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Lightning Distance Calculator

The Lightning Distance Calculator tells you how far away a lightning strike is by using the delay between seeing the flash and hearing the thunder. Light reaches you almost instantly, so the gap is set by the speed of sound — about 343 metres per second. Enter the seconds you counted and it returns the distance in kilometres and miles, with a plain safety prompt.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

How to use this calculator

  1. Start counting the moment you see the lightning flash.
  2. Stop when you hear the thunder and enter that number of seconds.
  3. Read the distance and the safety status, then act on the 30/30 rule.

How it works

The distance sound travels is time × speed of sound (343 m/s). That gives metres, which are converted to kilometres (÷ 1000) and miles (÷ 1.609). If the result is 10 km or less the storm is treated as within strike range, prompting the 30/30 safety rule.

Worked example

Worked example. A 9-second gap gives 9 × 343 = 3087 m = 3.087 km, about 1.919 miles away — well within strike range.

Common mistakes

  • Starting the count at the thunder instead of the flash.
  • Assuming a distant strike means you are safe; storms move fast and the next strike can be much closer.
  • Forgetting the 30/30 rule — shelter at a 30-second gap and wait 30 minutes after the last thunder.

Frequently asked questions

Why divide the seconds by 3 for kilometres?

Sound travels about 343 m per second, so roughly 3 seconds pass for each kilometre. Dividing the count by 3 gives a quick distance estimate.

Is a 10 km distance safe?

No. Lightning can strike more than 10 km from a storm's rain. If you can hear thunder you are within range — seek shelter.

Related tools

Explore more in Weather & Environmental Monitoring.

Tip: Enter any known values to calculate the remaining results.

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