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Lightning Distance Calculator

Distance to a lightning strike from the seconds between flash and thunder, at 5 seconds per mile, in miles or km, with what to do at that range. How it works

Use the Lightning Distance Calculator

Adds the distance at the speed of sound for this temperature: 331.3 + 0.606 × °C m/s.

Distance Unit
Distance
1.00 miles

⚡ DANGER
Go indoors now.
Counting down your delay
5s
A counting aid that repeats your delay. It does not predict the next strike.
0s5s
🛑

Safety Tips

  • •When thunder roars, go indoors.
  • •Avoid open fields, tall trees, and metal objects.

About the Lightning Distance Calculator

Count the seconds from the flash to the thunder and divide by 5 for miles: 10 seconds is 2.00 miles, or 3.22 km. Sound covers a mile in about 5 seconds while the light reaches you almost at once, so the delay gives the lightning distance. If you can hear thunder at all, you are within striking distance and should go indoors.

Flash-to-thunder delay and distance

DelayMilesKilometersBand shown
3 s0.600.97DANGER
5 s1.001.61DANGER
10 s2.003.22DANGER
15 s3.004.83DANGER
20 s4.006.44GO INDOORS
30 s6.009.66GO INDOORS
40 s8.0012.87GO INDOORS
50 s10.0016.09GO INDOORS
60 s12.0019.31STAY INSIDE

At the National Weather Service rule of 5 seconds per mile; kilometers are the miles × 1.609344. Every band tells you to go or stay indoors.

How to use this tool

  1. Start counting seconds when you see the flash and stop at the first sound of thunder.
  2. Enter the count and pick Miles or Kilometers. Decimals such as 7.5 are accepted. Optionally type the air temperature for a second figure at the speed of sound in that air.
  3. Read the distance and the band: Danger up to 15 seconds, Go Indoors from there to 50 seconds, Stay Inside beyond 50.

Frequently asked questions

How far away is lightning if the thunder comes 10 seconds later?
About 2 miles, or 3.22 km, by the 5-second rule, and the tool shows the Danger band. The National Weather Service counts anywhere you can hear thunder as within striking distance of the storm, so a 2-mile strike is a reason to be inside already.
Why divide by 5?
Sound takes about 5 seconds to travel a mile, a speed of about 322 m/s. In air at 20 °C sound travels about 343 m/s, so the real distance is about 6.6% more than the count gives; at 0 °C, 331 m/s, it is about 3% more. Type the air temperature and the tool shows that figure too: 10 seconds at 20 °C is 2.13 miles against 2.00. Light arrives in a few millionths of a second, so its travel time does not matter.
How far away can you hear thunder?
About 10 miles from the strike, according to the National Weather Service. That is a delay of about 50 seconds, which is why a count over 50 seconds gets the Stay Inside band and a prompt to check your count.
How long should I wait after the last thunder?
The National Weather Service says to wait 30 minutes after the last lightning or thunder before going back outside. The timer under the result only repeats your delay as a counting aid; it is not a 30-minute timer and does not predict when lightning will strike next.
Is a car a safe place in a thunderstorm?
A hard-topped vehicle is one of the safe options the National Weather Service lists, together with substantial buildings. Rain shelters, small sheds and open vehicles are not safe.

Flash-to-bang distance

Start counting when you see the flash and stop at the first sound of thunder: 10 seconds is 2.00 miles, or 3.22 km. The light arrives almost at once, so the delay is the time the sound takes to reach you.

A long count is not permission to stay outside. Whatever band the result falls in, if you heard thunder, go indoors.

How the distance is worked out

Miles: distance = seconds ÷ 5. Kilometers: distance = seconds ÷ 5 × 1.609344.

Both units use the NWS shortcut of 5 seconds per mile, about 322 m/s, so they always agree: 15 seconds gives 3.00 mi and 4.83 km. The true speed of sound is about 343 m/s in air at 20 °C, so the real distance is a little over 6% larger than shown; the shortcut errs on the side of showing the strike closer. Results show two decimals. Your reaction time adds error, so read the answer as an estimate to a few tenths.

Type an air temperature (°C or °F) and the page adds a second figure at the speed of sound for that air: c = 331.3 + 0.606 × °C m/s. At 20 °C that is 343.4 m/s, so 10 seconds is 2.13 miles instead of 2.00; at 0 °C it is 331.3 m/s and 2.06 miles. The headline and the band stay on the 5-second rule.

The status band is set from the delay in seconds with the mile rule, so a delay gets the same advice in either unit: up to 15 seconds (3 miles) is Danger and 16 to 50 seconds (up to 10 miles) is Go Indoors, both telling you to go indoors now. Beyond 50 seconds the band is Stay Inside, with a note that thunder is barely audible past about 10 miles and a prompt to check your count. A negative delay is rejected.

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