Seconds between flash and thunder to miles and kilometers, with National Weather Service safety guidance
Updated October 2026
Light from the flash reaches you almost instantly; the sound of thunder travels at the speed of sound, about 343 m/s in air at 68 °F (20 °C). That is 4.69 seconds per mile or 2.91 seconds per kilometer, which the National Weather Service rounds to 5 seconds per mile. Distance = seconds × speed of sound, with the speed from 331.3 × √(1 + T/273.15) m/s for air temperature T in °C. Worked example: a 15-second count at 68 °F is 15 × 343.2 = 5,148 m, or 3.2 miles (5.1 km); the NWS rule gives 3 miles.
| Seconds | Air (°F) | NWS rule (mi) | Calculated (mi) | Calculated (km) | Note |
|---|---|---|---|---|---|
| 0 | 68 | 0.0 | 0.0 | 0.0 | Thunder with the flash |
| 5 | 68 | 1.0 | 1.1 | 1.7 | NWS example |
| 10 | 68 | 2.0 | 2.1 | 3.4 | Common count |
| 15 | 68 | 3.0 | 3.2 | 5.1 | NWS example |
| 30 | 90 | 6.0 | 6.5 | 10.5 | Hot summer afternoon |
| 30 | 32 | 6.0 | 6.2 | 9.9 | Freezing air |
| 45 | 68 | 9.0 | 9.6 | 15.4 | Distant storm |
Sound is faster in warm air, about 0.6 m/s more per °C. A 30-second count is 6.5 miles on a 90 °F afternoon and 6.2 miles at freezing. The difference is small next to the error in counting seconds by ear, which is why the NWS uses a single rule. The speed of sound calculator covers other temperatures and materials.
The NWS states it plainly: there is no safe place outside when thunderstorms are in the area, and if you hear thunder you are likely within striking distance. Thunder can be heard only about 10 miles away, while lightning can strike as far as 10 miles from the storm; NWS myth-busting notes “bolts from the blue” 10 to 15 miles out. Safe places are substantial buildings with wiring and plumbing, or a hard-topped metal vehicle with the windows closed. Porches, dugouts, picnic shelters and tents are not safe.
Stay inside at least 30 minutes after the last thunder you hear; charge can linger in the clouds after a storm seems to have passed. An older approach, the 30-30 rule (the American Meteorological Society’s 2002 lightning safety statement, since expired), treated a flash-to-bang count under 30 seconds, about 6 miles, as the signal to take shelter. NWS research summaries note that successive flashes are often 6 to 8 miles apart rather than the 2 to 3 miles once assumed, and current NWS messaging uses the simpler rule: if you hear thunder, go indoors. Some local NWS pages still mention 30/30; the 30-minute wait is common to both.
The NWS glossary defines heat lightning as lightning far enough away that its thunder can no longer be heard. It is ordinary lightning from a distant storm, not a separate kind caused by heat, and that storm can move closer.
For organized activities such as a kids’ soccer game, the NWS recommends a written lightning safety plan followed without exception: check the forecast beforehand, name a lightning monitor who is not the coach or referee, stop play at the first thunder or lightning, allow time for everyone to reach a safe building or vehicle, and resume no sooner than 30 minutes after the last thunder. The NWS notes that about two-thirds of lightning deaths are associated with outdoor recreation.
It does not tell you whether you are safe; no flash-to-bang count does. It cannot see storms you cannot hear, and wind, traffic and rain can hide thunder. Use radar and lightning-detection apps and NWS warnings for planning, and call 911 for anyone struck: victims carry no electrical charge and may need CPR or an AED.
Counting from the start of a long rumble. Count to the first sound of thunder; the rumble comes from farther parts of the channel.
Matching the wrong flash to the thunder. In an active storm, flashes overlap; use a clear, isolated one.
Treating a large count as safe. Any audible thunder means the storm can reach you.
Dividing by 3 for miles. Divide by 5 for miles and by 3 for kilometers.