How Far Away Is the Lightning? Flash-to-Bang Seconds to km, and Whether the Storm Is Coming Closer

The short answer: divide the seconds by 3 for km

To find how far away lightning is, multiply the number of seconds between the flash and the thunder by the speed of sound. Sound travels about 340 m per second, so seconds × 0.34 gives you km — roughly, seconds ÷ 3. If the thunder comes 3 seconds after the flash, the lightning was about 1 km away; 10 seconds is about 3.5 km and 30 seconds about 10 km. In miles, divide the seconds by 5.

Knowing the distance is not the same as being safe, though. Lightning does not only strike directly under the storm; it can hit more than 10 km away. Think of the calculation not as a way to decide you are still fine, but as a way to know early whether it is time to start sheltering and whether the storm is coming towards you. The sections below explain how the calculation works and how to use the numbers you measure.

Why the gap between light and sound gives you the distance

Lightning is a sudden discharge of the electricity built up inside a thundercloud. The flash (light) and the thunder (sound) are created at the same moment. Thunder is a shock wave: the air along the lightning channel is heated to tens of thousands of degrees in an instant and expands violently. Light and sound start from the same place at the same time, but they take very different times to reach you, and that is where the gap comes from.

Light takes only 0.00003 seconds to arrive from 10 km away, so you can treat the moment you see the flash as the moment the lightning happened. The seconds until the sound arrives are then simply the time the sound took to travel from the lightning to you, and distance = speed × time does the rest.

What you measure is the distance to the part of the lightning channel closest to you. A lightning channel can be several km long, and sound from each part of it arrives a little later than the last, which is why thunder is a long rolling rumble rather than a single bang. When you time it, use the first sound you hear, not the end of the rumble.

The speed of sound depends on temperature (331.5 + 0.6 × °C)

People often say sound travels at 340 m/s, but strictly it depends on the air temperature. The approximation used in school science is speed of sound (m/s) = 331.5 + 0.6 × temperature (°C): 331.5 m/s at 0 °C, rising by about 0.6 m/s for every degree. 340 m/s is the value at around 15 °C, and the often-quoted 343 m/s is for 20 °C.

Between 0 °C and 35 °C the speed of sound differs by about 6%. For 30 seconds that is about 9.95 km at 0 °C and about 10.6 km at 35 °C — a difference of around 600 m. Thunderstorms are most common on summer afternoons, when it is usually 25–30 °C, so that is a fine default if you are not sure. It is colder on a mountain than in the valley, so when hiking a slightly lower temperature gives a closer value (see estimating the temperature at the summit).

Seconds-to-distance chart

Distance to the lightning (km) for each number of seconds between flash and thunder, by air temperature. Anything under 10 km is the range where the 30-second rule says to take shelter, and the 30-second row is roughly the boundary.

Working the other way — how many seconds until you hear thunder from lightning a given distance away — at 25 °C it is about 2.9 seconds for 1 km, 8.7 seconds for 3 km, 14.4 seconds for 5 km, 28.9 seconds for 10 km and 43.3 seconds for 15 km. Remember that under 15 seconds means under 5 km and under 30 seconds means under 10 km, and you can make the call while you count.

How to time it, and how to keep the error small

Start counting the moment you see the flash and stop the moment you hear the thunder. A phone stopwatch works, and so does counting out loud (“one-Mississippi, two-Mississippi…”). If you take several readings, note the time of each one along with the seconds so you can tell later whether the storm is getting closer.

1. 1

   Start at the flash

   You do not need to see the bolt itself; the moment the clouds light up is enough. Lightning inside the cloud also produces thunder, so you can time it the same way.

2. 2

   Stop at the first sound

   Even if the rumble keeps going, stop at the moment it begins. How long the rumble lasts depends on the length of the channel and on echoes, so it is not used for the distance.

