Lightning Distance Calculator
Tells you where the last strike was. Not the next one.
Work out Lightning Distance. Tells you where the last strike was. Not the next one. Free, with no account and nothing to install.
Distance to the strike
3.09 km
1.92 miles · sound travels at 343.4 m/s at 20 °C
Light arrives effectively instantly, so the whole interval is the sound travelling. At 20 °C that is 343 m/s, which makes each second almost exactly a third of a kilometre — the origin of the familiar rule. The speed of sound rises with temperature by about 0.6 m/s per degree, so a strike in the Arctic and one in the tropics at the same interval are genuinely different distances. The correction is a few percent and the arithmetic is free. The 30-30 rule is the one worth remembering: shelter if the interval is under 30 seconds, and stay sheltered for 30 minutes after the last thunder. Most lightning deaths happen before the storm arrives or after people think it has passed.
How the Lightning Distance Calculator works
Distance to a strike from the flash-to-bang interval, corrected for air temperature, with the divide-by-three rule shown alongside and whether the storm is closing.
Also known as: how far away is the lightning · count seconds between thunder and lightning · flash to bang rule · when is it safe after a thunderstorm
Frequently asked questions
How do I work out how far away lightning is?
Count the seconds between the flash and the thunder and multiply by the speed of sound — about 343 m/s at 20 °C, or roughly a third of a kilometre a second.
Is the divide-by-three rule accurate?
Within a few percent across ordinary temperatures. Cold air slows sound, so the same interval means a shorter distance in winter — a small correction that is free to make.
How close is too close?
If you can hear thunder at all, you are within range. The standard threshold is 10 km, and strikes routinely land 10 to 15 km from the parent storm, well outside the rain.
What is the 30-30 rule?
Shelter if the flash-to-bang interval is under 30 seconds, and stay sheltered for 30 minutes after the last thunder. Most deaths happen before a storm arrives or after people think it has passed.
Does this tell me if I am safe?
No. It tells you where a strike already landed, and the next one is the one that matters. Lightning kills people standing in sunshine, which is why the interval is a warning rather than a measurement of safety.
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