How do you tell how far away lightning is?

The flash-to-bang method is simple and genuinely accurate. When you see a lightning flash, start counting seconds until you hear the thunder, then divide by five to get the distance in miles (or divide by three for kilometers). This works because light reaches you essentially instantly while sound travels much slower, roughly one mile every five seconds, so the delay between seeing the flash and hearing the thunder tells you how far the lightning struck. A ten-second gap means the strike was about two miles away; a five-second gap means about one mile. If flash and thunder come almost together, the lightning is right on top of you.

This is not just a fun trick, it is a safety tool, and it feeds the rule lightning safety experts actually teach: when thunder roars, go indoors, and the 30-30 rule. If the time between flash and thunder is 30 seconds or less, the lightning is within about six miles, which means you are already in striking range and should get inside a substantial building or hard-topped vehicle immediately, because the next strike could hit your area. Lightning regularly strikes several miles from the center of a storm, even under blue sky ahead of or behind the rain (bolts from the blue), so if you can hear thunder at all, you are close enough to be in danger. The second 30 is to wait at least 30 minutes after the last thunder before going back outside, since strikes can trail a departing storm. So counting the gap tells you the distance, and if that gap is short, the honest takeaway is not how interesting but get inside now.