Thunderstorm Distance Calculator
The flash-to-bang method is a simple and reliable way to estimate how far away a thunderstorm is. When you see a lightning flash, start counting seconds until you hear the thunder. Divide that count by 5 to get the distance in miles (or divide by 3 for kilometers). The National Weather Service recommends seeking shelter immediately if a storm is within 6 miles (30 seconds or less between flash and bang). Enter the seconds between lightning and thunder to see the storm distance and safety status.
Flash-to-bang formula
Distance (miles) = Seconds / 5
Distance (km) = Seconds / 3
Sound travels approximately 1,100 feet per second (335 m/s) at typical thunderstorm conditions. This gives 5,280 feet per mile / 1,100 ft/sec = 4.8 seconds per mile, rounded to 5 for simplicity. For kilometers: 1,000 m / 335 m/s = approximately 3 seconds per kilometer.
NWS 30/30 rule for lightning safety
- 30 seconds or less (6 miles or closer): seek shelter immediately indoors or in a hard-topped metal vehicle.
- Wait 30 minutes after the last thunder before resuming outdoor activities.
- Avoid open fields, hilltops, trees, metal objects, and water during a thunderstorm.
- Lightning can strike 10+ miles from a storm's visible center, ahead of the rain.
- "When Thunder Roars, Go Indoors" is the official NWS lightning safety guidance.
Thunderstorm distance: frequently asked questions
How does the flash-to-bang method work?
The flash-to-bang method calculates thunderstorm distance by counting the seconds between seeing a lightning flash and hearing the corresponding thunder, then dividing by 5 to get miles (or dividing by 3 to get kilometers). Sound travels approximately one mile every 5 seconds (1 km every 3 seconds) at typical atmospheric conditions. Light reaches you virtually instantaneously.
What is the safe distance from a thunderstorm?
The NWS uses the '30/30 Rule' as a minimum safety guideline: if the time between lightning and thunder is 30 seconds or less (meaning the storm is 6 miles away or closer), immediately take shelter. Wait 30 minutes after the last thunder before resuming outdoor activities. 'When thunder roars, go indoors' is the NWS slogan.
Why does thunder have a delay but lightning is instantaneous?
Light travels at approximately 186,000 miles per second, so even a storm 10 miles away reaches you in 0.00005 seconds, imperceptible to humans. Sound travels at approximately 1,100 feet per second (about 0.21 miles per second) at sea level in standard conditions, creating a delay of about 5 seconds per mile. The delay between flash and bang is entirely due to the difference in propagation speeds.
Does temperature or altitude affect the calculation?
Yes, slightly. The speed of sound varies with temperature: at 0 C it is approximately 331 m/s, and it increases by about 0.6 m/s per degree Celsius. At 20 C, sound travels about 343 m/s. The 5-second per mile rule assumes roughly sea-level, moderate temperature conditions. The error from temperature variation is generally less than 5% under typical thunderstorm conditions.
Can thunder be heard from very distant storms?
Thunder is typically audible from storms within about 10-15 miles under quiet conditions. At greater distances, sound is absorbed and scattered by the atmosphere. Detecting lightning flashes from storms beyond 50-100 miles is possible visually, but thunder will not be heard from those distant storms. Lightning detectors and radar are needed for distant storm tracking.
Official sources
- National Weather Service: Lightning Safety: How Far Away?.
- NOAA Lightning Safety: Lightning Safety.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.