Tap the flash. Tap the thunder. The tool tells you exactly how far the strike was and whether you should still be outside.
The moment you see a lightning bolt, light has traveled from the strike to your eye instantly (at 299 million meters per second, we can call it instantaneous). Thunder, on the other hand, moves at the speed of sound — about 343 meters per second in typical air. That gap between the flash and the boom is directly proportional to how far away the strike was.
The classic rule of thumb is: count seconds, divide by 5 for miles (or by 3 for kilometers). This calculator does it a bit more precisely — it factors in air temperature, because sound moves about 0.6 m/s faster for every 1°C rise in temperature.
Even distant thunder is a warning. The National Weather Service estimates the average lightning strike can jump about 6 miles from the storm's core, and some documented "bolts from the blue" have struck 10, 15, even 25 miles from the parent thunderstorm. If you can hear thunder, you are within lightning range.
The odds of being struck in a given year are about 1 in a million in the US, but the odds go up dramatically if you're outdoors during a storm. Roughly 25 to 30 Americans are killed by lightning each year, and about 90 percent of survivors have lasting injuries.
More on staying safe: tornado & storm safety guide · lightning trivia quiz.