3. 3

   Don't mix up which flash goes with which thunder

   In an active storm, the next flash can come before the previous thunder arrives. If the seconds suddenly drop, check that you have not caught the thunder from the earlier strike, and only log clear pairs.

4. 4

   Log several readings with the time

   One reading cannot tell you whether the storm is approaching. Time it every few minutes and list the times and seconds side by side, and the trend will start to show.

People take about 0.2 seconds to react to a light or sound and press a button. At the speed of sound that is roughly 70 m of error, which hardly matters when lightning distances are measured in kilometres. Bigger sources of error are pairing the wrong flash and thunder, and missing the sound altogether. On windy days, in heavy rain, or inside a car or building, distant thunder is hard to hear and it is easy to think the storm is further away than it is.

How far away can you hear thunder?

How far thunder carries depends on the terrain, the wind and background noise, but it is generally heard up to about 15 km away — about 40–45 seconds. From further away the sound bends upwards or fades before reaching the ground, and you only see the light. The silent flickering on the horizon on summer nights, often called heat lightning, is usually just a storm too far away for its thunder to reach you.

Lightning, on the other hand, does not only strike directly under the storm. A discharge can angle out of the side of the cloud and strike clear sky more than 10 km away — a “bolt from the blue”. The range where you can hear thunder and the range where lightning can reach you are roughly the same. In other words, if you can hear thunder at all, assume you are already within striking distance.

• Light but no sound: probably more than 15 km away, though the storm may still be coming
• Faint rumbling: around 10–15 km. Start heading for shelter
• Clear thunder, under 30 seconds: within 10 km and striking distance. Finish sheltering now
• Thunder almost at the same time as the flash: about 1 km or closer. If you are not in a building or car, move immediately

How to tell whether the storm is approaching or moving away

Thunderstorms drift with the wind, usually at about 10–40 km per hour. A single reading cannot tell you whether a storm is coming towards you or passing by. Time it every few minutes and compare with the clock: if the seconds keep getting shorter it is approaching, and if they get longer it is moving away.

The catch is that strikes are scattered across the storm. One flash may come from the near side of the cloud and the next from the far side, so individual distances jump around by several km. Rather than comparing two readings, fit a straight line through three or more and look at which way they are moving on average; that way the scatter does not throw you off.

In this example the gap shrinks from 55 to 42 seconds over 9 minutes. A straight-line fit says the storm is closing in at about 0.51 km per minute (about 30 km/h). The latest distance on the line is about 14.5 km, so at this rate it will be within 10 km in about 9 minutes. If you start moving as soon as you hear the thunder, you have time to reach shelter before it gets that close.

In the second example the 2-minute strike was close (44 seconds) and the 4-minute one far (52 seconds), so comparing pairs makes it look like the storm came closer, backed off, then came closer again. Fitting a line through all five readings still shows it approaching at about 0.36 km per minute (about 22 km/h), reaching 10 km in roughly 14 minutes. Don't react to each individual rise and fall; look at the overall slope.

The 30-second rule and “30 minutes after the last thunder”

A widely used guide is the 30-second rule: if the gap between the flash and the thunder is under 30 seconds, take shelter. At 25 °C, 30 seconds is about 10.4 km. Most strikes happen within 10 km of the storm, so thunder within 30 seconds means you are close enough that the next strike could reach you. In the US this was paired with “stay indoors for 30 minutes after the last thunder” and known as the 30-30 rule.

Current safety advice puts “when thunder roars, go indoors — whatever the count” ahead of counting to 30. Think of the 30-second rule as a final check for anyone who has not yet sheltered: has it reached the point where you cannot wait any longer? For outdoor sports, club practice, camping, fishing, golf or events, agree in advance that play stops and everyone moves to shelter the moment thunder is heard, and nobody has to hesitate.

When deciding whether to go back out, go by the time since the last thunder, not by whether the rain has stopped or the sky has brightened. Lightning can still strike from the back of a storm that has just passed. Note the time of the last thunder, and if 20 minutes (ideally 30) pass with no new thunder, it is OK to go outside. If you hear thunder even once in that time, start counting again from there.

