Lightning distance calculator
Count the seconds from the flash until you hear thunder and enter them to see roughly how far away the strike was.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter seconds from flash to thunder and air temperature.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Distance = time × speed of sound, with the speed of sound in dry air ≈ 331.3 × √(1 + T ÷ 273.15) m/s, T in °C.
Example
Inputs: Seconds from flash to thunder = 5 s, Air temperature = 20 °C. Result: Distance to lightning: 1.72 km; In miles: 1.07 mi; In metres: 1,716 m.
Limits and caveats
- Counting by hand is the biggest error source; one second off is about 340 m.
- Wind and humidity change the speed of sound slightly; the formula is for dry air.
Questions about this tool
What is the quick rule for lightning distance?
Every 3 seconds is about one kilometre, and every 5 seconds about one mile.
When am I at risk from lightning?
If you can hear thunder you are close enough to be struck; go inside a building or a closed car.
Why do we see lightning before hearing thunder?
Light arrives almost instantly, while sound travels only about 343 m each second.