Distance from flash to thunder — time in any unit, distance in any unit.
Distance (km) = t ÷ 3 Distance (miles) = t ÷ 5
Light from a lightning bolt reaches your eyes nearly instantaneously (light travels at ~186,000 miles per second), so the delay you observe is almost entirely due to the travel time of sound. The speed of sound at sea level in air at ~20 °C (68 °F) is approximately 343 m/s (1,125 ft/s). Dividing this speed by the number of seconds you count gives the distance to the strike.
These are the standard approximations used by the National Weather Service.
Distance in km = 15 ÷ 3 = 5 km Distance in miles = 15 ÷ 5 = 3 miles
Result: The lightning strike was approximately 5 km (3 miles) away.
A 15-second gap between the flash and the thunder means the storm is about 3 miles (5 km) away. At this distance, the storm is dangerously close — lightning can strike up to 10 miles from the rain area. The National Weather Service recommends the 30-30 Rule: if the time between flash and thunder is 30 seconds or less (≤ 6 miles / 10 km), go indoors immediately and wait 30 minutes after the last thunder before going back outside.
Lightning is an enormous electrical discharge that superheats the surrounding air to roughly 30,000 K (five times hotter than the sun's surface) in a fraction of a second. This explosive heating causes the air to expand rapidly, creating a shock wave that we hear as thunder.
Because light travels at ~299,792 km/s, the flash reaches your eyes almost instantaneously regardless of distance. Sound, however, travels at only ~343 m/s (about 1,235 km/h) at 20 °C at sea level. This enormous difference in speed creates the delay between the flash and the boom — and that delay is your measuring tool.
| Time (seconds) | Distance (km) | Distance (miles) | Safety Status | |---|---|---|---| | 5 | ~1.7 km | ~1 mile | Extreme danger | | 10 | ~3.3 km | ~2 miles | Extreme danger | | 15 | ~5 km | ~3 miles | Danger — seek shelter | | 30 | ~10 km | ~6 miles | Seek shelter now | | 60 | ~20 km | ~12 miles | Storm nearby |
The NWS recommends:
Lightning kills an average of 20 people per year in the United States and injures hundreds more. It can strike up to 10–15 miles (16–24 km) from the storm's rain area — sometimes from a clear blue sky. Knowing the distance gives you precious time to act.
The division-by-3 (km) and division-by-5 (miles) rules are accurate to within about 10–15% under typical conditions. The primary sources of error are human reaction time (~0.2–0.5 seconds) and ambient temperature variations affecting the speed of sound. For safety purposes, this level of accuracy is more than sufficient.
The calculation uses 343 m/s (approximately 1,125 ft/s), which is the speed of sound in dry air at 20 °C (68 °F) at sea level. This gives the convenient approximations of 1 km per 3 seconds and 1 mile per 5 seconds.
According to the NWS, you should seek shelter if lightning is 6 miles (10 km) away or closer (a 30-second or shorter flash-to-thunder gap). Lightning can travel 10–15 miles from a storm, so even distances beyond 6 miles should prompt you to monitor conditions and be ready to move indoors quickly.
Yes — the flash-to-thunder method works equally well day or night. In fact, lightning flashes are easier to see at night, which may make nighttime detection slightly more reliable. The physics of sound travel is the same regardless of time of day.
The rumble of thunder occurs because a lightning bolt is several miles long. Different parts of the channel are at different distances from you, so sound from each segment arrives at slightly different times, creating a drawn-out rumble rather than a single sharp crack. For best results, start counting at the flash and stop at the first sound of thunder you hear.
Sound travels approximately 1 mile every 5 seconds. If you count 5 seconds between a lightning flash and thunder, the strike was about 1 mile away.
Sound travels approximately 1 kilometer every 3 seconds. A 3-second gap between flash and thunder means the lightning was roughly 1 km away.
The 30-30 Rule from the National Weather Service states: (1) If the time between lightning and thunder is 30 seconds or less (≈6 miles / 10 km), go indoors immediately; and (2) wait 30 minutes after the last thunder before going back outside.
Yes. If thunder cannot be heard but lightning is visible, the storm may still be within striking range. Lightning can travel 10–15 miles from the storm center. 'When thunder roars, go indoors' — even if you can't hear it yet.
The calculator uses the standard approximation (÷3 for km, ÷5 for miles) which assumes average sea-level conditions (~20 °C / 68 °F). In very cold or very high-altitude conditions, the speed of sound is slightly lower, so actual distances may differ by a small margin — but the approximation is accurate enough for safety decisions.
If you see the flash and hear thunder simultaneously (zero seconds delay), the lightning struck extremely close — within a few hundred meters. Take immediate cover. Do not wait to calculate anything.
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