How far away was that lightning?
Tap the big button at the flash, tap again at the thunder. You get the distance instantly, and after a few strikes you'll know if the storm is heading your way.
The quick rule: count to 5 for a mile
Light from a strike reaches you almost instantly. Thunder is slower: sound covers about 343 metres a second in mild air. So the gap between the two tells you the distance.
| Seconds counted | Miles | Kilometres | What to do |
|---|---|---|---|
| 3 | 0.6 | 1.0 | Get inside now |
| 5 | 1.1 | 1.7 | Get inside now |
| 10 | 2.1 | 3.4 | Get inside now |
| 15 | 3.2 | 5.1 | Get inside now |
| 30 | 6.4 | 10.3 | Still in range, go in |
| 45+ | 9.6+ | 15+ | Thunder may be too far to hear |
If you can hear thunder, you're in range
The National Weather Service puts it simply: when thunder roars, go indoors. Lightning regularly strikes several miles from the rain, and can reach 10 miles or more. A house or a hard-topped car is safe; a tent, picnic shelter or tree is not. Wait 30 minutes after the last thunder before heading back out.
Source: US National Weather Service lightning safety.
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How do you work out how far away lightning is?
Count the seconds between the flash and the thunder. Sound travels about 343 metres a second in mild air, so every 5 seconds is roughly 1 mile and every 3 seconds roughly 1 kilometre. Light arrives almost instantly.
Is the 5-second rule accurate?
It is close. Sound is a little slower in cold air and a little faster in warm air. Add the temperature on this page and it corrects for that, which changes the answer by a few percent.
How close is too close?
The US National Weather Service says that if you can hear thunder, you are close enough to be struck. Lightning can reach 10 miles or more from the storm. Go inside a building or a hard-topped car.
When is it safe to go back outside?
Wait 30 minutes after the last thunder you heard. This page starts that countdown for you each time you log a strike.
Can I tell if a storm is coming closer?
Yes. Log three or more strikes a minute or two apart. If the gaps between flash and thunder are getting shorter, the storm is heading your way, and the page estimates how fast.
Updated November 11, 2026.
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