Safe and unsafe places, and what to do with nowhere to go

Once you know the distance, the next question is where to go. You are protected where the lightning current can flow to the ground without passing through your body. Having a roof and walls is not enough: a small shed or a tent does not count.

If there is no building or car nearby, you can make use of tall objects such as utility poles, pylons or tall buildings. Guidance from the Japan Meteorological Agency and others describes a relatively protected zone around an object about 5–30 m tall: within the area where you look up at its top at 45° or more, and at least 4 m away from the object itself. Getting too close risks a side flash, so keeping 4 m away is part of the condition.

• Getting into a building or car comes first. Even if it is some distance away, start moving as soon as you hear thunder
• If you can't move, get into the protected zone of a tall object. With a tree, stay at least 2 m from the trunk, branches and leaves
• If you can't do that either, crouch with your feet together, keep your head low and cover your ears. Lying flat exposes you more to current flowing along the ground
• Don't hold anything long — umbrellas, fishing rods, golf clubs, trekking poles — above your body
• In a group, spread out and crouch several metres apart. Huddled together, one strike can hurt everyone

Bonus 1: distance to a fireworks launch site

The same calculation works for fireworks. Multiply the seconds between a shell bursting and hearing the boom by the speed of sound, and you get the straight-line distance to where it burst. Because fireworks burst high in the air, the distance over the ground to the launch site is found by subtracting the height with Pythagoras' theorem.

For example, at 25 °C, if you hear a 10-inch shell 3 seconds after it bursts, the burst was about 1,040 m away and the launch site is about 986 m away over the ground. With a burst height of about 330 m, you are looking up at about 18.5°. Watching a 30-inch shell from 2 seconds away (about 693 m) means looking up at about 60°, which gets tiring for your neck. If you are sitting, a spot where the viewing angle stays under about 30–45° is comfortable.

Heights and diameters by shell size are typical values (the sizes follow Japanese shell gauges) and vary with how the shell is made and with the maker. A single display may also have more than one launch site, so if your timings vary, measure the largest shells a few times and take the average.

Bonus 2: measuring the distance to a mountain with an echo

An echo is your voice bouncing off a facing mountain or cliff and coming back. Because the sound goes there and back, the one-way distance to whatever reflected it is seconds × speed of sound ÷ 2. The only difference from the lightning and fireworks calculation is dividing by 2 at the end.

Our ears can only hear an echo as a separate sound when the round trip takes at least 0.1 seconds — that is, when the reflecting surface is at least about 17 m away. A closer wall just makes your voice sound reverberant, not like a distinct echo. If you try this while hiking, combining it with an estimate of walking time (calculating hiking course times) gives you a real feel for how far away the next ridge is.

Common misconceptions about lightning distance and safety

• “It's only flashing in the distance, so it's fine”: storms move at 10–40 km/h, so a storm 20 km away can be overhead in 30 minutes to an hour. Shelter once you can hear thunder
• “It's more than 30 seconds, so I'm safe”: the 30-second rule is a final check. If you can hear thunder, you are already within striking distance
• “The rain has stopped, so I can go out”: go by the thunder, not the rain. Wait 20–30 minutes after the last thunder
• “Lightning never strikes the same place twice”: tall buildings and towers are struck many times, year after year
• “Taking off metal keeps you safe”: what matters is height. Small metal objects make almost no difference
• “A tree keeps off the rain and the lightning”: under a tree is one of the most dangerous places because of side flashes
• “Rubber boots or a raincoat insulate you”: lightning can be hundreds of millions of volts; a few millimetres of rubber won't stop it

The distance you calculate is only an estimate of where that particular thunder came from. There is no guarantee the next strike will be the same distance away, and it may be much closer. Check official sources such as thunderstorm warnings and real-time lightning maps from your national weather service as well, and when in doubt, shelter early.

