Lightning Distance Calculator — How Far Away Was the Lightning?
Count the seconds between a lightning flash and the thunder it produces, enter the air temperature for accuracy, and this calculator tells you the distance to the strike in kilometres and miles.
seconds
°C
Kilometres from you to where the bolt struck
- 1
Speed of sound
331.3 × √(1 + 20 ÷ 273.15) = 343.21The speed of sound in air rises with temperature. - 2
Distance (m)
5 s × 343.21 m/s = 1,716.1 - 3
Distance (km)
1,716.1 ÷ 1000 = 1.72
How does this calculator work?
Distance (km) = delay (seconds) × 0.343 (or more precisely, × 331.3 × √(1 + T/273.15) / 1000). Five seconds delay ≈ 1.7 km at 20 °C. Seek shelter if thunder is within 30 seconds of the flash (≈ 10 km). Under 3 km is an immediate danger zone.
Formula
How this is calculated
Light travels so fast (about 300,000 km/s) that the flash appears almost instantaneously regardless of distance. Sound, by contrast, travels through air at roughly 340 m/s — meaning thunder from a bolt 3 km away takes about 9 seconds to reach you. The delay between seeing the flash and hearing the thunder is therefore a direct measure of distance.
The speed of sound in air depends on temperature. The formula v = 331.3 × √(1 + T_C / 273.15) m/s gives the correct speed at any Celsius temperature, ranging from about 316 m/s at −30 °C to 352 m/s at 40 °C. The popular rules of thumb — 1 km per 3 seconds, 1 mile per 5 seconds — assume 20 °C and are accurate to about 1–2% at typical outdoor temperatures.
For safety, the 30-30 rule is widely recommended: if the delay between flash and thunder is 30 seconds or less (≈ 10 km), seek shelter immediately and wait 30 minutes after the last thunder before going back outside. Lightning can strike up to 16 km from its parent storm, well outside any visible rain. The calculator flags when the computed distance is within 3 km.
Frequently asked questions
Lightning and thunder are produced simultaneously, but light reaches you almost instantly while sound travels much more slowly (≈ 340 m/s). The delay between seeing the flash and hearing the thunder is the time sound needed to cross that distance, so dividing by the speed of sound gives the distance directly.
At 20 °C the speed of sound is about 343 m/s = 0.213 miles per second, making the exact rule 4.7 seconds per mile — close enough that "5 seconds per mile" is a useful approximation. The error is under 7% for air temperatures from −10 °C to 40 °C. The "3 seconds per km" rule is similarly accurate.
Most lightning safety organisations recommend taking shelter whenever thunder is heard — implying the storm is within about 10 km. Many use the 30-30 rule: seek cover if the flash-to-thunder gap is 30 seconds or less, and wait 30 minutes after the last thunder before going back outside. Never shelter under isolated trees, on hilltops, near water, or on open ground.
Also known as
TG we-Calculate Editorial Team. (2026). Lightning Distance Calculator — How Far Away Was the Lightning? [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/lightning-distance-calculator
TG we-Calculate Editorial Team. "Lightning Distance Calculator — How Far Away Was the Lightning?." TG we-Calculate. 2026. https://we-calculate.com/calculator/lightning-distance-calculator.
TG we-Calculate Editorial Team, "Lightning Distance Calculator — How Far Away Was the Lightning?," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/lightning-distance-calculator
@misc{wecalculate_lightning_distance_calculator, title = {Lightning Distance Calculator — How Far Away Was the Lightning?}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/lightning-distance-calculator}}, year = {2026}, note = {TG we-Calculate} }
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