Thunderstorm Distance Calculator

How to use the thunderstorm distance calculator

1. When you see a flash, start counting – “one-and-two-and…” or with a watch – and stop at the first sound of thunder. Enter the seconds.
2. Leave the mode on one count if you only want the distance and the danger level.
3. To see whether the storm is moving towards you, choose compare a second count, count again a few minutes later, and enter the second count and the minutes between the counts.

The result shows the distance in kilometres, a danger level (overhead, close – act now or in hearing range) with a short instruction, and a table of typical counts. With two counts you also get the approach speed, the time until the storm could be overhead and a small timeline – or, if the storm is moving away, its speed and a reminder of the 30-minute wait.

Worked examples

One count: 24 seconds

You see a flash over the next valley and count 24 seconds to the thunder. The calculator shows:

• Distance: 8.2 km (24 s × 0.343 km/s)
• Danger: Close – act now, with the note “30/30 rule: 30 s or less”
• The instruction: stop what you are doing and move to a building or a car, or off exposed ground

Eight kilometres sounds far, but under 30 seconds the storm is close enough to reach you.

Two counts: 24 seconds, then 15 seconds five minutes later

Five minutes after the first count you count again and get 15 seconds. With both counts entered:

• Distance: 5.1 km – the latest count decides the danger level, still Close – act now
• Approaching at 37 km/h – 3.1 km closer in 5 min
• Overhead in about 8 min, if it keeps this speed and track
• Timeline: now 5.1 km away, +5 min within 2 km, +8 min overhead

Eight minutes is not enough to walk off a long ridge. The time to leave was when the clouds were towering.

A distant storm: 60 seconds, then 40 seconds ten minutes later

Thunder is faint at first – 60 seconds – and ten minutes later the count is 40 seconds. The result: distance 13.7 km, danger level In hearing range, approaching at 41 km/h (6.9 km closer in 10 min) and overhead in about 20 min. The timeline: within 10 km – the 30-second zone – after about 5 minutes, within 5 km after 13 minutes, within 2 km after 17 minutes. Those twenty minutes are your chance to descend and reach a car or a building.

Moving away or standing still

A count of 20 seconds followed by 30 seconds ten minutes later gives a distance of 10.3 km and moving away at 21 km/h (3.4 km further in 10 min). The danger level is still Close – act now, and the calculator reminds you that a departing storm still strikes behind its rain: wait 30 minutes after the last thunder. Counts of 18 and then 17 seconds five minutes apart show 5.8 km and not clearly moving – the counts differ by only one second. Count again in a few minutes; a storm that seems to stand still can still grow and drift towards you.

Flash-to-bang at a glance

The calculator shows this table under every result and highlights the row nearest your count:

How it is calculated

Light reaches you practically instantly. Sound travels about 343 m per second in air at around 20 °C. The distance in kilometres is therefore the seconds × 0.343 – the familiar rule of thumb of about 3 seconds per kilometre.

The danger levels follow the 30/30 rule from the thunderstorm section of our weather guide:

• 5 seconds or less (under about 2 km): overhead.
• Up to 30 seconds (about 10 km): close – act now. Under 30 seconds the storm is close enough to strike you.
• Longer than 30 seconds: in hearing range. Many weather services simplify the rule: if you can hear thunder at all, you are within reach of lightning.

The second half of the rule does not need a calculator: wait 30 minutes after the last thunder before you return to exposed ground.

With two counts the tool uses the latest one for the danger level. It subtracts the newer distance from the older one and divides by the minutes between the counts to get the speed in km/h. If the two counts differ by 2 seconds or less (under 0.75 km), the storm is shown as not clearly moving, because a single count is only accurate to a second or two. The time until the storm is overhead assumes that it keeps its speed and its track – real storms often change both.

What the calculator cannot tell you

• No count means you are safe. Wind, rain, a stream or a ridge between you and the storm can swallow thunder. A quiet sky is not proof of safety when the clouds keep building.
• Lightning does not wait for the storm. Strikes can hit well away from the rain, ahead of the storm and behind it.
• The track. A storm passing to one side and one heading straight at you can give the same counts.
• Other storm hazards. Sudden gusts, hail, heavy rain and flash floods come with the same clouds – see the wind and Beaufort camp calculator for what strong wind means at camp.

Safety margin: treat every result as one level more serious than shown when you are on a ridge, on water or far from the car. The calculator is a planning aid, not a lightning warning.

What to do when you hear thunder

• Best: a solid building or a closed metal vehicle. Tents, tarps, open shelters and huts without lightning protection do not protect you.
• Leave summits, ridges, open fields and the water’s edge. Keep away from lone trees, fences and metal cables. On how to choose a lower, less exposed spot, see choosing a camp site in thunderstorm season.
• In a group, spread out several metres apart – about 15 m is often recommended – so one strike cannot hurt everyone and others can help.
• Last resort if a strike seems imminent (hair standing up, buzzing): crouch on your sleeping pad, feet together, head down. Do not lie flat. This only slightly reduces the risk.
• Someone hit? A person struck by lightning carries no charge. Call 112 and start first aid at once, including CPR if they are not breathing. Keep a first aid kit at hand.

The best protection is planning: check the forecast, start early on warm, humid days and be off peaks and ridges before the first thunder. Our trip planning guide shows how to build turn-around times into a route, and the daylight calculator tells you how much time you have left in the day. Everything about lightning itself – how it strikes and how injuries happen – is in the guide to lightning safety outdoors.