Lightning Distance Calculator
Tap when you see the lightning and again when you hear the thunder to find out how far away the storm is, and whether it’s getting closer.
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Tap Flash when you see lightning.
This storm
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How to tell how far away lightning is
- Tap Flash the moment you see the lightning.
- Tap Thunder when you hear it. The distance appears straight away.
- Time a few strikes: if the gap is getting shorter, the storm is coming towards you.
- No phone to hand? Count the seconds and divide by 3 for kilometres, or by 5 for miles.
How it works
Light arrives almost instantly, but sound travels at about 343 metres a second (at 20 °C), so every 3 seconds between the flash and the thunder is about a kilometre, and every 5 seconds about a mile. Sound is a little slower in cold air and faster in warm air, so the calculator adjusts for the temperature you enter. Your reaction time adds a little error, so treat the result as a guide.
The US National Weather Service’s advice is simple: when thunder roars, go indoors. Lightning can strike 10 miles (16 km) or more from a storm, often farther than you can hear thunder, so hearing it at all means you’re in range. Safe shelter is a building with wiring and plumbing or a hard-topped car with the windows shut, not a tent, shed or tree. Stay in for 30 minutes after the last thunder: the calculator counts this down from the last strike you timed.
The strikes you time stay on this page only; nothing is saved or sent.
Sources
Checked on 18 October 2026.
- NOAA JetStream: Lightning safety when thunder roars, go indoors; wait 30 minutes after the last thunder.
- National Weather Service: Lightning rules safe shelter, and how far from a storm lightning can strike.
Questions about lightning
How do you work out how far away lightning is?
Count the seconds between the flash and the thunder. Divide by 3 for kilometres or by 5 for miles: a 9-second gap is about 3 km (almost 2 miles).
How close is lightning dangerous?
If you can hear the thunder at all, you can be struck. Lightning can strike 16 km (10 miles) or more from the storm, so go indoors as soon as you hear it.
How long should I wait after a storm?
Thirty minutes after the last thunder you hear, as the back of a storm can still produce lightning.
Is a car safe in a thunderstorm?
A hard-topped car with the windows shut is safe, because the metal body carries the current around you. Convertibles, bikes and open vehicles aren’t.