How far is the lightning?

Worked example

You see a flash and count 10 seconds before the thunder, at 20 C. Sound travels at 331.3 + 0.606 x 20 = 343.4 m/s, so the strike was 10 x 343.4 = 3,434 m away: 3.4 km or 2.1 miles. Five minutes later the count is 6 seconds, 2.1 km. The storm closed 1.4 km in 5 minutes, about 16 km/h, and could be overhead in 8 minutes.

How it works

Light reaches you almost instantly, but sound travels at about 343 m/s in 20 C air, so distance = seconds x speed of sound, where speed of sound = 331.3 + 0.606 x temperature in C. The familiar shortcuts are about 3 seconds per kilometre and 5 seconds per mile.

The U.S. National Weather Service advice is "When thunder roars, go indoors": lightning can strike 10 miles (16 km) or more from a storm, so any thunder you can hear means you are within striking distance. A common version is the 30-30 rule: take shelter if the count from flash to thunder is 30 seconds or less (about 6 miles), and wait 30 minutes after the last thunder before going back out. Shelter means a substantial building or a hard-topped vehicle, not a tent, tree or open shelter. Not a substitute for official weather warnings or judgment.

Sources and basis: Speed of sound formula (331.3 + 0.606 x T C m/s); U.S. National Weather Service lightning safety guidance ("When thunder roars, go indoors" and the 30-minute wait after the last thunder). The 30-second flash-to-bang cutoff is a common rule of thumb, not an NWS figure. Checked 2026-10-05. Planning guide only, not medical or safety advice. Conditions, health and equipment change real needs; use your own judgment and local guidance.

FAQ

How do I calculate how far away lightning is?

Count the seconds between the flash and the thunder and divide by 5 for miles or by 3 for kilometres. The calculator does the same with the temperature adjusted speed of sound.

What is the 30-30 rule?

As a common rule of thumb (not an official NWS figure), a flash-to-thunder gap of 30 seconds or less means the storm is close: go indoors. The NWS advice is simply that if you can hear thunder you are close enough to be struck. Wait 30 minutes after the last thunder before resuming outdoor activity.

How far away can lightning strike?

Lightning can strike 10 miles (16 km) or more from the storm, even under blue sky. If you can hear thunder, you can be struck.

Does temperature change the answer?

Slightly. Sound travels about 331 m/s at 0 C and 349 m/s at 30 C, which changes a 10 second count by about 180 m. The 5 seconds per mile rule is a good approximation.

Where is safe in a thunderstorm?

A substantial building with plumbing and wiring, or an enclosed metal-roofed vehicle with the windows up. Tents, picnic shelters, trees and open fields are not safe.