Lightning Distance Calculator | Time the Seconds Between the Flash and the Thunder to See How Far Away the Storm Is, Whether It Is Approaching, the 30-Second Rule and 30 Minutes After the Last Thunder — Plus Fireworks and Echo Distance
The Lightning Distance Calculator is a free tool that works out how far away lightning struck from the number of seconds between the flash and the thunder (the flash-to-bang method). Tap once at the flash and once at the rumble and it times the gap like a stopwatch, converts it to distance using the speed of sound for your air temperature (331.5 + 0.6 × °C m/s) and logs each strike. As the log grows it draws a distance-over-time chart, tells you whether the storm is approaching or moving away, how many km per minute it is closing in, and how many minutes until it is within 10 km. It also checks the 30-second rule (take shelter if the gap is under 30 seconds, about 10 km) and counts the 20–30 minutes after the last thunder before it is considered safe to go back outside. Other tabs convert seconds to distance and distance to seconds with a chart by temperature (“3 seconds per km, 5 seconds per mile”), find the distance to a fireworks launch site from the delay of the boom with the burst height and viewing angle by shell size, and find the distance to a mountain from the time an echo takes to return. Your log is stored only in your browser.
Tap at the flash → tap again at the thunder
Tap once the moment the lightning flashes and again the moment you hear the rumble. The seconds it took the sound to arrive are turned into the distance to the lightning and logged. The space bar works too.
Speed of sound 346.5 m/s (about 347 m per second). If unsure, 25–30 °C is fine in summer
You can also log seconds you counted (“one-Mississippi, two…”)
Lightning safety guide
- If the thunder comes less than 30 seconds after the flash (within about 10 km), you are within striking distance (30-second rule)
- If you can hear any thunder, the storm is moving over you. Get into a building or a car
- Sheltering under a tree is dangerous. Stay at least 2 m away from the trunk, branches and leaves
- Wait 20–30 minutes after the last thunder before going back outside
Your taps include human reaction time (about 0.2 seconds). The distance is an estimate to where the sound came from and does not guarantee lightning safety. Your log is stored only in this device's browser.
ABOUT
About Lightning Distance Calculator | Time the Seconds Between the Flash and the Thunder to See How Far Away the Storm Is, Whether It Is Approaching, the 30-Second Rule and 30 Minutes After the Last Thunder — Plus Fireworks and Echo Distance
The Lightning Distance Calculator is a free tool that finds how far away lightning struck from the seconds between the flash and the thunder. Tap twice — at the flash and at the rumble — and it times the gap, converts it to distance using the speed of sound for the air temperature, and logs the strike. Once you have three or more strikes, it tells you whether the storm is approaching or moving away, how many km per minute it is closing in and how many minutes until it is within 10 km, and it shows the 30-second rule check and the time since the last thunder (with 20- and 30-minute targets). It also converts seconds and distance with a chart, and calculates fireworks and echo distances.
How to use Lightning Distance Calculator | Time the Seconds Between the Flash and the Thunder to See How Far Away the Storm Is, Whether It Is Approaching, the 30-Second Rule and 30 Minutes After the Last Thunder — Plus Fireworks and Echo Distance
- 1Open “Measure lightning” and enter the air temperature (if unsure, leave it at 25–30 °C in summer)
- 2Tap “Flash!” the moment you see the lightning (the space bar works too)
- 3Tap “Thunder!” the moment you hear the rumble; the seconds and distance are logged
- 4Repeat for each flash and check the distance chart, the approaching / moving away result and the 30-second rule
- 5After taking shelter, use “Since the last thunder” to check that 20–30 minutes have passed before going outside
- 6To convert seconds only, use “Seconds ⇄ distance”; for fireworks or echoes, enter the seconds in “Fireworks & echo”
Great for
- Time distant lightning while camping, hiking, fishing or golfing to decide whether to pack up
- Decide when to stop or evacuate a club practice, sports day or outdoor event using the 30-second rule
- See from a few timings and a chart whether a thunderstorm is coming towards you or moving away
- Count the time since the last thunder to know when 20–30 minutes have passed
- Investigate the speed of sound and distance from the gap between light and sound for a school science project
- Find the distance to the launch site and the viewing angle at a fireworks display from the delay of the boom
- Work out the distance to the facing mountain from how long an echo takes to come back
Good to know
- The speed of sound is calculated as 331.5 + 0.6 × air temperature (°C) m/s: about 340 m/s at 15 °C and about 349.5 m/s at 30 °C. The time light takes to arrive is tiny (about 0.00003 seconds over 10 km), so it is ignored.
