Lightning Safety Distance Calculator

Count the seconds between seeing the flash and hearing the thunder. That gap is almost entirely the time sound needed to reach you, because the light arrived in a few microseconds and your reaction time is the only other thing in it.

The physics. Light covers the distance at roughly 300,000 km/s, which for anything you can see is instantaneous. Sound moves at about 343 m/s in air at 20°C, so it takes 2.9 seconds per kilometre or 4.7 seconds per mile. This calculator uses that speed directly, which is why the kilometre and mile figures it gives always convert into each other properly.

The rules of thumb, and why they differ. You will hear “divide by 3 for kilometres” and “divide by 5 for miles”. Both are deliberately conservative, and they are not consistent with each other: divide-by-3 puts the strike about 3% closer than it really is, divide-by-5 about 6% closer. That is the right direction to be wrong in when the answer decides whether you run for cover, so use them freely in your head. Just do not expect them to agree.

The 30-30 rule:

1. If thunder follows the flash within 30 seconds, the strike was inside about 10 km (10.3 km at the real speed of sound) and you are already within range of the next one. Get to shelter.
2. Wait a full 30 minutes after the last thunder before going back out.

Why the 30-minute half is the part people skip. Lightning routinely strikes 10 miles from the rain, which is where “a bolt from the blue” comes from: clear sky overhead, storm well past, and a strike anyway. In the US, most lightning deaths happen either just before the rain starts or after it has stopped, not during the downpour. Waiting out the half hour is dull, and it is the single most effective thing on this page.

What counts as shelter: a substantial building with wiring and plumbing, which route the current around you, or a hard-top vehicle with the windows shut. The metal shell does the work, not the tyres, so a convertible or a golf cart is no protection at all.

What does not: open ground, hilltops, isolated trees, anywhere near water, and open-sided structures. A picnic shelter, a dugout and a bus stop are all just a roof, and a roof does nothing.

Scale, for context. The return stroke lasts 0.2 to 0.3 milliseconds and the channel reaches about 30,000 K, five times the surface temperature of the Sun. The thunder is that channel’s air exploding outward and then collapsing back.