Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Estimated distance
2.57
Distance (km)
4.13
Speed of sound used (ft/s)
1,129
Safety guidance
Danger — seek shelter now
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter seconds between flash and thunder.
- 2Enter air temperature (affects sound speed) (°F).
- 3Read your estimated distance on the right — it updates as you type.
- 4Hit Share to keep the scenario or send it to someone.
About this calculator
Light travels essentially instantaneously over storm-relevant distances, but sound travels at roughly 1,125 feet per second (about 343 m/s) at typical air temperature, so counting seconds between the lightning flash and the thunder clap gives a direct distance estimate. This is the same 'flash-to-bang' method used by lifeguards, golf courses, and outdoor event operators to decide when to suspend activity — the widely used 30-30 rule says if flash-to-bang is under 30 seconds (storm within about 6 miles), seek shelter immediately, and wait 30 minutes after the last strike before resuming outdoor activity, since lightning has struck as far as 10-12 miles from the parent storm cell.
Worked example
Using the values the calculator loads with:
Inputs
- Seconds between flash and thunder: 12
- Air temperature (affects sound speed): 70 °F
Results
- Estimated distance: 2.57
- Distance (km): 4.13
- Speed of sound used (ft/s): 1,129
- Safety guidance: Danger — seek shelter now
What each field means
Inputs
- Seconds between flash and thunder
- The seconds between flash and thunder used in the calculation. Starts at 12 so you have a working example on load. Accepted range: 0–120.
- Air temperature (affects sound speed) (°F)
- The air temperature (affects sound speed) used in the calculation, measured in °F. Starts at 70 °F so you have a working example on load. Accepted range: -10–110 °F.
Results
- Estimated distance
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Distance (km)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Speed of sound used (ft/s)
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Safety guidance
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why divide seconds by 5 for miles?
Sound travels roughly 1,125 feet per second at normal temperatures, and there are 5,280 feet in a mile, so 5,280 ÷ 1,125 ≈ 4.7 seconds per mile, commonly rounded to 5 for a quick mental estimate — this calculator uses the more precise temperature-adjusted speed instead of the rounded rule of thumb.
What is the 30-30 rule?
If the flash-to-bang count is 30 seconds or less (meaning the storm is within about 6 miles), go indoors immediately. Wait at least 30 minutes after the last observed lightning or thunder before resuming outdoor activities, since storms can produce strikes well ahead of and behind the visible rain core.
Can lightning strike without visible rain overhead?
Yes — 'bolts from the blue' can strike 10 miles or more from the parent thunderstorm, sometimes under apparently clear sky, which is exactly why the safety buffer in this calculator and the 30-30 rule extends well beyond where rain is falling.
Does air temperature meaningfully change the distance estimate?
Slightly — sound speed increases about 1.1 ft/s per degree Fahrenheit, so on a very hot day sound travels marginally faster than on a cold day, shifting distance estimates by a few percent at most over the ranges relevant to storm safety.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Lightning Distance Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/lightning-distance-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/lightning-distance-calculator" target="_blank" rel="noopener">Lightning Distance Calculator — RevenueLab</a> (2026).</p>
Source: [Lightning Distance Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/lightning-distance-calculator) (2026).