How do you calculate lightning distance?

Count the seconds between seeing the lightning flash and hearing the thunder. Divide by 3 to get the approximate distance in kilometres (sound travels at roughly 1km per 3 seconds). A 9-second gap means the lightning is about 3km away.

At what distance is lightning dangerous?

Lightning is dangerous within 10km (30 seconds flash-to-bang). The 30-30 rule recommends seeking shelter when the gap is 30 seconds or less, and staying sheltered for 30 minutes after the last thunder. Within 3km is very dangerous; within 1km is immediately life-threatening.

What should I do if caught in a lightning storm on a mountain?

Descend immediately from ridges, summits, and exposed ground. Move at least 30m away from the highest point. Avoid isolated trees, water, and metal objects (put trekking poles down away from you). If no shelter is available, crouch on the balls of your feet with feet together, hands on knees.

Is it safe to shelter in a tent during lightning?

A tent provides NO protection from lightning. If possible, move to a substantial building or vehicle. If stuck in a tent, avoid touching the metal poles. Crouch on your sleeping pad (insulation) in the centre of the tent. Do not lie flat — minimise your contact with the ground.

CampCalcs

Lightning Distance Calculator

Calculate lightning distance using the flash-to-bang method and assess safety.

Seconds Between Flash and Thunder

Count from lightning flash to thunder

How We Calculate This

Flash-to-Bang Method

Distance (km) = Seconds between flash and thunder / 3

Sound travels at approximately 343 m/s (1km every 2.9 seconds).

Safety Zones

10km+: Monitor. 6-10km: Warning. 3-6km: Dangerous. 1-3km: Very dangerous. Under 1km: Extreme danger.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify conditions and use proper safety equipment outdoors.