Lightning Distance Calculator (flash-to-thunder seconds to km & miles)

Understanding the lightning distance calculator — concept and principle

This calculator estimates how far away lightning struck from the number of seconds between seeing the flash and hearing the thunder. It adjusts the speed of sound for air temperature and also shows the quick counting rules. It is useful outdoors, for weather spotters and for teaching how sound travels.

Light arrives almost at once; sound covers about 343 m each second, so 9 s is roughly 3 km.

How it is calculated

Distance = speed of sound × seconds

Speed of sound c ≈ 331.3 × √(1 + T ÷ 273.15) m/s

Quick rules: seconds ÷ 3 ≈ km; seconds ÷ 5 ≈ miles

T is the air temperature in °C; distance is shown in km and miles.

The principle

Light from the flash arrives almost instantly, taking about 3 microseconds per kilometre. Thunder travels at the speed of sound, roughly a million times slower. So the delay you count is essentially the travel time of the sound, and multiplying by the speed of sound gives the distance. Sound travels faster in warmer air because the molecules move faster. The rules of thumb assume about 333 m/s for kilometres and about 322 m/s for miles.

Worked example

Default inputs: 9 seconds, 20 °C.

Speed of sound = 331.3 × √(1 + 20/273.15) = 343.2 m/s.
Distance = 343.2 × 9 = 3,089 m = 3.09 km, or 1.92 miles.
Quick rules: 9 ÷ 3 = 3 km; 9 ÷ 5 = 1.8 miles.
Safety: if you can hear thunder, go indoors now.

Each point is the calculator re-run with only “Seconds between the flash and the thunder” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Seconds between the flash and the thunder	Distance to the lightning (km)
2.25	0.77
4.5	1.54
6.75	2.32
9	3.09
11.2	3.86
13.5	4.63
18	6.18

How to read the result

The US National Weather Service advises that if you can hear thunder you are close enough to be struck. Go indoors or into a hard-topped vehicle, and wait 30 minutes after the last thunder before going back out. A shrinking count means the storm is approaching. The count usually measures the nearest part of the lightning channel, not necessarily the ground strike point.

Common mistakes

Starting to count late, after the flash.
Treating a long count as "safe"; lightning can strike far from the storm.
Using the result to decide to stay outside.

Keep in mind: There is no safe place outdoors during a thunderstorm. If you hear thunder, seek shelter immediately, whatever the calculated distance.

Frequently asked questions

How far away can thunder be heard?

The National Weather Service says usually about 10 miles (16 km).

Why divide by 3 for kilometres?

Sound travels about 1 km in 3 seconds.

Does temperature matter much?

Only a little; the speed of sound changes by about 0.6 m/s per °C.

How long should I wait after a storm?

The National Weather Service advises 30 minutes after the last thunder.

Sources: US National Weather Service, "Understanding Lightning: Thunder" and "Lightning Safety Overview" · O. Cramer, J. Acoust. Soc. Am. 93, 2510–2516 (1993)

Checked 9 Nov 2026. US National Weather Service, “Understanding Lightning: Thunder” (sound travels about a mile in 5 seconds; thunder can be heard only about 10 miles away) and “Lightning Safety Overview” (if you hear thunder you are likely within striking distance; wait 30 minutes after the last thunder). Speed of sound in air: O. Cramer, J. Acoust. Soc. Am. 93 (1993) 2510–2516, as used by our speed-of-sound calculator.

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