Lightning Storm Distance Calculator
Light from a lightning flash reaches you almost instantly, but the thunder travels at the speed of sound, about 343 metres per second in typical air. By counting the seconds between the flash and the bang you can work out how far away the strike was. This tool turns your flash-to-bang count into distance in kilometres and miles, and lets you adjust the speed of sound for temperature if you wish. The classic rule of thumb is that every 3 seconds is roughly one kilometre, or every 5 seconds is roughly one mile. The National Weather Service advises seeking shelter when thunder follows lightning by 30 seconds or less.
Lightning distance formula
distance (m) = seconds * speed of sound
distance (km) = distance (m) / 1,000
distance (miles) = distance (km) / 1.609344
The flash arrives effectively instantly because light travels about 900,000 times faster than sound. The thunder's travel time, multiplied by the speed of sound, gives the distance to the strike. The speed of sound rises by roughly 0.6 metres per second for each degree Celsius, so it is a user-editable input; 343 m/s corresponds to about 20 degrees Celsius.
Worked example
You see a flash and count 10 seconds until the thunder. At 343 m/s the sound travelled 10 * 343 = 3,430 metres, which is 3.43 km. Dividing by 1.609344 gives 2.13 miles. By the rule of thumb, 10 seconds is about 3.3 km or 2 miles, which agrees. With thunder this soon after the flash, the storm is close enough to warrant taking shelter.
Frequently asked questions
Why does counting seconds work for lightning?
Light travels at about 300 million metres per second, so the flash reaches you with no perceptible delay. Sound travels at only about 343 metres per second, so the thunder lags by a time proportional to the distance. Multiplying that delay by the speed of sound gives the distance to the lightning channel.
Is the five-seconds-per-mile rule accurate?
It is a good approximation. One mile is 1,609 metres, and at 343 m/s sound covers that in about 4.7 seconds, so five seconds per mile is close. The three-seconds-per-kilometre version is similarly accurate. This calculator gives the exact figure for the speed of sound you enter.
When is a thunderstorm dangerous?
The National Weather Service uses the 30-30 rule: if you count 30 seconds or fewer between flash and thunder, the storm is within about 10 kilometres and lightning can reach you, so move indoors. Wait at least 30 minutes after the last thunder before going back outside.
Does temperature change the result?
Yes, slightly. The speed of sound increases with air temperature, by roughly 0.6 metres per second per degree Celsius. On a hot day sound travels a little faster, so a given count maps to a slightly greater distance. Adjust the speed-of-sound input to match conditions if you need precision.
Official sources
- NOAA National Weather Service: lightning safety.
- NOAA National Severe Storms Laboratory: lightning basics.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.