Lightning Distance from Thunder Calculator

This calculator estimates how far away a lightning strike was from the time between seeing the flash and hearing the thunder. Enter the seconds counted and the air temperature; you get the distance in kilometres and miles.

Formula

Light arrives almost instantly, so the delay is the time sound takes to travel. Distance = time × speed of sound, and in dry air the speed of sound is about 331.3 × √(1 + T ÷ 273.15) m/s, with T in °C (343 m/s at 20 °C).

t: seconds between flash and thunder
T: air temperature, °C

Worked Example

6 seconds at 20 °C.

Speed of sound = 331.3 × √(1 + 20 ÷ 273.15) = 331.3 × 1.03596 = 343.2 m/s
Distance = 6 × 343.2 = 2,059 m = 2.06 km
2,059 ÷ 1609.344 = 1.28 mi

Limits and common mistakes

Rule of thumb: 3 seconds per kilometre, 5 seconds per mile.
Counting by voice is only accurate to about a second, so treat the result as roughly ±0.3 km.
If you can hear thunder at all, the storm is close enough to strike; go indoors and wait 30 minutes after the last thunder.

Formula used

in t | Seconds from flash to thunder | s | 6 | 0 | 300
in tc | Air temperature | °C | 20 | -40 | 50
out km | Distance | km | t * 331.3 * sqrt(1 + tc / 273.15) / 1000 | 2
out mi | Distance | mi | km * 1000 / 1609.344 | 2
out c | Speed of sound used | m/s | 331.3 * sqrt(1 + tc / 273.15) | 1

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Photo collage: a lightning bolt over a dark field at night and storm clouds. — Save this card: a 1000 × 1500 image of Lightning Distance from Thunder Calculator.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.

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