What this calculator does
Light reaches you almost instantly, but sound travels at a fixed, comparatively slow speed through air. Counting the seconds between seeing a lightning flash and hearing the thunder that goes with it, then multiplying by the speed of sound, gives a reasonable estimate of how far away the strike was.
The figure people usually remember is "one kilometre for every three seconds", which is a rounded version of the real speed of sound (about 343 m/s at 20°C). That speed shifts a little with air temperature, so this calculator leaves it as an adjustable field rather than baking in one fixed number.
The formula
Multiply the number of seconds counted between the flash and the thunder by the speed of sound. The result is the distance sound travelled in that time, which is roughly the distance to the strike (the small time light itself takes to arrive is negligible by comparison).
| Term | Meaning |
|---|---|
| Seconds | The time you counted from seeing the flash to hearing the thunder. |
| Speed of sound | How fast sound travels through air, around 343 m/s at 20°C and varying with temperature. |
The inputs explained
| Field | What to enter |
|---|---|
| Seconds between flash and thunder | Count from the flash to the start of the thunder, using a stopwatch or counting seconds aloud. |
| Speed of sound (m/s) | Defaults to the speed of sound in air at a typical 20°C. Colder air carries sound slightly slower, warmer air slightly faster. |
When to use it
Judging how close a storm is
A shrinking count between flash and thunder over successive strikes means the storm is approaching; a growing count means it is moving away.
Deciding when to take shelter
Many outdoor safety guidelines use a count of 30 seconds or fewer (roughly 10 km) as the point to head indoors, since lightning can strike well ahead of the visible storm front.
Teaching the physics of sound versus light
The gap between seeing and hearing an event is a simple, memorable demonstration that light and sound travel at very different speeds.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How far away is a strike for a given count?
Common counts and the estimated distance to the strike.
| Seconds counted | Estimated distance | In miles |
|---|---|---|
| 3 s | 1.03 km | 0.64 miles |
| 6 s | 2.06 km | 1.28 miles |
| 10 s | 3.43 km | 2.13 miles |
| 15 s | 5.15 km | 3.20 miles |
| 20 s | 6.86 km | 4.26 miles |
| 30 s | 10.29 km | 6.39 miles |
Does air temperature change the estimate much?
The same 6-second count at speeds of sound for a range of air temperatures.
| Speed of sound | Estimated distance | In metres |
|---|---|---|
| 330 m/s | 1.98 km | 1,980 m |
| 337 m/s | 2.02 km | 2,022 m |
| 343 m/s | 2.06 km | 2,058 m |
| 349 m/s | 2.09 km | 2,094 m |
| 355 m/s | 2.13 km | 2,130 m |
Questions
Why do I see the lightning before I hear the thunder?
Light travels at close to 300,000 km/s, effectively instant over these distances, while sound travels at only around 343 m/s. The gap between seeing and hearing the same event is the time sound needed to cover the distance.
Is the "3 seconds per kilometre" rule accurate?
It is a close approximation. The speed of sound at 20°C is 343 m/s, which works out to almost exactly 3 seconds per kilometre (2.92 seconds, to be precise), so the rule of thumb is a fair shortcut for quick estimates.
Does this tell me if I am in danger?
It gives a rough distance, not a safety guarantee. Lightning can strike more than 10 km from the storm centre, so a large count does not mean a strike nearby is impossible. Treat any thunder as a signal to consider shelter.
Why is the speed of sound adjustable here rather than fixed?
The speed of sound in air rises with temperature, so a hot summer storm and a cold front carry sound at slightly different speeds. Leaving it editable keeps the estimate honest rather than assuming one fixed condition.
For general everyday date and number tools, see the Day Number to Date Calculator.