Count the gap between the flash and the thunder to find out how far away lightning struck. Tap once when you see lightning and again when you hear thunder, or type the seconds; the calculator adjusts the speed of sound for air temperature and tracks whether the storm is coming closer.
Last updated:
0.0 s
Tap when you see lightning, tap again when you hear the thunder. Keyboard: Space or Enter.
Lightning distance–
Time a strike or enter seconds below
Strike log
Time two or more strikes to see whether the storm is coming closer.
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How to use the Lightning Distance Calculator
Tap Start (or press Space) the moment you see the lightning flash.
Tap Stop as soon as you hear the thunder. The distance appears immediately.
Set the air temperature for a slightly more accurate speed of sound.
Repeat for each strike. The log shows whether the storm is approaching and how fast.
Follow the safety banner: if the flash-to-bang is 30 seconds or less, get indoors now.
The flash-to-bang formula
Lightning heats the air to around 30,000 °C in a fraction of a second, and the expanding air creates thunder. The flash reaches you at the speed of light, effectively instantly, while the thunder travels at the speed of sound. So:
Distance = seconds × speed of sound
Speed of sound (dry air): c = 331.3 × √(1 + T / 273.15) m/s, where T is the air temperature in °C. At 20 °C (68 °F) that is about 343 m/s or 1,125 ft/s.
Rules of thumb: about 5 seconds per mile (4.7 s at 20 °C) and about 3 seconds per kilometer (2.9 s).
Example: you count 12 seconds at 25 °C. The speed of sound is 331.3 × √(1 + 25/273.15) ≈ 346 m/s, so the strike was 12 × 346 ≈ 4,150 m, or about 2.6 miles away. That is inside the 30-second danger zone, so it is time to go indoors.
Flash-to-bang reference table (at 68 °F / 20 °C)
Seconds
Miles
Kilometers
What to do
3
0.6
1.0
Very close: get indoors immediately
5
1.1
1.7
Very close: get indoors immediately
10
2.1
3.4
Dangerous: seek shelter now
15
3.2
5.1
Dangerous: seek shelter now
20
4.3
6.9
Dangerous: seek shelter now
30
6.4
10.3
30-30 rule limit: be indoors
40
8.5
13.7
Still within striking range
47
10.0
16.1
About the limit at which thunder is usually heard
Lightning safety tips
When thunder roars, go indoors. A fully enclosed building with wiring and plumbing, or a hard-topped vehicle, is the only safe place.
Not safe: open fields, hilltops, beaches, water, isolated trees, picnic shelters, tents, golf carts and open vehicles.
Indoors, stay off corded phones, avoid showers and baths, and keep away from windows and doors during the storm.
Wait 30 minutes after the last thunder before going back outside; the trailing edge of a storm can still produce strikes.
If someone is struck, call emergency services right away. People struck by lightning carry no electrical charge and can be helped immediately; CPR can save lives.
Frequently asked questions
How do you calculate how far away lightning is?
Light arrives almost instantly, while sound travels about 343 meters per second (1,125 feet per second) at 20 °C. Multiply the seconds between flash and thunder by the speed of sound. A quick rule: divide the seconds by 5 for miles or by 3 for kilometers.
Does temperature change the result?
Yes, slightly. The speed of sound in air is about 331 m/s at 0 °C and rises roughly 0.6 m/s per degree Celsius, so on a hot day thunder arrives a little sooner. The calculator uses c = 331.3 × √(1 + T/273.15) m/s for dry air.
What is the 30-30 rule?
If the time between the flash and the thunder is 30 seconds or less (about 6 miles or 10 km), the storm is close enough to be dangerous and you should get indoors. Then wait at least 30 minutes after the last thunder before going back outside.
How far away can lightning strike?
Lightning can strike 10 miles (16 km) or more from the storm that produced it, sometimes out of a clear sky. That is why weather services say: when thunder roars, go indoors. If you can hear thunder at all, you are within striking distance.
How far away can you hear thunder?
Usually up to about 10 miles (16 km), sometimes a little farther in quiet conditions. Beyond that, the sound is absorbed or bent upward by the atmosphere, so you may see distant lightning without hearing any thunder.
Is it safe to stay in a car during a thunderstorm?
A hard-topped metal car or truck is a reasonable shelter because the metal body carries the current around you, not because of the rubber tires. Close the windows and avoid touching metal parts. Convertibles, motorcycles and open vehicles do not protect you.
How accurate is the flash-to-bang method?
Your reaction time is usually the biggest error: a quarter of a second is about 85 meters (280 feet). Long lightning channels also make thunder rumble from several distances at once, so time to the first sound of thunder for the nearest point of the strike.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.