Lightning distance calculator
Count the seconds between a lightning flash and the first sound of thunder. Divide by five to estimate miles, or by about 3.1 for kilometres. Use the calculator to convert your count into both units.
Make observations from a safe place indoors. Thunder is a reason to take shelter, whatever distance the calculator shows.
Estimate the distance from a flash
Enter 0 to 120 seconds. Count from a safe place indoors.
If you hear thunder, go indoors. This estimate does not tell you whether it is safe outside.
Seconds between lightning and thunder: quick reference
| Seconds | Kilometres | Miles |
|---|---|---|
| 3 | 1.0 | 0.6 |
| 5 | 1.6 | 1.0 |
| 10 | 3.2 | 2.0 |
| 15 | 4.8 | 3.0 |
| 20 | 6.4 | 4.0 |
| 30 | 9.7 | 6.0 |
Why the flash arrives before the sound
Light from a nearby flash reaches you almost immediately. Thunder takes longer because sound travels much more slowly. The National Weather Service explains the five-seconds-per-mile rule as a way to estimate distance from that delay.
For example, a 15-second delay divided by five gives about 3 miles. Converting that result gives about 4.8 kilometres. This is an estimate, so a tenth of a kilometre in the display does not mean the location is known that precisely.
Why this can differ from the map distance
A lightning channel can extend through a large part of a cloud. You hear sound from different portions at different times, which is why thunder can rumble. The first thunder corresponds to the closest part you heard, rather than necessarily the point where lightning reached the ground.
Boltwatch’s nearby lightning map measures to reported detection coordinates. Satellite flashes can be inside clouds, and reports take time to arrive. That is a different measurement from counting the delay after a flash you saw. Read about satellite flashes and ground strikes before comparing the two.
Multiple flashes, distant rumbles and echoes can make it hard to pair a sound with the correct flash. The estimate cannot predict the next strike or show when a storm will reach you.
What to do when you hear thunder
Move into a substantial building or an enclosed, metal-topped vehicle. The National Weather Service lightning guidance advises remaining in safe shelter for at least 30 minutes after the last thunder. Follow your local weather service’s warnings.
Frequently asked questions
- How far away is lightning if thunder follows 15 seconds later?
- Using the National Weather Service rule of about five seconds per mile, 15 seconds is roughly 3 miles, or 4.8 kilometres. It is a rough estimate to the lightning channel you heard, not an exact ground-impact location.
- Do you divide the seconds by 3 or by 5?
- Divide by 5 for an approximate distance in miles. For kilometres, divide by about 3.1. This calculator divides by 5 first, then converts miles to kilometres. Air conditions and identifying the same flash and thunder affect the estimate.
- Can I use the distance to decide when to go outside?
- No. Hearing thunder means lightning is close enough to be a hazard. Go indoors and follow official weather warnings. The National Weather Service advises staying in safe shelter for at least 30 minutes after the last thunder.