Lightning Distance Calculator
Lightning distance calculator. Estimates how far away a strike is from the seconds between flash and thunder, with the 30-30 safety rule. Educational safety aid.
Lightning Distance Calculator
How to use the lightning distance calculator
Count the seconds
When you see a flash, start counting and stop when you hear the thunder from that flash. Count steadily — "one-thousand-one, one-thousand-two" — or use a stopwatch.
Enter the count
Type the number of seconds. A shorter gap means the strike was closer; thunder you can hear is generally within about 10 miles.
Read the distance
You get the distance in miles and kilometres, plus a risk indicator. Watch whether successive gaps are shrinking — that means the storm is approaching.
Act on the 30-30 rule
If the gap is 30 seconds or less, the storm is dangerously close — get to substantial shelter now, and wait 30 minutes after the last thunder before going back out.
Flash-to-bang — how far away is the lightning?
Light is instant, sound is slow
Lightning produces a flash of light and a clap of thunder at the same instant, but they reach you at very different speeds. Light travels about 300,000 kilometres per second — effectively instantaneous over any distance you can see — while sound crawls along at roughly 343 metres per second in air. That gap is what lets you estimate the distance to a strike: the longer the delay between seeing the flash and hearing the thunder, the further away the lightning was. The arithmetic is easy. Because sound covers about a kilometre every three seconds, or a mile every five, you divide the seconds by five to get miles, or by three to get kilometres. A ten-second gap means the strike was about two miles, or a little over three kilometres, away. This "flash-to-bang" method has guided outdoor people for generations and needs nothing more than the ability to count.
The technique also tells you which way a storm is moving. If you time several strikes and the gaps are getting shorter, the storm is coming toward you; if they're lengthening, it's moving away. Thunder is generally only audible out to about ten miles (sixteen kilometres), so if you can hear thunder at all, lightning is close enough to be a threat — the origin of the safety mantra, "when thunder roars, go indoors."
"Light is instant; sound takes five seconds a mile. Count from flash to bang, divide by five — and remember that if you can hear thunder at all, you're already within striking range."
A handy estimate, but lightning is the real risk
The flash-to-bang method is a good distance estimate, but a few things keep it honest, and none of them change the core safety message. The speed of sound varies slightly with air temperature, and thunder can echo off terrain or be muffled by wind and rain, so the timing is approximate — treat it as a rough guide, not a precise rangefinder. More importantly, the distance to the last strike says little about where the next one will hit. Lightning regularly strikes many kilometres from the rain core of a storm — so-called "bolts from the blue" can travel ten miles or more from a cloud into clear sky — which is why a comfortable-sounding gap is not a green light. The widely taught rule is the 30-30 rule: if the gap between flash and thunder is 30 seconds or less, the storm is within about ten kilometres and you should already be in substantial shelter (a building with wiring and plumbing, or a hard-topped vehicle, not a tree, shelter, or open ground), and you should wait a full 30 minutes after the last thunder before resuming activity, because strikes can continue from a departing storm. Use this calculator to understand and estimate distance, but make decisions on the conservative side: when thunder roars, go indoors, and let official weather warnings and common sense — not a number — govern your safety.
10 Facts About Lightning Distance
Light is instant; sound travels ~343 m/s.
Distance ≈ seconds ÷ 5 in miles.
Distance ≈ seconds ÷ 3 in kilometres.
A 10-second gap ≈ 2 miles / 3.4 km.
Shrinking gaps = the storm is approaching.
Thunder carries only about 10 miles.
"Bolts from the blue" strike 10+ miles from the storm.
The 30-30 rule: shelter at 30 s, wait 30 min after.
Shelter = a building or hard-topped vehicle.
"When thunder roars, go indoors."
Frequently asked questions
Count the seconds between seeing the flash and hearing the thunder, then divide by five for miles or by three for kilometres. Sound travels about a mile every five seconds, so a ten-second gap means the strike was roughly two miles, or a little over three kilometres, away. Light arrives effectively instantly, so the whole delay is the time the sound took to reach you. The calculator does this conversion and shows both units.
Because light and sound travel at vastly different speeds. Light covers any visible distance almost instantly, so you see the flash the moment it happens. Sound moves at only about 343 metres per second, so the thunder lags behind by the time it takes to travel from the strike to you. Measuring that lag and multiplying by the speed of sound gives the distance. It's the same principle as judging distance from an echo.
It's a widely taught lightning-safety guideline. The first 30: if the time between the flash and the thunder is 30 seconds or less, the lightning is within about 10 kilometres (6 miles) and you should already be in safe shelter. The second 30: after the last thunderclap, wait a full 30 minutes before going back outside, because strikes can still occur from a departing storm. The rule errs on the side of caution because lightning can strike well ahead of and behind the rain.
No. The gap tells you how far away the last strike was, not where the next one will hit. Lightning regularly strikes many kilometres from the rain — "bolts from the blue" can travel ten miles or more from the cloud into apparently clear sky. So a comfortable gap is not a green light to stay out. If you can hear thunder at all, lightning is close enough to be dangerous; seek shelter and use the 30-30 rule rather than relying on the distance alone.
It's a good rough estimate. The speed of sound changes slightly with air temperature, and thunder can echo off hills or be muffled by wind and rain, so the timing isn't exact — and human counting adds error. It's reliable enough to tell whether a storm is close or distant and whether it's approaching, which is what matters for safety. Don't treat it as a precise rangefinder; treat it as a quick way to judge risk and trigger the 30-30 rule.
A large, fully enclosed building with wiring and plumbing (which help carry a strike safely to ground), or a hard-topped metal vehicle with the windows up. Avoid open ground, hilltops, isolated trees, water, and small open shelters, picnic huts, or tents, which offer little or no protection. Inside, stay away from corded electronics, plumbing, and windows. If caught in the open with no shelter, minimise your contact with the ground and avoid being the tallest object — but reaching real shelter beforehand is the only reliable protection.
Time several strikes in a row. If the flash-to-thunder gaps are getting shorter, the storm is moving toward you and the danger is rising; if they're getting longer, it's moving away. Either way, while you can still hear thunder you remain at risk. Use the trend to decide how urgently to act, but don't wait for the gap to reach zero — by the 30-30 rule you should already be sheltered once it drops to 30 seconds.
Use it to understand and estimate distance, but make safety decisions conservatively and from official guidance. Lightning is lethal and unpredictable; the only reliable protection is to be in proper shelter before the storm arrives, follow the 30-30 rule, and heed official weather warnings. This calculator is an educational aid, not a substitute for caution. When thunder roars, go indoors — don't wait to do the maths.
Slightly. The speed of sound rises a little with temperature — roughly 343 m/s at 20 °C, a bit slower in cold air and faster in hot — so the distance can vary by a few percent. The calculator uses the standard 343 m/s, which is close enough for a safety estimate. Echoes off terrain and muffling by wind and rain add more uncertainty than temperature does, so treat the distance as approximate and act conservatively regardless.
No. The value you enter is processed entirely in your browser. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs on your device only.
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