Flash → thunder → distance
How far away was that lightning?
Light reaches you almost instantly at storm-scale distances. Thunder is much slower, so the delay tells you roughly how far the sound traveled.
Ready to time a flash.
Estimated distance
3.43 km2.13 miles · 3,432 mdistance = flash-to-thunder time × speed of sound = 10 s × 343.2 m/s
Quick mental check
The classic shortcuts are close
At 20 °C, the more detailed estimate is 2.13 miles / 3.43 km. The common rules give about 2 miles (seconds ÷ 5) or 3.33 km (seconds ÷ 3).
Do not use the number as a “safe distance.” The U.S. National Weather Service says that if you can hear thunder, you are likely within striking distance of the storm. Use substantial shelter rather than this calculator to make a safety decision.
Method + limits
Why the estimate works
NOAA/NESDIS teaches the same flash-to-thunder method using the rough rule that sound takes about five seconds to travel one mile. This calculator instead estimates dry-air sound speed from temperature with the ideal-gas relation used by NASA, then multiplies that speed by your measured delay.
Wind, humidity, temperature layers, terrain and the long shape of a lightning channel can all shift what you hear. Thunder may also come from a different part of the flash than the point you visually notice, so the result is an estimate rather than a strike-location service.