Lightning Distance Calculator
Turn the delay between a lightning flash and its thunder into the distance to the strike — the classic "flash-to-bang" count.
How it works
distance = speed of sound × delay · speed ≈ 331.3 + 0.606·T (m/s, T in °C)
Light from a lightning strike reaches you almost instantly, but its thunder travels at the speed of sound — about 343 m/s at 20 °C — so it lags behind by roughly three seconds for every kilometre, or five seconds per mile. Counting the "flash-to-bang" interval and multiplying by the speed of sound therefore gives the distance to the strike. Because sound speeds up slightly in warmer air, this calculator adjusts for temperature, though the effect is small over the range of a storm.
Worked example
At 20 °C the speed of sound is 331.3 + 0.606 × 20 ≈ 343 m/s. A 10-second delay between the flash and the thunder gives 343 × 10 = 3,430 m — about 3.4 km, or 2.1 miles. The rough rule "three seconds per kilometre" gives the same 3.3 km.
Sources: Lightning — Wikipedia
Frequently asked questions
How many seconds equal one kilometre or mile?
Sound covers roughly one kilometre in about 3 seconds and one mile in about 5 seconds. So divide the flash-to-bang count by 3 for kilometres, or by 5 for miles, to estimate the distance to the strike.
What is the 30/30 lightning safety rule?
If you hear thunder within 30 seconds of seeing the flash — meaning the strike is within about 10 km — go indoors. Then wait at least 30 minutes after the last thunder before going back outside, since strikes can occur far from the rain.
Why do you see lightning before you hear it?
Light travels about 300,000 km per second, so the flash reaches your eyes essentially instantly. Sound is nearly a million times slower, so the thunder arrives seconds later — and that delay is exactly what lets you gauge the distance.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.