Lightning Distance Calculator
Count the seconds between a lightning flash and the thunder to find how far away the strike was. Enter the delay in seconds (and the air temperature in °F or °C, which sets the speed of sound) to get the distance in miles and kilometers.
Example: with Seconds from flash to thunder 10 · Air temperature 68 · Temperature unit °F (Fahrenheit) → Distance to the strike: 2.13 miles.
- In kilometers and meters3.43 km (3,434 m)
- Rule-of-thumb checkAbout 2.0 mi by the 5-second rule, 3.3 km by the 3-second rule
- Safety noteUnder 30 s — the storm is within ~6 miles; the 30-30 rule says shelter now
Computed by the calculator below using its default values. Change any input to see your own numbers.
Distance = speed of sound × delay. Light reaches you almost instantly, so the gap before the thunder is essentially all sound-travel time — about 5 seconds per mile, 3 seconds per kilometer.
Why counting seconds works
Lightning gives off light and sound at the same instant. Light covers the distance to you almost immediately — about 300 million meters per second — so you see the flash with no meaningful delay. Sound plods along at roughly 343 meters per second in warm air, so the thunder arrives seconds later. That gap is almost entirely the time sound needed to reach you.
Multiply the delay by the speed of sound and you have the distance. Because sound travels about one mile every 4.7 seconds, dividing the count by five gives a quick estimate in miles; dividing by three gives kilometers.
Temperature and the 30-30 rule
The speed of sound rises with temperature, so cold air stretches the delay slightly and warm air shortens it. The effect is small — a few percent across normal weather — but this tool adjusts for it so the distance is honest.
Distance is only part of safety. The 30-30 rule says if thunder follows the flash by under 30 seconds, the storm is close enough to be dangerous, so go indoors, and wait 30 minutes after the last thunder before going back out. Lightning can strike miles from the rain.
How it’s calculated
Distance = speed of sound × delay. Speed of sound in air is taken as 331.3 + 0.606·T meters per second, with T in °C (Fahrenheit is converted first). Distance in meters is divided by 1,609.344 for miles and 1,000 for kilometers. The 5-second-per-mile and 3-second-per-kilometer rules use a fixed ~343 m/s.
Light travel time is treated as zero, which it effectively is. Wind, humidity, and terrain echoes can shift the arrival of thunder slightly; this is an estimate of the strike distance, not a safety guarantee.
Flash-to-thunder delay and distance
| Seconds | Distance (miles) | Distance (km) |
|---|---|---|
| 5 s | 1.07 mi | 1.72 km |
| 10 s | 2.13 mi | 3.43 km |
| 15 s | 3.20 mi | 5.15 km |
| 20 s | 4.27 mi | 6.87 km |
| 30 s | 6.40 mi | 10.30 km |
Computed with speed of sound 343 m/s at 20 °C (68 °F); light travel time is negligible.
Common mistakes
- Starting the count at the thunder instead of the flash — begin the moment you see the lightning.
- Assuming a short delay means you are safe; lightning can strike well ahead of the rain and thunder.
- Confusing the miles and kilometers rules — it is 5 seconds per mile but 3 seconds per kilometer.
- Treating a growing delay as an all-clear; wait 30 minutes after the last thunder before heading out.
Frequently asked questions
How do I calculate how far away lightning struck?
Count the seconds between the flash and the thunder, then multiply by the speed of sound (about 343 m/s). Dividing the seconds by five gives a fast estimate in miles, or by three for kilometers.
Why do I see lightning before I hear thunder?
Light travels about a million times faster than sound. The flash reaches you almost instantly, while the thunder lags by the time sound needs to cover the same distance — roughly 5 seconds per mile.
Does temperature really change the answer?
A little. Sound moves faster in warm air and slower in cold, changing the distance by a few percent. This calculator uses your temperature to keep the estimate accurate.
What is the 30-30 lightning safety rule?
If thunder follows the flash within 30 seconds, the storm is within about 6 miles and you should be indoors. After the last thunder, wait 30 minutes before going back outside.
Can I be struck if the storm seems far away?
Yes. Lightning can travel several miles from its parent cloud, striking under clear sky ahead of the rain. Distance is a guide, not a promise of safety.