Free lightning distance calculator
Count the seconds between the lightning flash and the thunder and get how far away it struck — in kilometres and miles — plus the NWS 30-30 safety read-out, updated live, as you type.
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An estimate, not safety advice — follow the NWS 30-30 rule. How accurate is this?
Results are estimates. Consult a professional.
How the lightning distance calculator works
This lightning distance calculator turns one easy observation — the seconds between the lightning flash and the thunder — into how far away the strike was, in both kilometres and miles. It works because light and sound travel at wildly different speeds. The flash of lightning reaches your eyes almost instantly, at about 300,000 kilometres per second, so you see it the moment it happens. The thunder is the sound of that same flash, and sound crawls along at only about 343 metres per second. The gap between the two — what storm spotters call "flash-to-bang" — is the time the sound spends catching up to the light.
So the moment you see a flash, start counting seconds until you hear the thunder. The longer the count, the farther away the lightning. As a quick rule of thumb, sound covers about one kilometre every three seconds, or one mile every five seconds. The calculator does the multiplication for you and adds a safety read-out based on the National Weather Service rule.
The flash-to-bang formula for lightning distance
Distance is simply the speed of sound multiplied by the flash-to-bang time. Because light arrives effectively instantly, the entire delay is the time the thunder takes to travel from the strike to you.
What the lightning distance calculator needs
There is just one thing to measure, and you do not need any equipment beyond a way to count seconds.
- Seconds between flash and thunder. The instant you see the lightning, start counting; stop the moment you hear the thunder. A steady "one-thousand-one, one-thousand-two" count or a stopwatch both work.
- Distance unit. Toggle the result between kilometres and miles — the calculator shows your chosen unit large and the other one beside it.
Always pair a flash with its own thunder. In an active storm with many strikes it is easy to match a flash to the wrong rumble, which throws the timing off. If you cannot tell which thunder belongs to which flash, the storm is close — treat it as such.
A worked example: counting 10 seconds
A storm is approaching and you see a bright flash. You count steadily — "one-thousand-one" up to "one-thousand-ten" — and the thunder arrives right as you reach 10 seconds. How far away did the lightning strike?
Step 1 — Multiply the seconds by the speed of sound
10 seconds × 343 m/s = 3,430 metres, which is 3.43 km.
Step 2 — Convert to miles
3.43 km × 0.621371 = 2.13 miles. The quick check agrees: 10 seconds ÷ 5 ≈ 2 miles.
Step 3 — Read the safety status
10 seconds is well under 30, so the strike is within the National Weather Service shelter threshold. Even at 3.43 km away, the next strike could be much closer — lightning regularly jumps several kilometres from the storm.
Lightning distance chart: seconds to kilometres and miles
This chart gives the distance to the strike for common flash-to-bang counts. Read down to the number of seconds you counted, then across to kilometres or miles. The figures come straight from the calculator's formula at the standard 343 m/s speed of sound.
| Seconds counted | Distance (km) | Distance (mi) |
|---|---|---|
| 1 second | 0.34 | 0.21 |
| 5 seconds | 1.72 | 1.07 |
| 10 seconds | 3.43 | 2.13 |
| 15 seconds | 5.14 | 3.20 |
| 20 seconds | 6.86 | 4.26 |
| 30 seconds | 10.29 | 6.39 |
Distance from the flash-to-bang time at a 343 m/s speed of sound. Note the 30-second row: about 10 km (6 miles) — the National Weather Service shelter threshold. Anything at or under 30 seconds means the storm is close enough to be dangerous.
The NWS 30-30 rule and when to seek shelter
Estimating the distance is interesting, but the reason it matters is safety. The U.S. National Weather Service sums it up in five words: "When Thunder Roars, Go Indoors." If you can hear thunder at all, you are already within striking range of lightning — there is no safe distance outdoors during a thunderstorm.
The 30-30 rule
- First 30 — the flash-to-bang count. If you count 30 seconds or fewer between the flash and the thunder, the lightning is within about 10 km (6 miles) — close enough to strike you. Go inside a substantial building or a hard-topped vehicle immediately.
- Second 30 — the wait. After the storm seems to pass, wait at least 30 minutes after the last clap of thunder before going back outside. Most lightning deaths happen before the rain starts or after it stops, when people head out too soon.
Why lightning distance is only an estimate
The flash-to-bang method gives a good ballpark, but it is an estimate, and a few things keep it from being exact. The biggest is the speed of sound itself. The 343 m/s figure is for air at 20 °C; sound travels a little faster in warm, humid air (around 349 m/s at 30 °C) and a little slower in cold air (about 331 m/s at 0 °C). That shifts the distance by a few percent either way — enough to matter for precision, but not for deciding whether to take shelter.
Human timing adds the rest of the error. Reaction time, the difficulty of pinning the exact instant the thunder begins, and the risk of matching a flash to the wrong thunder in a busy storm all introduce a second or two of slop. Because each second is worth about a third of a kilometre, a one-second error moves the estimate by roughly that much. Treat the result as "roughly this far away," not a surveyed measurement — and remember that for safety, the only number that counts is whether the flash-to-bang time is 30 seconds or less.
For other ways the same storm affects you, see our wind chill calculator for how cold and wind combine, or the heat index calculator for how hot a humid day really feels.
Frequently asked questions about the free lightning distance calculator
About this Lightning distance calculator
This lightning distance calculator runs entirely in your browser. Nothing you type is sent anywhere — the distance in kilometres and miles and the NWS 30-30 shelter read-out are computed locally from the flash-to-bang time as you adjust it.
It is one of our weather calculators, and a lightning-safety tool first — when thunder roars, go indoors. Browse the full set of free calculators for more weather and everyday tools.