Lightning Distance Calculator
The Lightning Distance Calculator tells you how far away a lightning strike is by using the delay between seeing the flash and hearing the thunder. Light reaches you almost instantly, so the gap is set by the speed of sound — about 343 metres per second. Enter the seconds you counted and it returns the distance in kilometres and miles, with a plain safety prompt.
Enter Values
Minimum 0
How to use this calculator
- Start counting the moment you see the lightning flash.
- Stop when you hear the thunder and enter that number of seconds.
- Read the distance and the safety status, then act on the 30/30 rule.
How it works
The distance sound travels is time × speed of sound (343 m/s). That gives metres, which are converted to kilometres (÷ 1000) and miles (÷ 1.609). If the result is 10 km or less the storm is treated as within strike range, prompting the 30/30 safety rule.
Worked example
Worked example. A 9-second gap gives 9 × 343 = 3087 m = 3.087 km, about 1.919 miles away — well within strike range.
Common mistakes
- Starting the count at the thunder instead of the flash.
- Assuming a distant strike means you are safe; storms move fast and the next strike can be much closer.
- Forgetting the 30/30 rule — shelter at a 30-second gap and wait 30 minutes after the last thunder.
Frequently asked questions
Why divide the seconds by 3 for kilometres?
Sound travels about 343 m per second, so roughly 3 seconds pass for each kilometre. Dividing the count by 3 gives a quick distance estimate.
Is a 10 km distance safe?
No. Lightning can strike more than 10 km from a storm's rain. If you can hear thunder you are within range — seek shelter.
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Cite this tool
Zerdly. (2026). Lightning Distance Calculator. Retrieved from https://www.zerdly.com/tools/lightning-distance-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.