Lightning distance calculator: flash to bang
Count the seconds from the lightning flash to the thunder. Light arrives almost at once; sound travels about 343 metres a second, so the delay tells you roughly how far away the strike was.
Distance to the lightning (example)
3.4 km · 2.1 miles
Go indoors now. A count of 30 seconds or less means the thunderstorm is close enough to be dangerous (NWS).
Rule of thumb: 10 ÷ 3 = 3.3 km; ÷ 5 = 2.0 miles.
When thunder roars, go indoors
The US National Weather Service's lightning safety advice is short: When Thunder Roars, Go Indoors. It says there is no safe place outside when thunderstorms are in the area, and that if you hear thunder, lightning is close enough to strike you. "Bolts from the blue" can strike 10 to 15 miles (16 to 24 km) from the thunderstorm.
- Go inside a substantial, completely enclosed building, or a hard-topped metal vehicle. A carport, open garage or covered patio is not enough.
- Stay in shelter at least 30 minutes after you hear the last sound of thunder.
- Do not wait for the rain: many people struck by lightning are not in the rain.
An older NWS rule, the 30/30 rule, put the same advice in numbers: when you see lightning, count until you hear thunder; if it is 30 seconds or less (about 10 km or 6 miles), the storm is close enough to be dangerous, so seek shelter at once; then wait at least 30 minutes after the last thunder before going out. The NWS said to use it only when you can see the storm clearly. Its current pages put it more simply: any thunder at all means go indoors.
How it is calculated
distance (km) = seconds × 343 m/s ÷ 1000 ≈ seconds ÷ 3 distance (miles) = seconds × 343 m/s ÷ 1609 ≈ seconds ÷ 5
343 metres a second is the speed of sound in air at about 20 °C. It is a little slower in cold air and a little faster in warm air, and the count itself is rarely better than a second or two, so the answer is a rough distance. The flash is treated as arriving instantly: light is nearly a million times faster than sound.
Worked example: 10 seconds
10 × 343 = 3,430 metres = 3.43 km = 2.13 miles rule of thumb: 10 ÷ 3 = 3.3 km; 10 ÷ 5 = 2 miles
Reference table
| Seconds | Kilometres | Miles |
|---|---|---|
| 1 s | 0.3 | 0.2 |
| 3 s | 1.0 | 0.6 |
| 5 s | 1.7 | 1.1 |
| 10 s | 3.4 | 2.1 |
| 15 s | 5.1 | 3.2 |
| 20 s | 6.9 | 4.3 |
| 30 s | 10.3 | 6.4 |
| 45 s | 15.4 | 9.6 |
| 60 s | 20.6 | 12.8 |
This calculator gives an estimate for learning and planning. It is not a forecast or a warning, and it cannot say whether conditions are safe. For decisions, follow your national weather service and local authorities.
Sources
- National Weather Service: Lightning Safety Tips and Resources: "When Thunder Roars, Go Indoors" and "no safe place outside"
- National Weather Service: Lightning Safety Tips: stay in shelter at least 30 minutes after the last thunder; hearing thunder means you can be struck
- National Weather Service: Lightning myths: "bolts from the blue" 10-15 miles from the storm
- NWS Public Information Statement, 15 June 2006: When Thunder Roars, Go Indoors: the 30/30 rule, enclosed shelter, rain
- NWS Melbourne: Lightning rules: "Five (5) seconds approximates one (1) mile of distance"
Questions
How do you work out how far away lightning is?
Count the seconds from the flash to the thunder. Sound travels about 343 metres a second, so divide the seconds by 3 for kilometres or by 5 for miles.
How far away is lightning dangerous?
If you can hear thunder, you are close enough to be struck. The US National Weather Service says lightning can strike 10 to 15 miles (16 to 24 km) away from a thunderstorm. Go indoors and stay there for at least 30 minutes after the last thunder.