Lightning Distance Calculator
Count the seconds between the lightning flash and the thunderclap, enter them below, and get the storm distance in miles and kilometres. You can adjust for air temperature to get a more accurate speed of sound, or reverse-solve to find the expected time delay for a known distance. Safety thresholds and the 30-30 rule are shown alongside your result.
How to measure the time between flash and thunder
The moment you see a lightning flash, start counting seconds - "one one-thousand, two one-thousand" works well if you do not have a stopwatch. Stop counting as soon as you hear the crack or rumble of thunder. Because light travels nearly instantaneously compared with sound, the gap between the two is almost entirely the travel time of the sound wave. Enter those seconds into the calculator above to get your distance.
The physics: why 5 seconds equals 1 mile
Sound travels through air at about 343 metres per second (1125 ft/s) at 20 degrees Celsius. Divide 1609 metres (1 mile) by 343 m/s and you get approximately 4.69 seconds - rounded to 5 for field use. For kilometres, divide 1000 m by 343 m/s and you get about 2.9 seconds - rounded to 3. These simple rules are accurate enough for safety decisions, though the calculator uses the precise temperature-adjusted formula (v = 331.3 x sqrt(1 + T/273.15)) for the best estimate.
Temperature matters: adjusting the speed of sound
The speed of sound rises by roughly 0.6 m/s for every degree Celsius of temperature increase. At 0 degrees Celsius it is about 331 m/s; at 20 degrees Celsius it is about 343 m/s; at 35 degrees Celsius it reaches roughly 352 m/s. On a very hot summer afternoon the storm is actually slightly closer than the 5-second rule suggests, and on a cold morning it is slightly further. The difference is small - about 3 percent across the common outdoor temperature range - but the calculator accounts for it automatically when you enter the air temperature.
The 30-30 rule and when to seek shelter
The National Oceanic and Atmospheric Administration (NOAA) and the National Lightning Safety Institute recommend the 30-30 rule: if the time between the flash and the thunder is 30 seconds or less, the storm is within roughly 10 km (6 miles) and you should seek shelter immediately in a substantial building or a hard-topped metal vehicle. After the last thunderclap, wait at least 30 minutes before going back outside. Lightning can strike from clouds that are more than 16 km away - the so-called bolt from the blue - so do not wait for heavy rain before acting.
Flash-to-thunder seconds and safety zones
| Seconds counted | Distance (km) | Distance (mi) | Safety action |
|---|---|---|---|
| 0 | 0 | 0 | Strike at your location - immediate danger |
| 5 | 1.7 | 1.1 | Extreme danger - shelter immediately |
| 10 | 3.4 | 2.1 | Danger - seek hard shelter now |
| 15 | 5.1 | 3.2 | High risk - seek shelter now |
| 20 | 6.9 | 4.3 | Risk - shelter before it gets closer |
| 30 | 10.3 | 6.4 | 30-30 rule threshold - shelter now |
| 45 | 15.4 | 9.6 | Monitor and stay ready |
| 60 | 20.6 | 12.8 | Distant storm - remain alert |
Distances calculated at 20°C (343 m/s). The 30-30 rule marks 30 seconds as the seek-shelter threshold.
Frequently asked questions
Why do I count seconds between the flash and the thunder?
Light reaches your eyes almost instantly - it travels the distance a lightning strike covers in a tiny fraction of a second. Sound is far slower, about 343 m/s, so the delay is almost entirely the travel time of the sound wave. Counting those seconds and multiplying by the speed of sound gives you the distance.
Is the "divide by 5 for miles" rule accurate?
It is accurate enough for safety decisions. Sound travels a mile in about 4.69 seconds at 20 degrees Celsius, so dividing by 5 is a slight overestimate - you are rounding a 4.69-second mile to a 5-second mile, meaning you think the storm is a little further than it is. For that reason the approximation errs safely on the side of caution. This calculator uses the precise formula instead.
What is the 30-30 rule?
The 30-30 rule has two parts. First: if the gap between a flash and its thunder is 30 seconds or less, the storm is close enough to be dangerous and you should seek shelter. Second: wait at least 30 minutes after the last thunderclap before leaving shelter, because the storm may still be within striking range even after rain has stopped.
Does temperature affect how far the sound travels?
Temperature affects the speed of sound, not whether it arrives - sound still reaches you regardless of temperature. But warmer air increases the speed, so the same time delay corresponds to a slightly longer distance on a hot day and a slightly shorter distance on a cold day. The difference across typical outdoor temperatures is a few percent; this calculator adjusts for it automatically.
Can lightning strike if I cannot hear thunder?
Yes. Thunder from a strike more than about 16-25 km away is often inaudible at ground level because sound disperses and is absorbed by the atmosphere. Lightning can travel this distance sideways from a storm cloud - often called a bolt from the blue. If you can see lightning, you are potentially at risk even if the thunder is too faint to hear.
What does the reverse-solve mode do?
In time-delay mode the calculator works backwards: you enter a known distance to the storm (from a weather radar or a map reading, for example) and it tells you how many seconds you should expect between the flash and the thunder. This is useful for verifying a radar estimate or for planning when a storm will arrive.