Short answer: A close lightning strike can produce thunder of roughly 120 dB or more (peak) near the strike, as loud as a siren at close range. A few kilometres away, thunder is much quieter and heard as a low rumble. To judge distance, count the seconds from flash to thunder: sound travels about 1 km every 3 seconds, or 1 mile every 5 seconds.
Thunder is the sound of lightning, so how loud it is tells you how close the danger is. This guide explains how loud thunder is at different distances, what causes it, why it rumbles instead of banging once, how to use the 30-second rule, whether thunder can harm your hearing, and how to help pets who are frightened by storms. You can log a storm from indoors with our sound decibel meter.
How Loud Is Thunder at Different Distances?
Thunder levels vary widely with the strike, the terrain and the weather, so treat this as a rough guide. The time between the flash and the sound is the most useful measure of all.
| Flash to thunder | Approximate distance | What you hear | What to do |
|---|---|---|---|
| 0–3 seconds | Within about 1 km (0.6 mile) | A sharp crack or bang; can exceed 120 dB peak very close | You are in danger: get into a building or car at once |
| 3–15 seconds | About 1–5 km (0.6–3 miles) | A loud boom followed by rumbling | Within striking distance: go indoors |
| 15–30 seconds | About 5–10 km (3–6 miles) | A rolling rumble | The 30-second rule says take shelter now |
| More than 30 seconds | More than about 10 km (6 miles) | A faint, low rumble | Watch the storm; if you can hear thunder, lightning can still reach you |
Thunder is rarely heard more than about 15–20 km away, because the sound is absorbed and bent upwards by the atmosphere, and it is easily drowned out by traffic or wind.
What Makes Thunder?
A lightning bolt heats a narrow channel of air to tens of thousands of degrees Celsius in a tiny fraction of a second. The air expands so violently that it creates a shock wave, which quickly becomes an ordinary sound wave travelling outwards in every direction. Lightning and thunder always happen together; if you hear thunder without seeing a flash, the lightning was hidden by cloud, rain or daylight.
Why Thunder Rumbles
A lightning channel can be several kilometres long and full of branches. Sound from the nearest part of the channel reaches you first, and sound from the farther parts arrives later, so a single strike is heard as a long, rolling rumble rather than one bang. Echoes from hills, clouds and buildings stretch it out further.
Distance also changes the character of the sound. The air absorbs high frequencies faster than low ones, so a nearby strike gives a sharp, bright crack, while distant thunder is a deep rumble made mostly of low frequencies. Low-frequency sound also has long wavelengths that bend around buildings and hills; the audio wavelength calculator shows that a 50 Hz tone has a wavelength of about 7 m. See dB vs Hz for the difference between loudness and pitch, or watch a storm’s low rumble on the frequency analyzer.
The 30-Second Rule: Counting the Seconds
Light arrives almost instantly, but sound travels at about 343 metres per second in air at 20 °C. That gives an easy rule of thumb:
- Count the seconds from the flash to the thunder.
- Divide by 3 to get the distance in kilometres, or by 5 for miles.
- Ten seconds means the strike was about 3 km (2 miles) away; 30 seconds means about 10 km (6 miles).
The traditional 30-30 rule says: if the thunder comes 30 seconds or less after the flash, get to shelter, and wait 30 minutes after the last thunder before going back outside. Many weather services now put it more simply: when thunder roars, go indoors. Lightning can strike well away from the heaviest rain, so hearing thunder at all means you are close enough to be at risk.
Safe shelter means a substantial building with plumbing and wiring, or a hard-topped metal vehicle with the windows closed. Open-sided shelters, tents, sheds and trees do not protect you.
How Thunder Compares with Other Sounds
The peak of a nearby thunderclap is comparable to an emergency siren a few metres away, about 120 dB, and a strike very close by can approach the 130 dB threshold of pain. Distant thunder is often no louder than heavy traffic at about 80 dB, and at night, against the quiet of a bedroom at around 40 dB, even a faint rumble stands out. The decibel chart puts these levels side by side.
Two points make thunder hard to compare. First, it is an impulse: the peak lasts a fraction of a second, so the peak level is much higher than the average. Second, most of its energy is at low frequencies, which A-weighting plays down, so a C-weighted reading is usually higher; dBA vs dBC explains why. Every 10 dB sounds about twice as loud, as the logarithmic decibel scale shows.
Can Thunder Damage Your Hearing?
For most people, no. You would have to be very close to a strike to receive a damaging peak, and at that distance the lightning itself is the far greater danger. Workplace rules treat very short sounds by their peak level: OSHA says impulse noise should not exceed 140 dB peak, as described in noise exposure time limits.
A very close strike can still cause pain, temporary ringing or muffled hearing, and people who are struck by lightning or are very near a strike can suffer ear injuries, including a perforated eardrum. If your hearing is muffled or ringing for more than a day after a storm, see a doctor promptly, as sudden hearing loss is treated as urgent. The online hearing test is a useful screening check afterwards. The safe noise levels chart shows safe times for steady noise.
Thunder, Pets and Fear
Many dogs, and some cats, are frightened of thunderstorms. They may also react to other storm cues such as wind, rain on the windows or flashes of light. A few general steps can help:
- Give your pet a quiet, enclosed space away from windows, such as a covered crate or an inner room.
- Close curtains to hide the flashes, and play steady background sound, such as a fan, the TV or soft music, to mask the rumble.
- Stay calm and nearby; do not punish fearful behaviour.
- If your pet panics, hides for hours or hurts itself, talk to your vet, who can suggest behaviour training or other help.
Children can find storms frightening too, especially at night. The same ideas help: a steady background sound, a calm explanation of counting the seconds, and a comfortable room. Our guide to baby room noise levels covers safe levels for white noise.
Measuring Thunder with Your Phone
Stay indoors: never go outside during a storm to take a reading. From inside, the noise monitor can log a storm over time and record the maximum and average levels. Indoor readings will be lower than outside, because walls and windows block much of the sound, particularly the higher frequencies. Before the storm arrives, run the background noise test so you know the room’s normal level.
Phone microphones respond reasonably well to steady sound but can miss or under-read very short, very loud peaks, and they are less sensitive to deep bass. Treat a thunder reading as a rough guide; online decibel meter accuracy explains the limits, and how to measure decibels covers good technique.
Frequently Asked Questions
How many decibels is thunder?
A nearby thunderclap can reach roughly 120 dB or more at its peak. Thunder several kilometres away is much quieter and heard as a low rumble.
How far away is lightning if I count 10 seconds?
About 3 km, or 2 miles. Divide the seconds by 3 for kilometres or by 5 for miles.
Is thunder louder than a gunshot?
Usually not where you are standing. A gunshot at the shooter’s ear can peak at 140 dB or more, while most people hear thunder from far enough away that it is much quieter.
Why does thunder sound louder at night?
The background is quieter, so the rumble stands out more. At night, cooler air near the ground can also bend sound back down towards the surface, helping it carry further.
Can you hear thunder without lightning?
No. Thunder is always caused by lightning, but the flash can be hidden by cloud, heavy rain or bright daylight.
Addito is an audio analysis and sound measurement writer specializing in decibel testing, noise level analysis, sound frequency measurement, and audio engineering tools for creators, audio professionals, students, and everyday users. He creates practical content focused on sound measurement, hearing safety, microphone testing, and acoustic analysis tools.