The dark band is the same count off by a second in either direction. Nobody counts a storm to better than that, which is why a single figure is the wrong output and why this page shows you the range it is really telling you.
Every temperature a person could be standing in, end to end. The line is not flat, but it is close enough to flat that the input you are most likely to get wrong is the one on the stopwatch.
“Divide by 3 for kilometres, by 5 for miles” is stated as a matched pair. It is not one. At 15 °C sound covers a kilometre in 2.94 seconds and a mile in 4.73. So dividing by 3 is out by about 2.0%, and dividing by 5 is out by about 5.4% — more than twice as far, in the same direction.
Both underestimate, which is the right direction to be wrong in: the storm is always at least as far away as the rule says, never nearer. But if you want the mile version to be as good as the kilometre one, the divisor is not 5 — it is 4.7, which nobody is going to do in their head during a thunderstorm. That is the real reason the kilometre form is the one to trust: 3 happens to be a much better rounding of 2.94 than 5 is of 4.73.
Sound is slower when it is cold, and the tool above lets you set the temperature because everybody expects it to. Here is what it buys: across the entire range from −10 °C to 40 °C, the seconds-per-kilometre figure moves it by 9.1%. One second of error on a nine-second count moves it by 11.1%.
So the input the page cannot see is worth more than the input it asks you for — and one second is a generous estimate of how well anyone counts while watching a storm. That is why the answer above is given as a band. A count is worth about half a kilometre either way, and no amount of decimal places changes that.
Light, for completeness, contributes nothing at all: it crosses ten kilometres in 33 microseconds, about one part in a million of the sound’s travel time. Every version of this rule that mentions light arriving “instantly” is right.
Nobody needs to know a storm is 7.2 km away rather than 6.8. The count answers one question: are you already in range? National weather services give the same answer in the same form — the 30/30 rule. If the flash-to-bang is under thirty seconds, go inside. Wait thirty minutes after the last thunder before coming back out.
Thirty seconds is 10.2 km, and it is not an arbitrary round number: strikes are recorded reaching 16 km from the parent storm, well outside the rain, which is where the phrase “a bolt from the blue” comes from.
And the asymmetry that matters most: thunder stops arriving before lightning stops reaching you. Beyond roughly fifteen kilometres, thunder is usually refracted upwards by warmer air aloft and never gets to you at all. So a storm can be visibly close, completely silent, and still in range. “I cannot hear anything” is not the test.
This page is arithmetic, not safety advice. The 30/30 guidance above is quoted from the people whose job it is; nothing here derives it, and nothing here should be used to talk yourself into staying outside.
This page has changed in 1 archived release, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.