Count the seconds between seeing the lightning flash and hearing the thunder. Divide by 5 to get the approximate distance in miles, or divide by 3 to get the approximate distance in kilometers. For greater precision, multiply seconds by 0.213 for miles or by 0.343 for kilometers (based on the speed of sound at 20°C / 68°F).

The flash-to-bang method is a simple technique used by meteorologists, outdoor enthusiasts, and safety professionals to estimate lightning distance. You observe the lightning flash ("flash") and count the seconds until you hear the thunder ("bang"). The time gap reveals how far away the lightning strike occurred, since light travels almost instantly while sound travels much slower.

Light travels at approximately 300,000,000 meters per second (186,000 miles per second) — essentially instantaneous over short distances. Sound, by contrast, travels at roughly 343 meters per second (about 1,125 feet per second). This massive speed difference means you see the lightning almost immediately, while the thunder takes time to reach your ears. The greater the distance, the longer the delay.

The commonly used rule of thumb is 5 seconds per mile. More precisely, at 20°C (68°F), sound travels 1 mile in approximately 4.69 seconds. The 5-second rule slightly underestimates distance (by about 6-7%), but it's easy to remember and provides a reasonable safety margin — it tells you the storm is slightly closer than it actually is, which errs on the side of caution.

The common rule of thumb is 3 seconds per kilometer. The precise value at 20°C (68°F) is approximately 2.91 seconds per kilometer. Like the 5-second-per-mile rule, this slight overestimate provides a built-in safety margin, making you think the lightning is marginally closer than it truly is.

The 30-30 rule is a critical lightning safety guideline: First 30: If the flash-to-bang time is 30 seconds or less (meaning lightning is within 6 miles or 10 km), immediately seek shelter in a substantial building or hard-topped vehicle. Second 30: Wait at least 30 minutes after the last observed lightning flash or thunderclap before returning outdoors. Lightning can strike from storms that appear to have passed, and "bolts from the blue" can travel many miles from the storm cloud.

The flash-to-bang method is fairly accurate for distances up to about 10-15 miles (16-24 km). Beyond that, thunder may not be audible due to atmospheric conditions, terrain, and sound wave refraction. Accuracy also depends on air temperature, which affects sound speed. At 0°C, sound travels at ~331 m/s; at 30°C, ~349 m/s — a difference of about 5%. For most practical safety purposes, the simple 5-second/mile or 3-second/km rules work well.

Yes. The speed of sound in air increases with temperature. At 0°C (32°F), sound travels at about 331 m/s. At 20°C (68°F), it's approximately 343 m/s. At 30°C (86°F), it reaches roughly 349 m/s. The formula is: speed (m/s) = 331.3 + 0.606 × temperature (°C). Use the advanced temperature setting in our calculator above for more precise results, especially in very hot or cold conditions.

Lightning can strike up to 10-15 miles (16-24 km) away from the parent thunderstorm, even under clear blue skies — a phenomenon known as a "bolt from the blue." This is why the 30-30 rule uses 30 seconds (approximately 6 miles / 10 km) as the threshold for seeking shelter, providing a conservative safety buffer. Lightning has been documented striking over 25 miles (40 km) from its originating storm in extreme cases.

If the flash-to-bang time is under 30 seconds: (1) Immediately seek shelter in a substantial enclosed building or a hard-topped metal vehicle. (2) Avoid open fields, hilltops, tall isolated trees, water, and metal objects. (3) If no shelter is available, crouch low with feet together, minimizing ground contact — do NOT lie flat. (4) Stay away from windows, plumbing, and electrical appliances. (5) Wait at least 30 minutes after the last thunder before leaving shelter.

No. Thunder is the sonic shockwave produced by the rapid heating and expansion of air along a lightning channel. Every thunderclap is caused by a lightning strike. However, you may hear thunder without seeing the corresponding lightning if the flash is hidden behind clouds, terrain, or buildings, or if the strike occurred far away and the light was scattered or blocked while the sound still reached you.

Thunder travels at the speed of sound: approximately 343 meters per second (1,125 feet per second, or about 767 miles per hour) at 20°C (68°F). This is roughly 1 mile every 4.7 seconds, or 1 kilometer every 2.9 seconds. For comparison, a commercial jet flies at about 550 mph — thunder travels significantly faster than that, but still far slower than light.