 3 gives an approximate distance in kilometres. NOAA and the National Lightning Safety Council recommend this method for estimating lightning proximity.
What is the 30-30 rule for lightning safety?
The 30-30 rule from NOAA: if the time between lightning and thunder is 30 seconds or less (about 6 miles or 10 km), seek shelter immediately. Stay sheltered for at least 30 minutes after the last thunder is heard. This rule is part of the NOAA Lightning Safety Awareness Week guidance and the American Meteorological Society policy statement on lightning safety.
Why does thunder sometimes not accompany lightning?
Thunder is always produced by lightning but may not be heard if the strike is more than 10 to 15 miles (16 to 24 km) away. Beyond that distance, the sound is too weak to hear. This distant lightning without audible thunder is called heat lightning (a colloquial term). NOAA notes that lightning can strike up to 10 miles from the nearest rain.
Does altitude affect the speed of sound?
Yes. The speed of sound decreases with temperature. At sea level and 20 deg C (68 deg F) it is 343 m/s (1,125 ft/s). At higher altitudes where temperatures are lower, sound travels slightly slower. For practical lightning safety purposes, the variation is small and the standard flash-to-bang formula (divide seconds by 5 for miles) is accurate enough.
When is it safe to go back outside after lightning?
NOAA recommends waiting at least 30 minutes after the last clap of thunder before resuming outdoor activities. Lightning can strike even after the rain has stopped and the sky appears to be clearing. The safest shelter is a substantial building or a metal-roofed vehicle with the windows up.
Official sources
- NOAA Lightning Safety: weather.gov/safety/lightning.
- National Lightning Safety Council: lightningsafety.com.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.