Lightning Distance Calculator
Storm distance in miles and km plus shelter verdict from flash to thunder seconds.
What it does
Flash-to-bang timing turns thunder delay into storm distance: light arrives instantly while sound crawls near a mile every five seconds. Ten seconds means about 2 miles, thirty means about 6.
The tool above takes the counted seconds. It returns miles with kilometers and a shelter verdict beside them.
The default of 10 seconds reads 2 mi (3.3 km): within 6 miles, seek shelter. Five seconds reads 1 mi: indoors now. Thirty seconds reads 6 mi: still shelter by the 30-30 rule.
The rule follows Weather Service lightning science with field office safety pages matching the divide by five math. Both add the 30-30 rule: shelter at 30 seconds or less, wait 30 minutes after the last thunder.
How to use
- Watch for the lightning flash and start counting.
- Stop at the thunder clap and enter the seconds.
- Read miles first for the headline distance.
- Read the verdict before deciding anything.
- Recount each flash: the next bolt can be closer.
Formula
- /5
- Sound covers about a mile every five seconds.
- /3
- Metric twin: about a km every three seconds.
- 30-30
- Shelter at 30 s, wait 30 min after the last clap.
One strike placed only; storms move and the next bolt starts its own count. Blue sky overhead never clears you while thunder sounds.
Worked example
Example: 10 seconds flash to bang
Default: 10 seconds counted.
- Miles: 10 / 5 = 2 mi.
- Km: 10 / 3 = 3.3 km.
- Rule: 10 sits under 30, so shelter now.
- Wait: stay inside 30 minutes past the last thunder.
- Tool shows 2 mi, 3.3 km, the seek shelter verdict.
Reference table
| Delay | Distance | Verdict |
|---|---|---|
| 5 s | 1 mi (1.7 km) | Under 1 mile: indoors now |
| 10 s | 2 mi (3.3 km) | Within 6 miles: seek shelter |
| 30 s | 6 mi (10 km) | Within 6 miles: seek shelter |
Limits
- One strike only; storms and distances move.
- Counting errors grow with wind and echoes.
- Rain free bolts strike past the audible range.
- No place outside is safe near a thunderstorm.
Common mistakes
- Waiting for rain before sheltering.
- Leaving shelter the minute rain ends.
- Sheltering under lone trees.
- Lying flat instead of getting inside.
- Trusting rubber shoes or tires for protection.
Try it yourself
Quick checks. Answers sit tucked under each question.
5 seconds?
1 mi (1.7 km): under 1 mile, indoors now.
30 seconds?
6 mi (10 km): still seek shelter by the 30-30 rule.
No thunder heard?
Unmeasurable and possibly safe, but watch building clouds.
Questions
How far is lightning?
Seconds divided by 5 in miles, by 3 in km. Ten seconds is 2 miles.
What is the 30-30 rule?
Shelter at 30 seconds or less; wait 30 minutes after the last thunder.
Can lightning hit without rain?
Yes, bolts reach 10+ miles past rainfall. Thunder heard means danger.
Is a car safe?
Hard topped metal vehicles protect through the shell, not the tires.
Why wait 30 minutes?
Trailing clouds hold charge and most strikes hit before or after the peak.
Heat lightning safe?
No such thing as safe: it is ordinary lightning too far for thunder to reach you.