Count the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles).
Sound travels ~343 m/s, so ~3 s per km and ~5 s per mile. Light arrives effectively instantly.
Only for the flash you can actually pair with its thunder. In a storm producing several strikes a minute you will pair the wrong pair, and thunder from beyond about 15 km usually never arrives at all — refracted upwards by warmer air aloft, which is why a visibly close storm can be silent.
At 15 °C the rule says 3 s and the exact answer is 2.939 s — 2.1% high. It holds to within 5% up to 31.7 °C, then drifts.
Sound speeds up as air warms, so the count-to-three is really a count-to-2.9 on a hot day and a count-to-3.1 in frost.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Count the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles). Sound travels ~343 m/s, so ~3 s per km and ~5 s per mile.