Tornado Distance Calculator (Flash-to-Bang)
Estimate tornado or storm distance using the flash-to-bang method.
Calculate how far away lightning and severe weather are.
The flash-to-bang method estimates how far away a storm is by counting the seconds between seeing lightning (the flash) and hearing thunder (the bang).
Basic formula:
Distance (miles) = Seconds between flash and bang / 5
Distance (km) = Seconds between flash and bang / 3
Why this works: Light travels at approximately 186,000 miles per second (300,000 km/s), so the flash is essentially instantaneous. Sound (thunder) travels at approximately 1,125 feet per second (343 meters per second), which is roughly 1 mile every 5 seconds or 1 km every 3 seconds.
Distance conversion table:
| Seconds | Distance (miles) | Distance (km) | Danger Level |
|---|---|---|---|
| 5 | 1.0 | 1.6 | Extreme, seek shelter immediately |
| 10 | 2.0 | 3.2 | Very dangerous |
| 15 | 3.0 | 4.8 | Dangerous |
| 20 | 4.0 | 6.4 | Moderate risk |
| 25 | 5.0 | 8.0 | Approaching, prepare |
| 30 | 6.0 | 9.7 | Monitor closely |
| 45 | 9.0 | 14.5 | Watch the sky |
| 60 | 12.0 | 19.3 | Storm in the area |
The 30-30 Rule for lightning safety:
- If the flash-to-bang time is 30 seconds or less (6 miles / 10 km), seek shelter immediately.
- Wait 30 minutes after the last flash of lightning before going back outside.
Factors affecting accuracy:
- Wind: Strong winds can carry thunder away from you, making the storm seem farther.
- Terrain: Hills and buildings can block or reflect sound.
- Temperature: Sound travels faster in warm air (~1,160 ft/s at 86°F / 30°C) and slower in cold air (~1,085 ft/s at 32°F / 0°C).
- Thunder range: Thunder is rarely audible beyond 10–15 miles (16–24 km).
Tornado-specific safety: If you see a tornado and can hear it (roaring sound like a freight train), it is very close (within 1–2 miles). Seek underground shelter immediately. If no basement is available, go to the lowest floor interior room away from windows.
Tip: Count slowly (“one-Mississippi, two-Mississippi…”) to estimate seconds between flash and bang. If the interval is getting shorter, the storm is approaching. If it is getting longer, the storm is moving away.