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 at 15 degrees Celsius). A 9-second gap means the lightning is about 3km away. The exact speed varies slightly with air temperature.

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.

How can I track if a storm is approaching or receding?

Take multiple flash-to-bang readings a few minutes apart. If the gap is getting shorter, the storm is approaching. If the gap is getting longer, it is moving away. This calculator can estimate storm speed and time to arrival from two readings.

What should I do if caught in a lightning storm while hiking?

Descend from ridges and summits immediately. Avoid isolated trees, water, and metal objects (put trekking poles down away from you). If in a group, spread out 6+ metres apart. If no shelter is available, crouch on the balls of your feet with feet together, hands on knees — do NOT lie flat.

CampCalcs

Lightning Distance Calculator

Calculate lightning distance using the flash-to-bang method and assess safety based on terrain and shelter.

Seconds Between Flash and Thunder

Count from lightning flash to thunder

Your Terrain

Your current terrain affects lightning risk level

Available Shelter

Best shelter option available to you

Group Size

Number of people in your group

How We Calculate This

Flash-to-Bang Method

Distance (km) = Seconds x Speed of sound / 1000

Speed of sound = 331.3 + (0.606 x Temperature in degrees C)

At 15 degrees C, sound travels at 340.4 m/s (roughly 1km every 2.9 seconds).

Storm Tracking

Storm speed = Distance change / Time between readings

Two readings taken minutes apart reveal whether the storm is approaching or receding and at what speed.

Frequently Asked Questions

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Last updated: March 2026

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