Calculate how far away lightning struck from the flash-to-thunder delay, with safety zone assessment, storm tracking, and temperature-corrected speed of sound.
When you see a lightning flash, the light reaches you almost instantly — but the thunder takes several seconds because sound travels much slower than light. By counting the seconds between the flash and the thunder, you can calculate the distance to the lightning strike with surprising accuracy.
The speed of sound in air depends on temperature (approximately 343 m/s at 20°C), so this calculator uses the actual temperature to refine the distance estimate. It also includes a safety assessment: lightning within 3 km (about 9 seconds) means you should immediately seek shelter, while the 30/30 rule recommends going indoors when the delay is 30 seconds or less and staying inside for 30 minutes after the last thunder.
This Lightning Distance Calculator provides the distance in multiple units, a color-coded safety zone indicator, storm trending from successive measurements, and a comprehensive reference table mapping delay times to distances. Whether you are a storm chaser, outdoor enthusiast, or simply curious during a thunderstorm, this tool helps you stay informed and safe.
Use this calculator when you want a fast distance estimate that is better than a rough "seconds divided by five" guess.
It is useful for hikers, event staff, coaches, and anyone outdoors who needs a quick read on whether a storm is closing in and whether shelter is already overdue.
Distance = Speed of Sound × Time Delay Speed of Sound: c = 331.3 + 0.606 × T (m/s), T in °C Quick rule: Distance (km) ≈ seconds ÷ 3 Quick rule: Distance (mi) ≈ seconds ÷ 5
Result: Distance = 1.72 km (1.07 miles)
A 5-second flash-to-thunder delay at 20°C means the lightning is about 1.7 km away — in the caution zone. Be prepared to seek shelter.
Flash-to-thunder timing works because the light arrives essentially immediately while the sound moves at a few hundred meters per second. That makes the method reliable enough for field decisions, especially when you repeat the measurement over several strikes to see whether the storm is moving toward you or away from you.
Do not treat the calculator as a substitute for lightning safety guidance. Wind, terrain, and echoes can make thunder harder to time cleanly, and some dangerous storms produce cloud-to-ground strikes before the heaviest rain arrives. If you can hear thunder at all, you are already close enough to be at risk.
Light travels at 300,000 km/s (nearly instant), while sound travels at ~343 m/s. The delay directly gives you distance via the speed of sound.
If the flash-to-thunder delay is 30 seconds or less, go indoors. Stay inside for 30 minutes after the last thunder. This is the most widely recommended lightning safety guideline.
Approximately. At 20°C, sound takes about 4.7 seconds to travel 1 mile. "5 seconds per mile" is close enough for a quick estimate.
Warmer air speeds up sound: from ~331 m/s at 0°C to ~349 m/s at 30°C. A 10°C change shifts the distance by about 2%.
Rarely — thunder is typically audible up to about 10–15 miles (16–24 km). Beyond that, the sound dissipates in the atmosphere.
The lightning is likely more than 15–20 km away. This "heat lightning" is too far for the sound to reach you, but the storm could be approaching.