Lightning Distance Calculator
How far the strike was, from the count and the air temperature — because five seconds to the mile is right in the cold and 7% out in the warm.
This is a distance, not a safety margin. A strike can reach many kilometres from the cloud that made it, so the fact that the last one was far away says little about where the next one lands. Nothing is uploaded.
The count measures the air as much as the distance
Sound travels at 331.3 m/s at freezing and rises with the square root of absolute temperature — pressure cancels out, so altitude barely matters and cold does. The familiar "five seconds to the mile" is almost exactly right at -20 °C (5.05 s) and about seven percent optimistic at 20 °C, where a mile takes only 4.69.
| Air | Speed of sound | Per km | Per mile | A 5-count is |
|---|---|---|---|---|
| -20 °C | 318.9 m/s | 3.14 s | 5.05 s | 1595 m |
| -10 °C | 325.2 m/s | 3.08 s | 4.95 s | 1626 m |
| 0 °C | 331.3 m/s | 3.02 s | 4.86 s | 1657 m |
| 10 °C | 337.3 m/s | 2.96 s | 4.77 s | 1687 m |
| 20 °C | 343.2 m/s | 2.91 s | 4.69 s | 1716 m |
| 30 °C | 349.0 m/s | 2.87 s | 4.61 s | 1745 m |
| 40 °C | 354.7 m/s | 2.82 s | 4.54 s | 1774 m |
So the same five-count means 1595 m on a freezing night and 1774 m on a hot afternoon — 179 metres of difference from nothing but the air. The kilometre version of the rule fares better: three seconds is right to within a hundredth at freezing, and merely a little slow when warm.
The flash, meanwhile, is instantaneous for every practical purpose
Light is 873 thousand times faster than sound, so it covers a kilometre in about 3.34 millionths of a second. Even at ten kilometres the flash's own travel time is under a ten-thousandth of a second — thousands of times smaller than the twitch in your thumb on the stopwatch.
| Count | Distance | In miles | "Divide by 3" says |
|---|---|---|---|
| 1 s | 0.34 km | 0.21 mi | 0.33 km |
| 2 s | 0.69 km | 0.43 mi | 0.67 km |
| 3 s | 1.03 km | 0.64 mi | 1.00 km |
| 5 s | 1.72 km | 1.07 mi | 1.67 km |
| 10 s | 3.43 km | 2.13 mi | 3.33 km |
| 15 s | 5.15 km | 3.20 mi | 5.00 km |
| 30 s | 10.30 km | 6.40 mi | 10.00 km |
Which means the count is a measurement of sound alone, and the only real error in it is your reaction time and the air temperature. The "divide by three" rule is consistently a little short in warm air, for the same reason the mile rule is: it was set at a colder temperature than most thunderstorms happen in.
How to use
- Count the seconds between the flash and the thunder.
- Enter the air temperature.
- Read the distance in kilometres and miles.
- Compare it with what the rule of thumb would have said.
Frequently asked questions
How far away was the lightning?
Multiply the count by the speed of sound. At 20 C that is 343 m/s, so a five-second count is about 1.7 km — just over a mile.
Is five seconds to the mile right?
In cold air, almost exactly — 5.05 seconds at minus 20 C. In warm air it is about seven percent optimistic: at 20 C a mile takes 4.69 seconds, so the rule puts the strike closer than it really is.
Why does temperature change it?
The speed of sound rises with the square root of absolute temperature. Pressure cancels out of the physics, which is why altitude barely matters and the air temperature does.
How much difference does that make?
The same five-count means 1595 m on a freezing night and 1723 m at 40 C — well over a hundred metres, from nothing but the air you are standing in.
Is the three-seconds-per-kilometre rule better?
It is exact at freezing, to within a hundredth of a second, and only a little slow when warm. Of the two familiar rules it is the more robust, though both were set at temperatures colder than most thunderstorms.
Does the flash take time to arrive too?
Technically, and never enough to matter. Light is about 873,000 times faster than sound, so it covers a kilometre in three millionths of a second — thousands of times smaller than your reaction on the stopwatch.
Does a big count mean I am safe?
No, and this is worth being plain about. It is a distance, not a safety margin. A strike can reach many kilometres from the cloud that made it, so where the last one landed says little about the next.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.