You see a flash of lightning and hear thunder several seconds later. That delay offers a simple distance estimate: count the seconds and divide by three to obtain approximate kilometres. The method can help you understand whether lightning activity is moving closer or farther away, but it does not replace official warnings or turn any particular distance into a safe boundary.
The quick rule: divide the seconds by three
At human scales, the light from a flash reaches you almost instantly. Sound travels much more slowly through air, at roughly 343 metres per second in conditions near 20°C. Thunder therefore covers about one kilometre every three seconds.
Approximate distance in kilometres = seconds between lightning and thunder ÷ 3.
- 3 seconds: about 1 km.
- 9 seconds: about 3 km.
- 15 seconds: about 5 km.
- 30 seconds: about 10 km.
For example, an interval of 12 seconds suggests that the discharge was about 4 km away: 12 ÷ 3 = 4.
How to time the interval
- Start counting as soon as you see the flash.
- Stop when you hear the beginning of the thunder, not when the rumble ends.
- Repeat with several flashes, but only from a protected place.
- Watch the trend: shorter intervals suggest that recent discharges are closer; longer ones may indicate that they are moving away.
Each result describes only one discharge. A storm covers a broad area, so the next strike can occur much closer or in a different direction. Clouds, mountains and buildings can also reflect sound and stretch or distort the rumble.
Why the result is only an estimate
The speed of sound varies slightly with air temperature and other atmospheric conditions. Wind can alter how well sound travels, and the first part of the thunder is not always easy to identify. It is therefore sensible to round the result rather than claim metre-level accuracy.
The calculation estimates the distance from you to the discharge that produced that thunder. It does not measure the distance to the edge or centre of the storm and cannot predict where the next strike will occur.
If you hear thunder, stop measuring and seek shelter
Do not use the interval to decide how long you can stay outdoors. Audible thunder means electrical activity is close enough to require caution. Move into a substantial enclosed building or a fully enclosed vehicle, and avoid water, ridges, open ground, isolated trees, poles and towers.
MeteoSwiss’s official thunderstorm recommendations advise avoiding bodies of water, exposed terrain and prominent objects, and planning outdoor activities around available shelter and alternative routes. See our practical guide to staying safer during a thunderstorm outdoors for the next steps.
Combine the count with radar and warnings
The flash-to-thunder interval is a local, immediate observation. Radar shows how precipitation is moving, while official warnings describe hazards expected or observed across a wider area. Each answers a different question, so use them together.
Check current warnings before setting out, monitor developments during your activity and do not wait for a short interval before seeking protection. A storm can change course, intensify quickly or produce lightning outside the most obvious rain area.
In brief
Count the seconds from the flash to the start of thunder and divide by three for approximate kilometres. Repeat only from shelter and use the sequence to understand the trend. The key limitation remains: the distance to the last strike tells you nothing certain about the next one. When thunder is audible, check current information and take cover.