Lightning Distance Calculator
How far away lightning struck, from flash-to-thunder seconds.
About this tool
Tap the button when you see the flash and again when you hear the thunder, or type the seconds in. The tool converts the gap into a distance in kilometres and miles, adjusted for air temperature, and tells you whether you're within striking range.
The flash-to-bang method
Light from a lightning flash arrives almost instantly, but thunder travels at the speed of sound, about 340 metres per second. So every 3 seconds between flash and thunder is roughly 1 kilometre, and every 5 seconds roughly 1 mile. A 6-second gap means the strike was about 2 km (1.3 miles) away. The calculator uses the temperature-adjusted speed of sound for a little more accuracy. The 30-30 rule: if the gap is 30 seconds or less (about 10 km or 6 miles), you're within range of the next strike, so get inside a substantial building or a hard-topped vehicle, and wait 30 minutes after the last thunder before going back out. Honest limits: it tells you the distance of one strike, not where the storm is heading; lightning can strike 15 km or more from the storm's core; if you can hear thunder at all, no outdoor place is safe. This is not safety advice; follow your local weather service's warnings. The speed of sound calculator and the Beaufort scale converter are related.
Frequently asked questions
How do you calculate how far away lightning is?
Count the seconds from flash to thunder. Divide by 3 for kilometres or by 5 for miles.
How close is too close for lightning?
If thunder follows the flash within 30 seconds (about 10 km), you're at risk. Go indoors immediately.
How long should I wait after a storm?
Wait 30 minutes after the last thunder you hear before going back outside.
Why do you see lightning before hearing thunder?
Light travels about a million times faster than sound, so the flash arrives almost instantly and the thunder follows.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.