- Remember “about 3 seconds per km” (or “5 seconds per mile”) and you can estimate the distance by counting even without a stopwatch.
- The result is the distance to where that thunder came from — the closest part of the lightning channel. A lightning channel is several km long, which is why the rumble lasts a while.
- The “30-second rule” says to take shelter if the gap between flash and thunder is under 30 seconds (within about 10 km), because the next strike could reach you. Lightning can strike more than 10 km from the storm, so if you can hear thunder at all, move to a safe place regardless of the count.
- For returning outside, the Society of Atmospheric Electricity of Japan recommends waiting at least 20 minutes after the last thunder and the US National Weather Service at least 30 minutes. This tool shows the time left to both.
- Approaching or moving away is judged by fitting a straight line to the logged times and distances. Strikes are scattered across the storm, so the result is shown only after three or more timings over at least 2 minutes, and it is only a guide.
- Your taps include human reaction time (about 0.2 seconds). That is about 70 m per second of error, which hardly affects a lightning distance.
- Firework heights and burst diameters by shell size are typical values and vary with shell construction and the maker.
- This tool is a guide to help you decide when to shelter and does not guarantee safety. Check official thunderstorm warnings and lightning maps too. Your log is stored only in this device's browser.
Questions this tool answers
Use Lightning Distance Calculator | Time the Seconds Between the Flash and the Thunder to See How Far Away the Storm Is, Whether It Is Approaching, the 30-Second Rule and 30 Minutes After the Last Thunder — Plus Fireworks and Echo Distance for searches like these:
- How many km away is lightning if the thunder comes after so many seconds?
- Is the thunderstorm getting closer or moving away?
- Should I take shelter under the 30-second lightning rule?
- How many minutes after thunder stops can I go outside?
- How far away can thunder be heard?
- How does the speed of sound change with temperature?
- How far away are the fireworks if the boom is delayed by a few seconds?
- How far away is the mountain if the echo takes 2 seconds?
FAQ
- If the thunder comes 10 seconds after the flash, how far away is the lightning?
- About 3.4 km (2.1 miles). Sound travels about 340 m per second at 15 °C, so 10 × 340 m = 3,400 m. At 25 °C the speed of sound is about 346.5 m/s, giving about 3.5 km. That is within 10 km, so move into a building or a car right away.
- What is the 30-second rule for lightning?
- If the gap between the flash and the thunder is under 30 seconds, the lightning is within about 10 km (6 miles) and the next strike could reach you, so take shelter. It was originally paired with “stay inside until 30 minutes after the last thunder” as the 30-30 rule. Today the basic advice is “when thunder roars, go indoors” straight away.
- How can I tell if a thunderstorm is approaching or moving away?
- Time the gap for each flash: if the seconds get shorter, the storm is getting closer. In this tool, log a few “Flash!” / “Thunder!” taps and it charts the distance, shows how many km per minute the storm is approaching (or moving away) and how many minutes until it is within 10 km at that rate.
- How far away can you hear thunder?
- Usually up to about 10–15 km. Lightning farther away is often seen as a flash with no sound (heat lightning). If you can hear thunder, assume you are close enough to be struck.
- How long should I wait after thunder before going outside?
- Wait 20–30 minutes after the last thunder (at least 20 minutes according to the Society of Atmospheric Electricity of Japan, 30 minutes according to the US National Weather Service). This tool shows the time since the last logged thunder and how long is left until 20 and 30 minutes.
- If I hear a firework 3 seconds after it bursts, how far away is it?
- At 25 °C, about 1,040 m to the burst. A 10-inch shell bursts at about 330 m, so the ground distance to the launch site is about 990 m and you look up at about 18°.
- If an echo comes back in 2 seconds, how far away is the mountain?
- About 340–350 m. Sound travels to the mountain and back, so the one-way distance is 2 seconds × speed of sound ÷ 2 (about 347 m at 25 °C).
ARTICLES
Guides for Lightning Distance Calculator | Time the Seconds Between the Flash and the Thunder to See How Far Away the Storm Is, Whether It Is Approaching, the 30-Second Rule and 30 Minutes After the Last Thunder — Plus Fireworks and Echo Distance
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