Light arrives almost instantly, while thunder travels at the speed of sound, so the delay reveals the distance.
How to use the Lightning Distance Calculator
- Seconds between flash and thunder
- Air temperature
- Want to work backwards? Type a value into Distance to strike and Seconds between flash and thunder is calculated for you.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Multiply the delay by the speed of sound at the current air temperature.
Inputs explained
| Field | What to enter |
|---|---|
| Seconds between flash and thunder | A number |
| Air temperature | Units: °C, °F, K |
What each result means
- Speed of sound = 331.3×sqrt(1 + [Air temperature]÷273.15)
- Distance to strike = [Seconds between flash and thunder] × [Speed of sound]
Example values
How the main result changes when Seconds between flash and thunder changes and everything else stays at the example values above:
| Seconds between flash and thunder | Distance to strike (km) |
|---|---|
| 3 | 1.03 |
| 4.5 | 1.54 |
| 6 | 2.06 |
| 7.5 | 2.57 |
| 9 | 3.09 |
| 12 | 4.12 |
Formula
Distance = seconds × speed of sound (≈ 343 m/s)Worked example
Inputs — Seconds between flash and thunder: 6; Air temperature: 20 °C.
Results — Speed of sound = 343.2 m/s; Distance to strike = 2.06 km.
Frequently asked questions
What is the 30-30 rule?
If thunder follows lightning within 30 seconds, go indoors and wait 30 minutes after the last thunder.
Is the 3-second rule accurate?
Each 3 seconds is about 1 km; 5 seconds is about 1 mile.
What formula does the Lightning Distance Calculator use?
Distance = seconds × speed of sound (≈ 343 m/s).
Can I work backwards from the result?
Yes. Type the result you want into Distance to strike and the calculator finds the matching seconds between flash and thunder.