Time the flash to bang
0.0s
Tap when you see the flash
If you cannot hear thunder within about 30 seconds, the strike is likely beyond the range where counting is reliable.
Sound moves faster in warm air. Temperature is why the classic five second rule is an approximation and not an exact conversion.
Estimated lightning distance
2.13miles
3.43 km · 11,267 ft · sound at 1,127 ft/s
Take shelter now
The storm is within striking distance
Anything under a 30 second count puts the strike roughly six miles out or closer. Successive strikes from the same storm can jump several miles, so the next one could land where you are standing.
The classic five second rule would estimate 2.00 miles. Adjusted for an air temperature of 68°F, the answer is 2.13 miles.
How to use the lightning distance calculator
Measuring the time between seeing lightning and hearing thunder is the oldest storm gauge there is. It can help estimate how far the lightning is from us because sound travels at a speed we can measure. Our lightning distance calculator helps give you a rough estimate of how far away lightning is using this same method. Here’s how:
Click the I see the flash button to start the timer, and when you hear the thunder, click I hear the thunder.
After that, enter the air temperature and the calculator will give you an estimate of how far the lightning is away from you.
How Does Lightning Detection Work?
What Is the Distance Between Lightning and Thunder?
Lightning heats the air in its channel to roughly 50,000 degrees Fahrenheit, about five times hotter than the surface of the sun. That air explodes outward, then collapses back, and the pressure wave it creates is thunder.
What you are actually measuring is the gap between two messengers arriving from the same event.
Light covers about 186,000 miles per second, so the flash reaches your eyes with no meaningful delay at any distance you could care about. Sound covers about 1,126 feet per second on a 68 degree afternoon. Over a mile, that is a difference of roughly four and a half seconds. Over five miles, it is closer to twenty-four.
That gap is the entire basis of the flash to bang method, and it is why the same storm can look like it is on top of you and sound like it is a county away.
How to Calculate Lightning Distance: The Flash-to-Bang Equation
The calculation is a rearranged speed formula. Two equations do all the work.
1
Find the speed of sound at your temperature
Sound moves faster through warm air because warm molecules are moving faster and pass the pressure wave along more quickly.
v = 331.3 + (0.606 × T)
v Speed of sound, in meters per second
T Air temperature, in degrees Celsius
Worked at 20°C, or 68°F
v = 331.3 + (0.606 × 20)
v = 331.3 + 12.12
v = 343.4 meters per second, or 1,126 feet per second
Every degree Celsius of warming adds roughly 0.6 meters per second.
|
32°F morning 1,087 ft/s |
68°F baseline 1,126 ft/s |
95°F afternoon 1,157 ft/s |
That swing across the year is small, but it is the reason a fixed conversion factor can only ever be an approximation.
2
Multiply speed by time
d = v × t
d Distance to the strike
v Speed of sound, from Step 1
t Seconds you counted between flash and thunder
Worked at ten seconds, 68°F
d = 343.4 m/s × 10 s
d = 3,434 meters
Sound travels at a fixed speed for a given temperature, so the count is the only variable you have to measure in the field.
3
Convert to the unit you want
Divide meters by 1,609.34 for miles, or by 1,000 for kilometers.
Converting 3,434 meters
3,434 ÷ 1,609.34 = 2.13 miles
3,434 ÷ 1,000 = 3.43 kilometers
Ten seconds means the lightning struck a little over two miles away.
Why the Five Second Lightning Rule Is an Approximation
The shortcut almost everyone learns is to count the seconds and divide by five. The National Weather Service uses it, OSHA and NOAA reference it in their joint guidance for outdoor worksites, and it is close enough to be genuinely useful.
But the real figure at 68 degrees Fahrenheit is one mile every 4.69 seconds, not five. Dividing by five underestimates the distance by about six percent.
Ten seconds becomes 2.00 miles instead of 2.13. Thirty seconds becomes 6.00 miles instead of 6.40.
The error runs in the conservative direction, which is why nobody worries about it. The shortcut tells you the storm is slightly closer than it really is, and for a safety decision, erring toward closer is the right kind of wrong.
Lightning Thunder Seconds to Distance Chart
All figures calculated at 68 degrees Fahrenheit.
| Seconds counted | Actual distance | Five second rule | What it means |
|---|---|---|---|
| 1 second | 0.21 miles | 0.20 miles | The strike hit within a few hundred yards |
| 3 seconds | 0.64 miles | 0.60 miles | Immediate danger, get inside now |
| 5 seconds | 1.07 miles | 1.00 mile | Immediate danger |
| 10 seconds | 2.13 miles | 2.00 miles | Well inside striking distance |
| 15 seconds | 3.20 miles | 3.00 miles | Well inside striking distance |
| 20 seconds | 4.27 miles | 4.00 miles | Inside striking distance |
| 25 seconds | 5.34 miles | 5.00 miles | Inside striking distance |
| 30 seconds | 6.40 miles | 6.00 miles | The standard threshold to stop outdoor work or play |
| 40 seconds | 8.53 miles | 8.00 miles | Inside the 10 mile radius most policies use |
| 45 seconds | 9.60 miles | 9.00 miles | At the outer edge of audible thunder |
| 50 seconds or more | 10.67 miles or more | 10.00 miles or more | Beyond the range where counting is reliable |
The 30 second mark is the one that shows up in written policy. A 30 second count puts the strike roughly six miles out, and that is the threshold behind the 30-30 rule referenced by NOAA, OSHA, and the National Lightning Safety Council: if thunder arrives within 30 seconds of the flash, stop, take shelter, and wait 30 minutes after the last thunder before resuming.
