Lightning Distance Calculator (Flash to Thunder)
How far away a lightning strike was from the seconds between the flash and the thunder, with the speed of sound corrected for temperature.
Results
Distance to the lightning
2.06
km
Distance in miles
1.28
mi
Speed of sound at that temperature
343.2
m/s
Advice
Thunder is audible — you are within striking range: go indoors
The dashed line marks the value you entered.
What this tool does
Light arrives almost instantly while sound covers roughly a kilometre every three seconds, so counting the seconds gives the distance — divide by three for kilometres, or by five for miles. Lightning can strike more than 15 km from the storm that produced it, which is about as far as thunder can be heard; weather services' advice is that if you can hear thunder you are close enough to be struck, and should stay indoors until 30 minutes after the last thunder.
Formula
d = v × t · v = 331.3 × √(1 + T ÷ 273.15) m/s
Variables
| Symbol | Meaning | Unit |
|---|---|---|
s | Seconds between flash and thunder | s |
t | Temperature | °C |
D | Distance to the lightning | km |
MI | Distance in miles | mi |
V | Speed of sound at that temperature | m/s |
RK | Advice | — |
Worked example
- Seconds between flash and thunder6 s
- Temperature20 °C
- Distance to the lightning2.06 km
- Distance in miles1.28 mi
- Speed of sound at that temperature343.2 m/s
- AdviceThunder is audible — you are within striking range: go indoors
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.