Lightning Distance Calculator
Light reaches you almost instantly, but the sound of thunder lags behind at about 343 meters per second. Counting the seconds between the lightning flash and the thunderclap, then multiplying by the speed of sound, tells you roughly how far away the strike was. The common shortcut is to divide the seconds by 5 for miles or by 3 for kilometers. Enter the seconds you counted and this calculator returns the distance in kilometers and miles, along with a safety note based on the National Weather Service 30-30 rule.
Lightning distance formula
Distance (m) = seconds x speed of sound
Distance (km) = distance (m) / 1,000
Distance (miles) = distance (m) / 1,609.344
Rule of thumb: miles = seconds / 5, km = seconds / 3
The flash arrives effectively instantly. Multiplying the count of seconds by the speed of sound gives the distance the thunder traveled, which is the distance to the strike.
Lightning safety notes
- The NWS 30-30 rule: shelter if thunder follows the flash within 30 seconds (about 6 miles).
- Wait at least 30 minutes after the last thunder before resuming outdoor activity.
- The speed of sound rises slightly with temperature; 343 m/s suits about 20 C.
- Thunder is seldom heard beyond about 10 miles.
- When thunder roars, go indoors: no place outside is safe in a storm.
Lightning distance: frequently asked questions
How does the flash-to-bang method work?
Light from a lightning flash reaches you almost instantly, but sound travels much slower, about 343 meters per second. By counting the seconds between seeing the flash and hearing the thunder and multiplying by the speed of sound, you estimate the distance to the strike.
What is the rule for lightning distance?
A handy rule is to divide the seconds by 5 for the distance in miles, or by 3 for kilometers. So thunder 10 seconds after the flash means the strike was about 2 miles or 3.3 kilometers away. This calculator uses the speed of sound for a precise figure and shows the rule-of-thumb result too.
When is lightning close enough to be dangerous?
The National Weather Service advises the 30-30 rule: if you hear thunder within 30 seconds of the flash (about 6 miles), the storm is close enough to be dangerous, so seek shelter. Wait at least 30 minutes after the last thunder before going back outside.
Does the speed of sound change with conditions?
Yes, slightly. The speed of sound rises with air temperature, from about 331 meters per second at 0 degrees Celsius to around 343 at 20 degrees. This calculator uses 343 meters per second, a typical value, but the variation is small over the distances involved.
Why can I see lightning but not hear thunder?
Thunder is rarely heard beyond about 10 miles because the sound is refracted upward by temperature gradients and attenuated by the air. Distant lightning seen without thunder is sometimes called heat lightning. If you can see the flash but the storm is far, conditions can still change quickly.
Official sources
- NOAA National Weather Service: Lightning Safety.
- NIST physical constants: Fundamental Constants.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.