Lightning Distance Calculator
Calculate lightning strike distance in miles and km by counting seconds between flash and thunder.
Returns distance and 30-30 rule shelter guidance.
Lightning distance calculation uses the time difference between seeing a lightning flash and hearing the resulting thunder. Because light travels nearly instantaneously (at 186,000 miles/sec), the delay between flash and thunder represents only the travel time of sound.
Distance Formula:
Distance (miles) = Time (seconds) / 5
Distance (km) = Time (seconds) / 3
These constants represent the approximate time for sound to travel 1 mile (5 seconds) and 1 kilometer (3 seconds) at standard air temperature.
More precise formula: Speed of sound at 20°C (68°F) = 343 m/s = 0.343 km/s = 0.213 miles/s
Distance (km) = Time (sec) × 0.343 Distance (miles) = Time (sec) × 0.213
Worked example: You see a lightning flash, then hear thunder 8 seconds later. Distance = 8 / 5 = 1.6 miles (approximate) Distance = 8 × 0.213 = 1.70 miles (precise)
The 30-30 rule (National Weather Service recommendation):
- First 30: If thunder is heard within 30 seconds of a flash (6 miles or less), seek shelter immediately
- Second 30: Wait 30 minutes after the last thunder before going back outside
Safety reference by distance:
| Time Delay | Distance | Safety Level |
|---|---|---|
| Under 5 sec | Under 1 mile | Extreme danger, take cover NOW |
| 5–15 sec | 1–3 miles | Very dangerous, indoors immediately |
| 15–30 sec | 3–6 miles | Dangerous, seek sturdy shelter |
| 30–60 sec | 6–12 miles | Caution, move toward shelter |
| Over 60 sec | 12+ miles | Monitor, storm may be moving toward you |
Lightning can strike up to 10 miles from visible rainfall. “Bolts from the blue” occur in seemingly clear conditions when a storm is nearby — never assume safety based on cloudless skies alone.