About the Lightning Distance Calculator
This lightning distance calculator turns the gap between seeing a lightning flash and hearing its thunder into a distance. Light arrives almost instantly, while sound travels at roughly 343 m/s, so every second of delay puts the strike about a third of a kilometre — or a fifth of a mile — farther away. Enter the seconds you counted and the air temperature, and you get the distance in kilometres, miles and metres.
It is handy for hikers, campers, sports coaches, pool lifeguards and anyone watching a storm roll in who wants to know whether it is approaching. Count again a minute later: if the gap is shrinking, the storm is moving towards you. The table gives a quick reference for common counts.
The speed of sound depends mainly on air temperature, so the calculator adjusts for it (about 331 m/s at 0 °C and 343 m/s at 20 °C). The estimate assumes still air and that you start counting at the flash; human reaction time, wind and the length of the lightning channel each add some uncertainty. Lightning can strike more than 10 miles from a storm, so if you can hear thunder at all, you are close enough to be struck.
With the default inputs, the distance to the strike is 3.43 km. Change any value above to recalculate instantly.
How to use the lightning distance calculator
- 1Start counting seconds the moment you see the flash.
- 2Stop counting when you first hear the thunder.
- 3Enter the seconds and the approximate air temperature.
- 4Read the distance in kilometres or miles.
- 5Repeat with later flashes — a shrinking count means the storm is approaching.
Formula and method
The lightning flash reaches you at the speed of light, effectively instantly over a few kilometres, while the thunder travels at the speed of sound. The distance is therefore the speed of sound multiplied by the delay you counted. The speed of sound in dry air rises with temperature: about 331.3 m/s at 0 °C, scaling with the square root of the absolute temperature, which gives roughly 343 m/s at 20 °C.
Dividing by 1,000 gives kilometres and by 1,609.344 gives miles. That leads to the field rules of thumb: about 3 seconds per kilometre and 5 seconds per mile. Humidity changes the speed by well under 1%, and wind can add or subtract a few metres per second, so treat the result as an estimate to within about 10%.
- d
- Distance to the lightning strike
- v
- Speed of sound in air (m/s)
- t
- Seconds between the flash and the thunder
- T
- Air temperature in °C
Worked examples
10 seconds on a 20 °C day
At 20 °C sound travels about 343.2 m/s, so 10 seconds means 3,432 m — about 3.4 km or 2.1 miles. That is well within striking range, so it is time to go inside.
5 seconds at 68 °F
68 °F is 20 °C, so the speed of sound is again about 343 m/s. Five seconds puts the strike roughly 1.7 km — just over a mile — away.
30 seconds on a freezing day
At 0 °C sound travels 331.3 m/s, so a 30-second delay means 9,939 m — about 9.9 km or 6.2 miles. Thunder that far away still means the storm can reach you.
Frequently asked questions
How do you calculate how far away lightning is?+
Count the seconds between the flash and the thunder, then divide by 3 for kilometres or by 5 for miles. For example, 15 seconds is about 5 km or 3 miles. This calculator refines that using the actual speed of sound at your temperature.
How far away can you hear thunder?+
Thunder is usually audible up to about 10 miles (16 km) away, depending on terrain, wind and background noise. Because lightning can strike that far from its parent storm, hearing thunder means you are within striking distance.
What is the 30-30 rule for lightning?+
The older 30-30 rule said to seek shelter if thunder follows lightning within 30 seconds and wait 30 minutes after the last thunder. Current US National Weather Service guidance is simpler: when thunder roars, go indoors, and wait 30 minutes after the last thunder.
Why does temperature change the result?+
Sound travels faster in warmer air, about 0.6 m/s faster for every 1 °C. Between a cold 0 °C day and a hot 35 °C day the speed changes by about 6%, which changes the distance by the same proportion.
Why do I see lightning but hear no thunder?+
So-called heat lightning is simply lightning too far away for its thunder to reach you, often more than 15–25 km. The storm is real; it is just distant or the sound is blocked by terrain or refracted upward.