The Flash-to-Bang Method
When a thunderstorm is in the area, one of the most important things you can do is estimate how far away the lightning is striking. This tells you whether you are in immediate danger and whether you need to seek shelter. The technique for doing this is called the flash-to-bang method, and it relies on a simple physical fact: light travels almost instantaneously, while sound takes a measurable amount of time to cover the same distance.
If you want current map context, check the closest-strike checker for an estimated distance to the nearest recent satellite detection, the last-24-hours lightning strike map, or search your area on the live lightning map before relying on sound alone.
Use this method only from a safe location; do not remain outdoors watching a storm to collect a timing measurement. Here is how the estimate works:
Step-by-Step Instructions
- Note a visible flash from shelter. Do not delay moving indoors to time lightning.
- Start counting immediately. The moment you see the flash, begin counting seconds. Use the "one-one-thousand, two-one-thousand" method, or count using a watch or phone timer for greater accuracy.
- Stop when you hear thunder. As soon as you hear the first rumble, crack, or boom of thunder, stop counting.
- Divide by five. Take the number of seconds you counted and divide by five. The result is the approximate distance to the lightning strike in miles. (For kilometers, divide the seconds by three instead.)
For example, if you count 15 seconds between a flash and the thunder, the lightning struck approximately 3 miles away. If you count only 5 seconds, the strike was about 1 mile away and you should already be inside a safe structure.
Thunder Distance Calculator
If thunder is audible, head indoors regardless of the calculated distance. For an estimate to the nearest detected strike, use the closest-strike checker.
Distance Reference Table
The following table provides quick conversions from the flash-to-bang count to distance. Use this as a reference during thunderstorms to assess your level of risk.
| Seconds | Distance (Miles) | Distance (Kilometers) | Risk Level |
|---|---|---|---|
| 0–5 | 0–1 | 0–1.6 | Extreme: take cover immediately |
| 5–10 | 1–2 | 1.6–3.4 | Very high: you should already be sheltered |
| 10–15 | 2–3 | 3.4–5.1 | High: seek shelter now |
| 15–20 | 3–4 | 5.1–6.8 | Thunder audible: go indoors |
| 20–25 | 4–5 | 6.8–8.5 | Thunder audible: go indoors |
| 25–30 | 5–6 | 8.5–10.2 | Thunder audible: go indoors |
| 30+ | 6+ | 10.2+ | Thunder audible: go indoors |
The Science: Why Five Seconds Equals One Mile
The flash-to-bang method works because of the enormous difference between the speed of light and the speed of sound. Light from a lightning bolt travels at approximately 300,000 kilometers per second (186,000 miles per second), reaching your eyes in essentially zero time regardless of distance. The flash is effectively instantaneous.
Sound, however, travels at approximately 343 meters per second at sea level and standard atmospheric conditions (20 degrees Celsius, or 68 degrees Fahrenheit). Converting to more familiar units, this is about 1,125 feet per second, or roughly one mile every 4.7 seconds. Rounding to 5 seconds per mile provides a practical and sufficiently accurate rule of thumb.
The speed and path of sound vary with temperature, humidity, wind, altitude, terrain, and buildings. Five seconds per mile is therefore a practical approximation, not an exact safety measurement.
When to Seek Shelter: If You Hear Thunder
The National Weather Service guidance is simpler than waiting for a calculated distance: when thunder roars, go indoors. If thunder is audible, move to a substantial building or hard-topped vehicle.
Flash-to-bang timing can explain approximate distance, but it should not delay shelter. Lightning can occur outside the heaviest rain and beyond the apparent storm core, while storm motion and new development can change quickly.
Remain sheltered for at least 30 minutes after the last thunder. A map, app alert, or temporary gap in detections does not provide an all-clear.
What Counts as "Safe Shelter"?
Not all structures provide adequate lightning protection. A safe shelter is either:
- A substantial building with plumbing and electrical wiring. The wiring and plumbing provide paths for lightning current to flow to the ground, reducing the risk to occupants. Homes, offices, stores, and schools all qualify.
- A hard-topped vehicle with the windows closed. The metal body of the car acts as a Faraday cage, directing lightning current around the exterior and into the ground. This is why convertibles, golf carts, ATVs, and motorcycles do not provide protection.
Structures that do not provide protection include open pavilions, picnic shelters, tents, carports, dugouts, sheds without wiring, and bus stop shelters. No outdoor posture makes you safe; continue toward a substantial building or hard-topped vehicle without sheltering under an isolated tree.
Limitations of the Flash-to-Bang Method
While the flash-to-bang method is the best tool available to individuals in the field, it does have limitations you should be aware of:
- Multiple storms: When several thunderstorm cells are active simultaneously, it can be difficult to match a specific flash with its corresponding thunder, especially when multiple rumbles overlap.
- Continuous thunder: During intense storms, thunder from multiple strikes can blend into continuous noise, making it impossible to isolate individual flash-to-bang intervals.
- Intra-cloud lightning: In-cloud flashes are visible but may produce thunder that sounds different from or arrives at unexpected times compared to cloud-to-ground strikes.
- Terrain and buildings: Sound can be reflected or blocked by terrain, buildings, and vegetation, potentially making thunder seem closer or farther than it actually is.
- Wind: Strong winds can carry sound toward you (making the storm seem closer) or away from you (making it seem farther), though the effect on five-second-per-mile accuracy is generally small.
Flash-to-bang timing can add context, but audible thunder itself is the safety trigger. Combine official forecasts and warnings with recent detection data, and never use a calculated distance as permission to remain outdoors.
Need a refresher on why the flash arrives before the sound? Read our thunder vs lightning guide before using flash-to-bang timing.
Beyond Counting Seconds
NOAA GOES-19 East and GOES-18 West GLM provide broad optical lightning coverage for the United States, but no sensor detects every flash and a marker is an approximate reported flash location rather than an exact ground-impact point. Learn more in our article on GOES-19 GLM lightning detection and the Lightning Tracker methodology.
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