To tell how far away lightning is in kilometers, you can use the "flash-to- bang" method:

1. As soon as you see the lightning flash, start counting the number of seconds until you hear the thunder.
2. Divide the number of seconds counted by 3.

The result gives you the approximate distance to the lightning in kilometers. For example, if you count 15 seconds between the flash and the thunder, the lightning is about 5 kilometers away (15 ÷ 3 = 5 km)

. This works because light travels almost instantaneously to your eyes, while sound travels much slower—about 0.33 to 0.35 km per second depending on air temperature. Since sound takes roughly 3 seconds to travel 1 kilometer, dividing the time interval by 3 converts seconds into kilometers

Summary:

• Count seconds between lightning flash and thunder.
• Distance in km = seconds ÷ 3.

If the time between flash and thunder is less than 30 seconds, the storm is close enough to be dangerous, so seek shelter

. This simple method is widely recommended for estimating lightning distance safely outdoors.