Count the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles).
Sound travels ~340 m/s, so ~2.94 s per kilometre and ~4.73 s per mile. Light arrives effectively instantly — 33 microseconds over ten kilometres. The two divisors are not equally good: 3 is a close rounding of 2.94 and is out by 2%, while 5 is a poor rounding of 4.73 and is out by 5.4%. Trust the kilometre form; both underestimate, which is the safe direction.
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.
Estimate with the rule, then check it against the calculator that models it properly.
Open Lightning Distance →Count the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles). Sound travels ~340 m/s, so ~2.94 s per kilometre and ~4.73 s per mile.