110 seconds delay at 70°F
Inputs
Delay10 seconds
Temperature70°F
Result
Distance2.1 miles
Sound speed at 70°F = 344.5 m/s. 10 × 344.5 = 3,445 m = 2.14 miles. DANGER zone—seek shelter.
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Lightning Distance Calculator
Distance
2.1 mi
Km
3.4 km
Safety
Danger
Lightning Distance
2.1 miles
3.4 km
Safety Zone
Danger
Storm ETA
5 min
Danger
Seek shelter immediately. Lightning can strike within a 6-mile radius of the last thunder.
Details
Distance11289 ft / 3441 m
Sound Speed344 m/s
Storm Arrival (at 25 mph)5 min
What You'll Need
AcuRite 02020 Portable Lightning Detector Black, 2½L x 1W x 2¾H
$50.49
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StrikeAlert HD Personal Lightning Detector
$203.99
View on Amazon
Robic Strike Alert Personal Lightning Detector Black
$72.99
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Price as of Jul 24, 2026. Product prices and availability are accurate as of the date indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
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Example Calculations
225 seconds delay at 85°F
Inputs
Delay25 seconds
Temperature85°F
Result
Distance5.4 miles
Sound speed at 85°F = 349 m/s. 25 × 349 = 8,725 m = 5.4 miles. Still in danger zone—head for shelter.
Frequently Asked Questions
QHow do you calculate lightning distance from thunder?
Count the seconds between the lightning flash and thunder, then divide by 5 for miles or by 3 for kilometers. Light travels instantly; sound travels about 1,125 ft/sec (343 m/s), so a 10-second delay means roughly 2 miles.
- 1 second = ~1,125 feet
- 5 seconds ≈ 1 mile
- 3 seconds ≈ 1 kilometer
- Sound speed varies with temperature
| Seconds | Miles | Kilometers | Safety |
|---|---|---|---|
| 5 sec | 1.0 mi | 1.6 km | DANGER |
| 15 sec | 3.0 mi | 4.8 km | DANGER |
| 30 sec | 6.0 mi | 9.7 km | Warning |
| 60 sec | 12.0 mi | 19.3 km | Alert |
QAt what distance is lightning dangerous?
Lightning can strike up to 10–15 miles from the storm center (sometimes called "bolts from the blue"). The NWS 30-30 rule says: if the flash-to-thunder time is under 30 seconds (6 miles), seek shelter immediately.
- Under 6 miles: seek shelter NOW
- 6–10 miles: move toward shelter
- 10–15 miles: be aware, have a plan
- Wait 30 min after last thunder to resume
QDoes temperature affect the calculation?
Yes. Sound speed depends on air temperature: 331.3 + 0.606 × T(°C) meters per second. At 32°F (0°C), sound travels 1,087 ft/sec. At 90°F (32°C), it is 1,152 ft/sec—a 6% difference.
- Cold air: slower sound, underestimates distance
- Hot air: faster sound, slightly further
- Difference is ~6% across typical weather range
- The 5-second rule is accurate enough for safety
QHow fast does a thunderstorm approach?
Typical thunderstorms move at 25–35 mph. A storm 10 miles away could reach you in 15–25 minutes. Fast-moving squall lines can travel 50–60 mph, arriving in under 10 minutes from the same distance.
- Average storm speed: 25–35 mph
- 10 miles away: 15–25 min to arrival
- Fast squall lines: 50–60 mph
- Always seek shelter before the storm arrives
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Last Updated: Jul 24, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.