Lightning Distance Calculator
Calculate the distance to a lightning strike from the flash-to-bang time delay. Includes temperature-adjusted speed of sound and safety guidance.
About this calculator
Distance is driven overwhelmingly by Flash-to-Bang Time — the count of seconds between seeing the flash and hearing the thunder — since distance is simply the speed of sound multiplied by that delay. Air Temperature plays a smaller, secondary role: it adjusts the speed of sound itself using the standard approximation (331.3 m/s at 0C, rising by about 0.6 m/s per degree Celsius), so warmer air carries the thunderclap slightly faster, giving a marginally larger calculated distance for the same flash-to-bang count.
The calculator also reports a Rule of Thumb figure (seconds divided by 2.94, based on a fixed ~340 m/s speed of sound with no temperature adjustment) alongside the temperature-corrected Distance, so you can see how much the two methods diverge — the gap widens as air temperature moves further from roughly 15C, where the two nearly agree. Safety Assessment layers plain-language guidance on top of the calculated distance using standard storm-safety distance bands (under 1 km, under 3 km, under 10 km, under 16 km), and Storm Arrival estimates time until the storm reaches your location assuming a typical 40 km/h storm movement speed — actual storm speed varies considerably and can be much faster, so lean toward seeking shelter sooner than that figure alone would suggest.
Inputs
Results
Distance
3.46 km
≈ 10 Eiffel Towers
Safety Assessment
Danger — Move indoors, lightning within striking range
How to Use This Calculator
- Count the seconds between the lightning flash and the thunder clap and enter as Flash-to-Bang Time (seconds).
- Optionally enter Air Temperature (°C) to refine the speed of sound calculation.
- Review Distance (km) and Distance (miles) to the lightning strike.
- Check Safety Assessment: under 10 km (6 miles) = move indoors or seek shelter.
- Monitor Storm Arrival (at 40 km/h) to estimate how quickly the storm is approaching your location.
How the result changes with Flash-to-Bang Time
| Flash-to-Bang Time | Distance | Safety Assessment |
|---|---|---|
| 5 | 1.73 km | Very Dangerous — Lightning is very close |
| 7.5 | 2.6 km | Very Dangerous — Lightning is very close |
| 15 | 5.2 km | Danger — Move indoors, lightning within striking range |
| 25 | 8.66 km | Danger — Move indoors, lightning within striking range |
What each input means
- Flash-to-Bang Time
- Count the seconds between seeing the lightning flash and hearing the thunder. Start counting at the flash.
- Air Temperature
- Current air temperature in °C. Sound travels faster in warmer air, affecting the distance calculation.
How this is calculated
Worked example, using the default values
- Identify Input ParametersFlash-to-Bang Time = 10, Air Temperature = 25 = 2 input(s) provided
- Calculate DistanceDistance = speedOfSoundKmPerS3.46 = 3.46
- Calculate Safety AssessmentSafety AssessmentDanger — Move indoors, lightning within striking range = Danger — Move indoors, lightning within striking range
- Calculate DistanceDistance2.15 = 2.15
- Calculate DistanceDistance3460 = 3460
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does Flash-to-Bang Time matter so much more than Air Temperature?
Distance is the speed of sound multiplied directly by Flash-to-Bang Time, so every additional second of delay adds a full increment of distance (roughly a third of a kilometer per second at typical temperatures). Air Temperature only nudges the speed of sound itself by a fraction of a percent per degree, so its effect on the final distance is real but much smaller than the flash-to-bang count.
Why does the calculator show two different distance figures — Distance and Rule of Thumb?
Distance uses the temperature-adjusted speed of sound (331.3 m/s at 0C, plus about 0.6 m/s per degree Celsius), while Rule of Thumb uses the commonly taught shortcut of dividing seconds by 2.94, which assumes a fixed speed of sound near 340 m/s regardless of actual temperature. The two agree closely around 15C and diverge more as the actual air temperature moves further from that point.
How reliable is the Storm Arrival estimate?
It assumes a fixed storm movement speed of 40 km/h, dividing the calculated Distance by that speed to estimate arrival time. Real thunderstorms can move considerably faster or slower than 40 km/h depending on the weather system, so treat Storm Arrival as a rough planning figure, not a precise countdown — err on the side of seeking shelter earlier than the estimate suggests.
What do the Safety Assessment distance thresholds mean?
The calculator steps through standard storm-safety guidance bands: under 1 km is flagged as extreme danger requiring immediate shelter, under 3 km as very dangerous, under 10 km as within striking range (lightning can strike well outside the rain area itself), and under 16 km as a caution to prepare for shelter as the storm approaches. Distances beyond 16 km are reported as safe under this guidance.
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