Lightning Distance Calculator
Free estimate distance to lightning strike using the flash-to-bang method. Get instant, accurate results with our easy-to-use calculator.
Input Parameters
Time between seeing lightning flash and hearing thunder
For more accurate calculation (default: 20°C / 343 m/s)
Results
Enter time delay to calculate
What is the Flash-to-Bang Method?
The flash-to-bang method is a simple technique to estimate the distance to a lightning strike by measuring the time delay between seeing the lightning flash and hearing the thunder. Since light travels much faster than sound, this delay represents the time sound takes to travel from the lightning to you.
Light from lightning reaches you almost instantly (traveling at 300,000 km/s), while sound travels at about 343 m/s at 20°C. By counting the seconds between flash and thunder, you can estimate the distance using: Distance = Speed of Sound × Time.
This method is useful for safety - if the delay is 30 seconds or less (about 10 km or 6 miles), you're in immediate danger and should seek shelter immediately. Lightning can strike up to 10 miles from a thunderstorm.
Lightning Distance Formula
Where:
- • Distance = Distance to lightning (m or km)
- • Speed of Sound ≈ 343 m/s at 20°C (varies with temperature)
- • Time Delay = Seconds between flash and thunder
Quick approximations:
Distance (km) ≈ Time (seconds) / 3
Distance (miles) ≈ Time (seconds) / 5
Speed of sound: c_sound = 331.3 + 0.606 × T(°C) m/s
How to Calculate
-
1
Observe the lightning flash
When you see lightning, start counting seconds immediately.
-
2
Count until thunder
Stop counting when you hear the thunder. This is your time delay.
-
3
Calculate distance
Distance = Speed of Sound × Time. Use 343 m/s at 20°C, or calculate from temperature.
Practical Examples
Example 1: 5 Second Delay
You count 5 seconds between flash and thunder. How far is the lightning?
Solution:
Distance = 343 m/s × 5 s = 1,715 m
≈ 1.7 km (≈ 1.1 miles) - Close! Seek shelter!
Example 2: 10 Second Delay
10 seconds between flash and thunder.
Solution:
Distance = 343 m/s × 10 s = 3,430 m
≈ 3.4 km (≈ 2.1 miles) - Still dangerous!
Applications
Safety
Determining if lightning is close enough to be dangerous. If delay ≤ 30 seconds (10 km), seek immediate shelter.
Weather Monitoring
Tracking thunderstorm distance and movement, understanding storm proximity and intensity.
Education
Teaching about speed of sound, light vs sound travel times, and practical physics applications.
Outdoor Activities
Assessing lightning risk during hiking, camping, sports, and other outdoor activities.
Frequently Asked Questions
Why does sound travel slower than light?
Light is an electromagnetic wave traveling at 300,000 km/s. Sound is a mechanical wave requiring a medium (air) and travels at ~343 m/s - about 880,000 times slower!
How accurate is this method?
Reasonably accurate for single strikes. Factors affecting accuracy: temperature (affects sound speed), wind, multiple strikes, and your counting precision. Typically accurate to within 10-20%.
What if I don't hear thunder?
If lightning is more than about 15-20 km away, thunder may be inaudible. However, you can still be struck by lightning from distant storms - if you see lightning, you're at risk!
How does temperature affect the calculation?
Sound speed increases with temperature: c = 331.3 + 0.606×T(°C) m/s. At 0°C: 331 m/s, at 20°C: 343 m/s, at 30°C: 349 m/s. Warmer air = faster sound = slightly longer distance for same delay.
What is the 30-30 rule?
If flash-to-bang is ≤ 30 seconds, wait 30 minutes after the last thunder before resuming outdoor activities. This ensures the storm has passed and reduces lightning risk.