Where the Time Between Lightning and Thunder Method Breaks Down
Flash to bang is good physics. As the operational basis for a lightning safety policy on a job site, a golf course, a park system, a refinery, or a festival ground, it has four failure points, and they compound.
Thunder only carries about 10 miles
According to the National Weather Service, thunder can be heard for a distance of only about ten miles from the strike. Past that, the sound refracts upward through the atmosphere and never reaches the ground where you are standing. That gives the method a hard ceiling.
Beyond roughly ten miles, silence and a storm forty miles away produce the exact same reading: nothing.
You cannot tell the difference between a clear afternoon and a cell that is closing fast. Since ten miles is also the radius most written policies use as their outer trigger, flash to bang runs out of range at almost precisely the distance where your policy needs data.
This makes having a lightning detection system even more valuable. Lightning detection systems like Perry Weather can detect lightning miles and miles away from your location. Set up a lightning radius, telling the system when to send you an alert, and you’ll be notified when lightning is nearing your facilities.
You have to see the flash that made the thunder
A single count requires someone to be looking at the sky at the moment lightning strikes, then correctly match that flash to the thunder that follows it. In daylight, in haze, behind a treeline, from inside a cab or a trailer, or with cloud cover between the observer and the storm, plenty of strikes are never seen at all.
The next strike can be miles from where the last one landed
This is the part that makes flash to bang genuinely risky rather than just imprecise. The National Athletic Trainers’ Association position statement on lightning safety notes that the distance between successive flashes may be as great as nine miles or more, depending on local geography and atmospheric conditions.
If you observe a flash to bang count of 30 seconds, the next strike could be at your location.
Your count tells you roughly where lightning was. It does not tell you where the next one is going. A comfortable ten second reading and a strike on your property are entirely compatible outcomes from the same storm.
That risk is not distributed evenly. Cranes, scaffolding, towers, light poles, open water, and equipment on high ground all raise the odds that a strike in the area becomes a strike on your site.
NOAA specifically flags cranes as vulnerable in its guidance for outdoor worksites.
Someone has to be counting, every single time
Flash to bang requires a designated person watching the sky continuously, counting accurately, converting to miles, comparing that number to a written policy, and then making an unpopular call in front of a crew, a client, a general contractor, or paying customers.
It requires that person to be present at every site, on every shift, all season.
Even then, flash to bang is still and estimate and a unreliable method of measurement.
The Most Accurate Way to Know The Lightning Distance
Lightning distance measured, not estimated
Perry Weather runs on the National Lightning Detection Network, the same network relied on by the National Weather Service, NASA, the FAA, and the U.S. Military. The system works by triangulating strikes and sending that information directly to your phone. This method gives you coverage anywhere you are in the US and is much more accurate than free lightning apps.
- 187+ ground based sensors across the continental United States
- Strikes located to within 84 meters, about three quarters of a football field
- Triangulated in roughly 12 seconds
- Classified cloud to ground or in cloud with 95 percent accuracy
It also matters what does not set off an alert. Older lightning prediction systems read static field changes and have been tripped by power lines, cell phones, and lawn mowers. False alarms teach crews to ignore alerts, which is its own hazard.
A lightning delay should mean that your location is in danger. Older systems can often be the boy that cried wolf.
With Perry Weather, you set the lightning detection radius and let the system alert your entire team via sirens, strobes and mobile notifications. Get alerts as lightning moves in closer to your location so you’ll never be caught off guard.
Sirens, strobes, and A PA system the whole site hears
The outdoor warning system covers the people who are not looking at a phone. Here’s how it works:
- Lightning strikes, the National Lightning Detection Network sends the location data to Perry Weather in 12 seconds
- Loud audible sirens sound, alerting your whole team whether they have Perry Weather or not
- Strobe lights flash the entire duration of the delay
- Text to speech, so a supervisor types a message and broadcasts it through the siren
Mobile notifications with the instruction already attached
When Perry Weather detects a lightning strike, it then sends alerts to your entire team, so everyone’s on the same page about halting activities or work. The alert becomes everyone’s sources of truth. No more arguing about which app to trust.
- Push notifications and texts sent to your whole team
- The lightning alert is accompanied with custom instructions that detail where to take shelter.
- You can configure the alerts to by sent to different teams or groups so no one is getting bombarded by every lightning notification
Each team gets only the alert that apply to them.
With Perry Weather, you know exactly when and where lightning struck and what to do.
Lightning countdown timers and an automatic all clear
This is the piece flash to bang cannot replicate at all—when to get back to work.
- When a strike is detected, Perry Weather starts a countdown timer for that zone
- Every new strike inside the zone resets the clock automatically
- The timer is live on the mobile app, the web dashboard, and public facing widgets and QR links so everyone knows how long the delay with last, even visitors
- When it expires, the all clear goes out as both a text and an audible siren alert
A significant number of lightning deaths happen after the storm appears to have passed.
The National Weather Service is explicit about this: do not leave shelter just because the rain has stopped.
Blue sky is not an all clear.
Resetting a 30 minute clock by hand, across multiple zones, during a storm that keeps flaring back up, with a client asking when work restarts, is exactly the kind of task people get wrong under pressure. Automating it removes the early return and the argument about whether the clock restarted.
A timestamped record of every lightning delay
- Every delay logged automatically with timestamps
- Exportable report of total hours spent in delay
- Built for claims, compliance, and lost time documentation
A count in someone’s head leaves no record.
When an insurer, an owner, a general contractor, or a regulator asks what your threshold was and when you acted on it, the log answers for you.
Try Perry Weather free
Counting seconds is a fine way to satisfy your curiosity about a storm. It is a poor way to run a lightning safety policy across multiple sites, shifts, and crews.
Start a free trial to see how Perry Weather handles lightning distance, alerting, and the all clear automatically.