How to use the Lightning Distance Calculator

With the Lightning Distance Calculator you just tap twice — at the flash and at the thunder — and it times the gap, converts it to distance and logs it with the time. As the log grows, it automatically tells you whether the storm is approaching or moving away and how long until it is within 10 km.

1. 1

   Enter the air temperature

   On the “Measure lightning” tab, enter the current temperature. If you are not sure, 25–30 °C is fine in summer. The speed of sound and the distance per second are shown.

2. 2

   Tap at the flash → tap again at the thunder

   Tap “Flash!” the moment you see lightning and “Thunder!” the moment you hear it. The space bar works too. If you didn't hear any thunder, tap “Cancel”. You can also log seconds you counted yourself.

3. 3

   Check the distance trend

   Log 3 or more strikes over at least 2 minutes to see a chart of the distance, an “Approaching” or “Moving away” result, how many km per minute it is closing in, and how many minutes until it is within 10 km at this rate.

4. 4

   Check the time since the last thunder

   Once you have sheltered, check “Since the last thunder” and how long is left until 20 and 30 minutes. Logging new thunder restarts the count.

5. 5

   Use the chart, fireworks and echo tabs

   The “Seconds ⇄ distance” tab converts seconds to distance and back, with a chart by temperature. On the “Fireworks & echo” tab, choose a shell size to get the distance to the launch site and the viewing angle, or enter echo seconds to get the distance to a mountain.

FAQ

The thunder came 10 seconds after the lightning. How far away was it?

At 25 °C, about 3.5 km (about 2 miles). Sound travels about 346.5 m per second, so 10 seconds is about 3,465 m. Under 5 km is very close — the next strike could land anywhere around you — so get into a building or car right away.

How do you calculate the distance to lightning?

Multiply the seconds between the flash and the thunder by the speed of sound (about 0.34 km per second). In your head, seconds ÷ 3 gives km and seconds ÷ 5 gives miles. More precisely, the speed of sound is 331.5 + 0.6 × temperature (°C) m/s, a little faster when it is warmer.

Is “3 seconds per km” accurate?

It is a good rule of thumb. At 0 °C, 3 seconds is about 0.99 km and at 25 °C about 1.04 km, so temperature changes it by only a few percent. For a quick decision, about 3 seconds per km (or 5 seconds per mile) is plenty.

How can I tell if a thunderstorm is getting closer?

Time the gap between flash and thunder every few minutes and note the time. If the seconds keep getting shorter, it is approaching. Strikes are scattered across the storm, so don't judge from just two readings — take 3 or more over at least 2 minutes and look at the overall trend.

What is the 30-second rule?

If the thunder comes less than 30 seconds after the flash (within about 10 km), you are within striking distance and should take shelter. Current advice, though, is to shelter as soon as you hear thunder at all, whatever the count.

How long should I wait after the thunder stops before going outside?

Wait 20–30 minutes after the last thunder. The Society of Atmospheric Electricity of Japan and others say 20 minutes; the US National Weather Service says 30. If you hear thunder even once in that time, start counting again. Whether the rain has stopped is not a guide.

How far away can you hear thunder?

It depends on the terrain, wind and background noise, but generally up to about 15 km (about 10 miles) — around 40–45 seconds. Beyond that you only see the light. The range where you can hear thunder is roughly the same as the range where lightning can strike, so start sheltering once you hear it.

Am I safe from lightning in a car?

A car with a metal roof is fairly safe: the current flows through the body to the ground. Close the windows and avoid touching metal parts of the car or steering wheel. Convertibles, motorbikes and bicycles offer no protection.

Can I find the distance to fireworks the same way?

Yes. Multiply the seconds between the burst and the boom by the speed of sound for the straight-line distance to the burst. To get the distance over the ground to the launch site, subtract the burst height (about 330 m for a 10-inch shell) using Pythagoras' theorem.