Lightning Distance Calculator
Count the seconds between the lightning flash and the thunder to find how far away it struck, allowing for the air temperature.
Sound travels a little faster in warm air. Leave as it is if you're not sure.
Distance to the lightning
2.06 km
- In miles
- 1.28 miles
- Speed of sound used
- 343.4 m/s
The lightning struck about 2.1 km away. If you can hear thunder, you're close enough to be struck: go indoors.
How it's worked out
- Speed of sound
331.3 + 0.606 × 20 °C343.4 m/s - Distance
343.4 m/s × 6 s2,061 m
- Quick rule: every 3 seconds is about 1 km, every 5 seconds about 1 mile.
- Shelter in a proper building or an enclosed car with a metal roof and the windows up, and stay there for 30 minutes after the last thunder.
How it is worked out
Light reaches you almost instantly, but sound takes time. The speed of sound in air is about 331.3 + 0.606 × temperature (°C) metres per second, and the distance is that speed times the seconds you counted.
Stay safe
Lightning can strike far from the storm itself. Don't wait for rain before going indoors: when you hear thunder, head inside.
Questions people ask
How far away is lightning if the thunder comes 6 seconds later?
About 2.1 km (1.3 miles) at 20 °C. Sound covers about 343 m a second at that temperature.
What's the quick rule?
Divide the seconds by 3 for kilometres, or by 5 for miles.
Why does temperature matter?
Sound travels a little faster in warm air: about 331 m/s at 0 °C and 343 m/s at 20 °C. The difference is small, so the quick rule works well enough.
Am I safe if the lightning is a few miles away?
No. If you can hear thunder, you're close enough to be struck. Go inside a proper building or an enclosed car with a metal roof and the windows up, and stay there for 30 minutes after the last thunder.
About this calculator
Formula and rules last checked 10 November 2026Written and tested by the CalculatorDex teamReport a problem
Sources: NOAA: Lightning safety · US National Weather Service: Lightning safety overview