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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.