How flash-to-bang works
Light from a lightning flash reaches you almost instantly. Its thunder travels at the speed of sound, about 343 metres per second in 20 °C air. So the gap between the two, in seconds, times the speed of sound, is the distance to the strike.
distance (m) = seconds × c
c (m/s) = 331.3 × √(1 + T / 273.15) T in °C
miles = metres / 1,609.344That gives the familiar rule of thumb the National Weather Service teaches: about 5 seconds per mile, or 3 seconds per kilometre. Temperature matters a little: sound covers 331 m/s at 0 °C and 352 m/s at 35 °C, so a 30-second count is 6.2 miles on a cold day and 6.6 miles on a hot one.
Flash-to-bang distance table
At 20 °C (68 °F). Lightning can strike about 10 miles from a storm, roughly a 47-second gap.
| Seconds | Miles | Kilometres |
|---|---|---|
| 3 s | 0.6 | 1.0 |
| 5 s | 1.1 | 1.7 |
| 10 s | 2.1 | 3.4 |
| 15 s | 3.2 | 5.1 |
| 20 s | 4.3 | 6.9 |
| 30 s | 6.4 | 10.3 |
| 40 s | 8.5 | 13.7 |
| 47 s | 10.0 | 16.1 |
| 60 s | 12.8 | 20.6 |
What the guidance says at that distance
The distance tells you how close the last strike was. It is not the signal to move. Current guidance acts on thunder itself:
- NWS: “When thunder roars, go indoors.” Thunder is normally heard about 10 miles from a strike, and lightning can strike about 10 miles from a storm, so audible thunder means you are in range.
- NFHS (high schools): suspend play and move everyone to a safer location as soon as thunder is heard or lightning is seen. Where detection data is available, suspend when lightning is within 10 miles.
- NATA: if you can hear thunder, you are close enough to be struck; get to a safe place immediately.
- The old 30-30 rule: go in when the gap is 30 seconds or less (about 6 miles). Its first half has been retired; its second half, waiting 30 minutes after the last thunder, still stands.
Read the full story in the 30-30 lightning rule, explained, the side-by-side of NFHS and NCAA lightning guidelines, and how far lightning can strike. For when to go back out, see the lightning all-clear rule.
Why counting is not enough
Flash-to-bang only works when you can see the flash, hear its thunder and match the two. In a stadium crowd, under a roof or behind a hill, you may hear nothing until the storm is overhead. And by the time thunder is audible at all, the strike was within about 10 miles (a 47-second gap), which is already striking distance. The point of prediction is to start the move before that.
Clear the field before the first thunder
Flash Lightning Prediction forecasts strikes up to 60 minutes ahead for each 1×1 km cell, and runs the 30-minute all-clear clock for every site.
Sources
- NWS — Understanding Lightning: Thunder (5 seconds per mile; thunder heard about 10 miles away)weather.gov/safety/lightning-science-thunder (opens in a new tab)
- NWS — Lightning Safety: When thunder roars, go indoorsweather.gov/safety/lightning (opens in a new tab)
- NFHS SMAC — Guidelines on Handling Practices and Contests During Lightning or Thunder Disturbances (October 2024)
- NATA — Position Statement: Lightning Safety for Athletics and Recreation, J Athl Train 2013;48(2):258–270
- NCAA — Sports Medicine Handbook, Guideline 1d: Lightning Safety
- Speed of sound in dry air: c = 331.3 · √(1 + T/273.15) m/s (standard acoustics)
Frequently asked
Count the seconds from the flash to the thunder and divide by 5 for miles, or by 3 for kilometres. Sound travels about 343 metres per second at 20 °C, so each second is roughly 0.21 miles.
About 2.1 miles, or 3.4 km. That is well inside the roughly 10 miles from a storm that lightning can strike.
Only its second half. Current NFHS, NATA and NWS guidance says to go inside as soon as you hear thunder, not after a 30-second count. The 30-minute wait after the last thunder or lightning still stands.
The NWS says thunder is normally heard about 10 miles from a strike, and lightning can strike about 10 miles from a storm. If you can hear thunder, you are within striking distance.