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Lightning Distance Calculator (Flash-to-Bang)

Determine how far away a lightning strike is by timing the gap between the flash and thunder, corrected for air temperature.

Seconds between lightning flash and thunder clap. Count "one-one-thousand, two-one-thousand..."
Air temperature affects the speed of sound (warmer air = faster sound).

Results

Distance to Lightning Strike1.72 km
Distance (miles)1.07 miles
Speed of Sound at Current Temperature343.2 m/s speed of sound
Safety Assessment (1 = safe >3 km, 0 = seek shelter)0 (1=safe >3km, 0=seek shelter)

📖What is it?

Lightning and thunder happen simultaneously, but light travels almost instantly while sound travels at ~343 m/s. By counting the seconds between the flash and the bang, you can calculate how far away the strike occurred. Temperature significantly affects the speed of sound.

🎯How to use

Start counting seconds when you see the flash and stop when you hear the thunder. Enter that count and the current air temperature. The calculator shows how far away the strike was and whether you should seek shelter.

💡Example scenario

You count 8 seconds between flash and thunder on a 25°C summer afternoon. Speed of sound ≈ 346 m/s. Distance ≈ 346 × 8 = 2,768 m ≈ 2.77 km. Safety output is 0 — seek shelter immediately!

🏆Pro tip

The classic "30-30 rule": if flash-to-bang is under 30 seconds (~10 km), head indoors. Wait 30 minutes after the last thunder before resuming outdoor activities. Lightning can strike up to 16 km (10 miles) from the visible storm — "bolt from the blue." Avoid tall trees, open fields, and bodies of water.