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 by the rule of thumb Distance = 8 × 0.213 = 1.70 miles precisely, which is what this page returns
Why the ÷5 rule reads low, and why that is deliberate. Dividing by 5 assumes sound covers a mile in 5 seconds, which works out to 1,056 ft/s. Sound at 68°F actually travels 1,125 ft/s, about 6% faster, so the rule places every storm slightly closer than it really is. For a safety rule that is the right direction to be wrong in. The ÷3 rule for kilometres needs no such fudge: 343 m/s really is very nearly one kilometre every three seconds.
This calculator uses the true speed, and refines it further if you give it a temperature. Cold air slows sound down, so the same count means a nearer storm on a winter night than on a summer afternoon.
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 delay (distances are the precise ones, at 68°F):
| Time delay | Distance | What to do |
|---|---|---|
| Under 5 sec | Under 1.1 miles | Extreme danger, take cover now |
| 5-15 sec | 1.1-3.2 miles | Very dangerous, get indoors |
| 15-30 sec | 3.2-6.4 miles | Inside the 30-30 rule, seek sturdy shelter |
| 30-60 sec | 6.4-12.8 miles | Move toward shelter and keep counting |
| Over 60 sec | Beyond 12.8 miles | Monitor, and note the storm may be tracking toward you |
Distance from the rain is not distance from the danger. Lightning strikes up to 10 miles from the visible rainfall, which is well beyond where most people think the storm ends. “Bolts from the blue” happen under apparently clear sky, from a storm cell that is nowhere near overhead. A cloudless patch above you proves nothing.
Counting only helps for lightning. A tornado offers no flash to start the clock on, so flash-to-bang cannot range one. If you can hear a tornado you are already inside a mile or two of it, and the answer is shelter rather than arithmetic.
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.
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