Lightning Distance Calculator
Measure the gap between lightning and thunder, and get lightning distance in km, miles, meters, and feet — with air temperature correction and a built-in stopwatch.
⚡ Mode 1 — Distance from the flash-to-bang gap
0.0 sec
Press Flash! the moment lightning is seen, then Thunder! when its sound is heard. Or enter manually below.
Speed of sound at this temperature: 343.2 m/s
3.089 kilometers
1.919
miles
3,088.9
meters
10,134.3
feet
3.00
km (3-sec rule)
Rule of thumb: 9 sec ≈ 3.00 km (3 sec ≈ 1 km) • ≈ 1.80 miles (5 sec ≈ 1 mile). Precise result above: 3.089 km.
⚠️ Gap under 30 seconds — lightning is close enough to be dangerous. Seek shelter inside an enclosed building immediately. See the 30-30 guide below.
🔁 Mode 2 — Time from distance (inverse)
Enter a known distance, get the thunder gap you should hear.
8.7 seconds of thunder gap
Pada 343.2 m/s (20°C — follows the slider in mode 1).
🛡️ Thunderstorm safety guide (30-30 rule)
General educational content, not medical advice. Lightning is a real weather hazard — always prioritize your personal safety.
- First 30 rule: if the flash-to-bang gap is under 30 seconds, the storm is close enough to be dangerous — get inside an enclosed building (not an open shelter) or a metal-topped vehicle with windows closed.
- Second 30 rule: wait at least 30 minutes after the last thunder before going back outside. Half of lightning victims are struck after the storm appears to have passed.
- Indoors: avoid corded electrical equipment, faucets and water pipes, and areas near windows.
- Outdoors with no shelter: stay away from open fields, tall isolated trees, and puddles/water. If trapped, crouch low with feet together as a last resort.
Methodology
c(T) = 331.3 × √(1 + T/273.15) m/s → d = t × c
The speed of sound in air rises with temperature. At 20°C: c = 343.2146 m/s, so a 9-second gap → d = 9 × 343.2146 = 3088.93 m ≈ 3.089 km. Inverse mode uses t = d / c(T). Unit conversions use exact international definitions: 1 mile = 1609.344 m, 1 foot = 0.3048 m.
Frequently asked questions (FAQ)
How do you correctly measure lightning distance?
Start counting (or press the Flash button on the stopwatch above) the moment you see lightning, then stop when the thunder is heard. The difference is the flash-to-bang gap. Enter that figure as the time input — the calculator multiplies it by the speed of sound at the air temperature you choose.
Why do the "3 seconds ≈ 1 kilometer" and "5 seconds ≈ 1 mile" rules exist?
The speed of sound in air is about 343 m/s at 20°C, so in 3 seconds sound travels ±1,030 m (≈1.03 km) and in 5 seconds ±1,716 m (≈1.07 mi). These rules of thumb round those figures for easy mental math; the calculator above gives a more precise result with temperature correction.
Does air temperature affect the result?
Yes. The speed of sound rises with temperature: c(T) = 331.3 × √(1 + T/273.15) m/s. At −20°C the speed of sound is ≈318.9 m/s, while at 40°C it is ≈354.7 m/s — a ±5% difference versus the 20°C value. Drag the temperature slider above to see the correction live.
How far away can thunder still be heard?
In calm conditions, thunder is generally audible up to 15–25 km; when atmospheric conditions help (e.g. a nighttime temperature inversion), thunder can be heard up to about 30 km or more. Beyond that the sound is usually too faint for the human ear.
Does wind affect the speed of sound?
Wind shifts how fast sound travels relative to the ground: sound moving downwind arrives slightly sooner, and vice versa. This calculator uses the speed of sound in still air, so in strong wind actual results can deviate by a few percent — still fine for a safety estimate.
What is the 30-30 rule for lightning safety?
The 30-30 rule is a practical safety guide: if the flash-to-bang gap is under 30 seconds, lightning is close enough to be dangerous — immediately seek shelter inside an enclosed building or a metal-topped vehicle. Then wait at least 30 minutes after the last thunder before going back outside.
How accurate is this lightning distance calculator?
Accuracy is mainly limited by how precisely you press the stopwatch (±0.2–0.5 s for human reaction time, equivalent to ±70–170 m) and wind conditions. The formula and its temperature correction are themselves mathematically exact. For safety decisions, always round toward the more cautious side.
Is it true that lightning light is treated as arriving instantly?
Yes, for practical purposes. Light travels at ±300,000 km/s, so from 10 km away the flash arrives in 0.00003 seconds — far below what humans can measure. The entire gap you measure is practically pure sound travel time.
How do you use the "time from distance" mode?
This mode is the reverse of the main calculation: enter a known distance (e.g. lightning seen 5 km from you according to a map), and the calculator computes how many seconds of thunder gap you should hear at the chosen air temperature. Useful for verifying the direction of an approaching storm.
See also
- Speed of Sound Calculator — compute c(T) in more detail with altitude, humidity, and other-medium corrections.
Free tool by NRTechStudio