Lightning Distance Calculator — How Far Away Is the Storm? (Flash-to-Bang Method)

How to Calculate the Lightning Distance?

Light travels at 186,291 miles per second (~300,000 km/s), while the speed of sound in dry air is only about 0.21–0.22 miles per second (0.33–0.35 km/s) — varying with air temperature. This seven-orders-of-magnitude difference makes it easy to estimate how far away a lightning strike is.

You see the lightning flash almost instantly, but the thunder takes time to travel. The formula is simple:

Storm distance = Time × Speed of sound

Where Time is the number of seconds between noticing the flash and hearing the thunder, and Speed of sound is the speed of sound in air at a specific temperature.

Storm Distance — How Far Away Is Lightning? ⛈️

The method of counting the seconds between lightning and thunder is sometimes called the flash-to-bang technique. Here's how to use it step-by-step:

1. Watch the stormy sky. Hopefully you are at some distance from the thunderstorm, not in the middle of it!
2. As soon as you notice the lightning strike, count the number of seconds until you hear the thunder. Use your watch or phone, or count as accurately as possible.
3. Calculate the storm distance:
   • In miles: divide the seconds by 5
   • In kilometers: divide the seconds by 3
   For example, 15 seconds gives about 3 miles or 5 km.
4. Or simply use our Lightning Distance Calculator, which computes the distance more accurately using the exact speed of sound at a given air temperature!

Speed of Sound and Temperature

The speed of sound in air depends on temperature. The standard formula is:

v = 331.3 + 0.606 × T

where v is the speed of sound in m/s and T is the air temperature in °C. At 20 °C (68 °F), the speed of sound is approximately 343.4 m/s (1,126 ft/s or 768 mph).

Colder air slows down sound — hot summer air speeds it up slightly. Our calculator uses this formula automatically when you enter the air temperature.

FAQs

How do I calculate the distance of lightning in km?

1. Note the time in seconds between the lightning flash and the thunderclap.
2. Divide the number of seconds by 3.
3. That's the approximate lightning distance in kilometers.

How do I calculate the distance of lightning in miles?

1. Measure the time in seconds between the lightning flash and the thunderclap.
2. Divide the number of seconds by 5.
3. That's the approximate lightning distance in miles.

What is the 30-30 rule for lightning?

When you see lightning, start counting to 30. If you hear the thunder before you finish counting, the thunderstorm is close enough to be dangerous — seek shelter immediately. Go outside only after 30 minutes from the last thunderclap.

How long do I have to wait between thunder?

You must wait for at least 30 minutes after the last thunderclap, as there may still be electric charges lingering in the clouds. This means there is still a chance of another lightning strike.

Is lightning distance affected by humidity?

High humidity slightly slows down sound because water vapor is lighter than dry air molecules — but the effect is small (typically less than 1%). For practical lightning distance estimation, air temperature is the most important factor.