How far away is that echo?
Sound travels 343 metres a second, so an echo returning after one second came off something 171 metres away.
Almost exactly 3 s — 1 km away
3 s is longer than 3 of the 7 references on this page.
Reference figures are measurements and are rounded; the unit conversion behind them is exact.
| Reference | Size | Yours ÷ theirs | Note |
|---|---|---|---|
| 0.1 s — 17 m away | 0.1 s | 30× | The shortest delay heard as an echo |
| 0.5 s — 86 m away | 0.5 s | 6× | |
| 1 s — 171 m away | 1 s | 3× | |
| 3 s — 1 km away | 3 s | 1× | Lightning one kilometre off |
| 5 s — 1 mile away | 5 s | 0.6× | |
| 10 s — 3.4 km away | 10 s | 0.3× | |
| 30 s — 10 km away | 30 s | 0.1× |
Worth knowing
- Count the seconds between the flash and the thunder and divide by three for kilometres, or by five for miles.
- The ear needs about 100 milliseconds of separation to hear a reflection as a distinct echo rather than as reverberation.
Frequently asked
How far away is lightning?
Divide the seconds between flash and thunder by 3 for kilometres or by 5 for miles. Ten seconds is about 3.4 km.
Why do echoes need a minimum distance?
Below about 17 m the reflection returns within 100 ms and the brain merges it with the original sound. That merging is what makes a room sound "live" rather than echoey.