Lightning Distance Calculator
Estimate how far away lightning struck by counting the seconds until you hear thunder.
About the Lightning Distance Calculator
Count the seconds from the flash to the thunder and enter the air temperature. The calculator gives how far away the lightning is and whether you're within striking distance.
Light arrives almost instantly, but sound travels at about 343 m/s at 20 °C, so each 3 seconds is about 1 km and each 5 seconds about 1 mile. The US National Weather Service advises: when thunder roars, go indoors, and stay inside until 30 minutes after the last thunder.
The formula
distance = speed of sound × seconds; speed = 331.3 × √(1 + T ÷ 273.15) m/s
Worked example
9 seconds at 68 °F (20 °C)
- Sound travels about 343 m/s
- 9 × 343 = 3.09 km (1.92 miles)
- Within striking distance: go indoors
Questions people ask
How far away is safe?
If you can hear thunder, you're close enough to be struck.
Is a car safe?
A hard-topped metal vehicle is; convertibles and bikes aren't.