How is this calculated?
Lightning (light) arrives almost instantly, but thunder travels much more slowly, at about 343 meters per second (the speed of sound). This time difference can be used to roughly calculate how far away the lightning struck.
Handy rules of thumb
- About 3 seconds per km: if thunder arrives 3 seconds after the flash, the lightning struck about 1 km away.
- About 5 seconds per mile: in miles, that works out to roughly 5 seconds per mile.
β οΈ Safety Notice
If the gap between lightning and thunder is 30 seconds or less, that's a sign you're within lightning strike range, so you should head indoors to safety immediately. This calculator is for reference only β always follow official guidance from weather authorities during an actual storm.
Want more Earth science facts?
If you're curious about how Earth's rotation and revolution shape our seasons and calendar, check out Earth's Rotation and Revolution Facts too.
Frequently Asked Questions
Is this calculation always accurate?
Since the speed of sound varies slightly with temperature, this isn't a precise physical measurement, but it's a useful everyday estimate for gauging your distance from lightning.
Why do we see lightning before we hear the thunder?
Because the speed of light (about 300,000 km/s) is overwhelmingly faster than the speed of sound (about 343 m/s), even though the lightning and thunder happen at the same instant, we see the flash first and hear the thunder afterward.