How to use
- When you see a flash, count seconds until you hear thunder.
- Type that count and press Calculate.
- The hero is miles at 1,125 feet per second. The other line is seconds divided by 5.
- If you can hear thunder, treat the storm as close enough to leave exposed ground. The arithmetic does not change that.
How it's calculated
Feet = seconds × 1,125. Miles = feet ÷ 5,280. The five-second rule reports seconds ÷ 5 as miles, which is a trail shorthand and a slightly different distance.
Sound speed changes with temperature. 1,125 is a round figure, not a reading from a thermometer on the mountain.
Worked example
Fifteen seconds: 15 × 1,125 = 16,875 feet, which is 3.20 miles. The five-second rule says 15 ÷ 5 = 3.0 miles. Both are on the result so you can see the gap.
Assumptions
A straight path for the sound and a speed of 1,125 feet per second. Echoes off cliffs can confuse the count. This page does not locate the next strike.
FAQ
What is the five-second rule?
Divide the seconds by five and call that miles. It is the coarser number shown beside the 1,125-feet-per-second result.
Is 1,125 exact?
No. It is a round speed of sound near room temperature. Cold air is slower. The page does not take a temperature.
How close is too close?
Outdoor safety guidance treats audible thunder as a signal to get off exposed ridges and out of open water. Do not wait for a small number on this page.
Does wind chill belong here?
Wind chill is a different question. ClearQuip has that calculator. This page is only flash-to-thunder distance.