How this instrument works
A lightning bolt and its thunderclap are born at the same instant and place. What separates them by the time they reach you is nothing but the speed of two different messengers: light at roughly 300,000 kilometres per second and sound at about 343 metres per second in typical air. Across a storm's working range — a few hundred metres to a dozen kilometres — light's travel time is a fraction of a millisecond, too small for a person to notice. Every second you count between seeing the flash and hearing the boom is, for practical purposes, entirely the sound catching up.
That is why the formula is just distance equals speed times time, run in reverse: d = t × v_sound. You supply the one thing a stopwatch or a mental count can give you, the delay t, and the instrument multiplies it by the fixed speed of sound and divides by a thousand to hand back kilometres instead of metres. There is no calculus here and no hidden correction — the strike's sound wave crossed the distance between you and it at a steady pace, and t is simply how long that crossing took.
The speed of sound is not a universal constant the way light's is; it depends on the air it travels through. This calculator fixes v_sound at 343 m/s, the accepted figure for dry air at 20 degrees Celsius, so a reading taken on a freezing night or at altitude will drift a few percent from the true distance. It also assumes the boom you timed belongs to the flash you watched — in an active storm throwing multiple bolts a minute, catching the wrong pair of flash and thunder for each other is the real source of error, not the physics.
- Watch the sky and start a count the instant you see a lightning flash.
- Stop counting when the matching thunderclap arrives, and type that number of seconds into Time delay (flash to thunder).
- Read Distance to the strike — the instrument multiplies your count by 343 m/s and reports the result in kilometres by default.
- Switch the unit menu on Distance to the strike to metres or miles if that scale suits you better.
- Repeat for each new flash during a storm to see whether the count, and so the distance, is shrinking or growing.
Worked example — a five-second flash-to-bang count
You see the flash and start counting: one, two, three, four, five — the thunder arrives right as you reach five. Enter t = 5 s and the instrument works d = (5 × 343) ÷ 1000 = 1.715 km. The strike hit about 1.715 kilometres away, roughly the length of seventeen football pitches laid end to end, which is exactly the golden figure this page is built and checked against.
Storm spotters lean on the same arithmetic through the 30-30 rule: count 30 seconds or fewer between flash and bang — about 10.29 km by this formula — and the guidance is to be indoors already, then wait 30 minutes after the last flash before going back out. A five-second count of 1.715 km sits well inside that danger threshold; whoever timed it should already be under cover, not reaching for a stopwatch.
Questions
Why can the flash be treated as instant but not the thunder?
Because light is about 900,000 times faster than sound in air. Over a storm's working range of a few kilometres, light arrives in a fraction of a millisecond — far below what anyone can perceive — while sound needs a full second to cross roughly 343 metres. Practically the entire delay you count between seeing a flash and hearing its thunder is sound catching up, not light lagging.
Does the calculator ever adjust the speed of sound for weather?
No, it holds v_sound fixed at 343 m/s, the standard value for dry air at 20 degrees Celsius. Real air varies: sound speed rises roughly 0.6 m/s per degree Celsius of warming and shifts slightly with humidity and altitude. On a cold night near freezing it drops to about 331 m/s, which nudges a computed distance by a few percent — usually smaller than the error in counting seconds by hand.
What is the 30-30 rule storm spotters use?
It is a safety threshold built on this exact formula: if thunder follows a flash within 30 seconds — about 10.3 km away — treat the storm as close enough to be dangerous and get indoors immediately, then stay there until 30 minutes pass without another flash. The rule turns the same flash-to-bang arithmetic this calculator performs into a simple go or stay-inside decision.
Why might the estimate be wrong for a nearby storm?
Usually because the wrong flash and boom got paired. An active storm can fire several bolts a minute, and if the thunder you time actually belongs to a different, closer or farther strike than the flash you watched, the count no longer describes that bolt's distance. Terrain, buildings, and temperature layers near the ground can also bend the sound path and add a small error at short range.
Can this formula tell me which direction the lightning struck?
No — a single flash-to-bang count only fixes a radius, not a bearing, because it measures travel time along one path, not the angle it arrived from. Locating the actual strike point needs either a second observer some distance away, comparing counts by triangulation, or a dedicated lightning-detection network; a lone stopwatch tells you how big a circle to worry about, nothing more.
Why does the result come out in kilometres rather than metres?
Because the formula divides t × v_sound by 1000 before reporting the answer, turning a raw metre figure into the more storm-scaled kilometre. A 5-second count produces 1715 metres internally, which the instrument displays as 1.715 km; switching the output unit to metres shows that same 1715 figure directly, with no change to the underlying arithmetic.