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Lightning distance: how far away?

Count the seconds between the flash and the thunder, then type them with the air temperature. The lightning distance calculator gives how far away the strike was, next to the 5-seconds-a-mile rule. If you can hear thunder, go indoors.

Your numbers

Units
Distance to the lightning
2.13 mi

The lightning was about 2.13 mi away (seconds ÷ 5 gives 2 miles).

Within striking distance: go indoors now

  1. Within striking distance: go indoors now
  2. Lightning can still strike more than 10 miles away
Distance (km)
3.429
Distance (ft)
11,250
Rule of thumb (miles)
2
Rule of thumb (km)
3.333
Speed of sound (m/s)
342.9
Speed of sound (ft/s)
1,125

Distance to the lightning: 2.13 mi. The lightning was about 2.13 mi away (seconds ÷ 5 gives 2 miles).

How to calculate

Lightning distance calculator: how far away a lightning strike was from the seconds between the flash and the thunder, using the speed of sound at the air temperature and the 5-seconds-a-mile rule.

Example with the default inputs (Seconds from flash to thunder 10, Air temperature 67.9999999999999 °F): The lightning was about 2.13 mi away (seconds ÷ 5 gives 2 miles).

Method: distance = v × t, with v = 331 m/s × √(1 + T_C ÷ 273) the speed of sound at air temperature T_C (°C) and t the seconds from flash to thunder; rule of thumb t ÷ 5 miles or t ÷ 3 km.

  • Light arrives at once, so the delay is the travel time of the sound. Wind and humidity are ignored.
  • Thunder can be heard only about 10 miles away, but lightning can strike more than 10 miles from the storm (NWS, NOAA).

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Seconds from flash to thunder 5, Air temperature 68 °F gives Rule of thumb (miles) 1, Speed of sound (m/s) 342.910261, Distance to the lightning 1.07 mi, Distance (km) 1.714551.Source: National Weather Service, Understanding Lightning: Thunder (count the seconds from flash to thunder and divide by 5 for miles: 5 seconds = 1 mile, 15 seconds = 3 miles), https://www.weather.gov/safety/lightning-science-thunder (retrieved 2026-10-18); OpenStax, University Physics Volume 1, §17.2 Speed of Sound (Equation 17.7, v = 331 m/s × √(1 + T_C ÷ 273 °C); 343 m/s at 20.0 °C; Example 17.1: 348.7 m/s at 30.0 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-18)
  2. Seconds from flash to thunder 15, Air temperature 68 °F gives Rule of thumb (miles) 3, Rule of thumb (km) 5, Distance to the lightning 3.2 mi.Source: National Weather Service, Understanding Lightning: Thunder (count the seconds from flash to thunder and divide by 5 for miles: 5 seconds = 1 mile, 15 seconds = 3 miles), https://www.weather.gov/safety/lightning-science-thunder (retrieved 2026-10-18); NOAA JetStream, Lightning Safety (divide by 3 for kilometres: 15 seconds is 3 miles, 5 km), https://www.noaa.gov/jetstream/lightning/lightning-safety (retrieved 2026-10-18); OpenStax, University Physics Volume 1, §17.2 Speed of Sound (Equation 17.7, v = 331 m/s × √(1 + T_C ÷ 273 °C); 343 m/s at 20.0 °C; Example 17.1: 348.7 m/s at 30.0 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-18)
  3. Seconds from flash to thunder 10, Air temperature 86 °F gives Speed of sound (m/s) 348.712877, Distance to the lightning 2.17 mi.Source: OpenStax, University Physics Volume 1, §17.2 Speed of Sound (Equation 17.7, v = 331 m/s × √(1 + T_C ÷ 273 °C); 343 m/s at 20.0 °C; Example 17.1: 348.7 m/s at 30.0 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-18) (Example 17.1: (331 m/s)√(303/273) = 348.7 m/s)
  4. Seconds from flash to thunder 10, Air temperature 32 °F gives Speed of sound (m/s) 331, Distance to the lightning 2.06 mi, Distance (ft) 10,859.580052.Source: OpenStax, University Physics Volume 1, §17.2 Speed of Sound (Equation 17.7, v = 331 m/s × √(1 + T_C ÷ 273 °C); 343 m/s at 20.0 °C; Example 17.1: 348.7 m/s at 30.0 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-18): v = 331 m/s at 0 °C
  5. Seconds from flash to thunder 0, Air temperature 68 °F gives Distance to the lightning 0 mi, Rule of thumb (miles) 0.Source: National Weather Service, Understanding Lightning: Thunder (count the seconds from flash to thunder and divide by 5 for miles: 5 seconds = 1 mile, 15 seconds = 3 miles), https://www.weather.gov/safety/lightning-science-thunder (retrieved 2026-10-18) (0 seconds = very close)

How it works

  • Speed of sound at air temperature T (in °C): v = 331 × √(1 + T ÷ 273) m/s (OpenStax Equation 17.7). A temperature typed in °F is first changed to °C exactly: T = (°F − 32) × 5/9.
  • Distance = v × t, with t the seconds from flash to thunder. It is shown in miles (km in Metric), and also in km and feet: 1 mile = 1,609.344 m and 1 ft = 0.3048 m.
  • Rules of thumb: t ÷ 5 miles (National Weather Service) and t ÷ 3 km (NOAA).
  • Safety band: under 10 miles the result reads "Within striking distance: go indoors now"; at 10 miles or more it reads "Lightning can still strike more than 10 miles away". Either way, if you hear thunder, go indoors and stay there until 30 minutes after the last thunder.

Rules

  • Seconds from 0 to 300; air temperature from −60 °C to 60 °C (−76 °F to 140 °F).

Output format

The distance shows to 3 significant figures; the other numbers to 4. The rules of thumb are exact (t ÷ 5 and t ÷ 3 of the typed seconds, rounded once); the speed of sound has a square root, so it and the distance are 64-bit floats.

Worked examples by hand

5 seconds at 20 °C. v = 331 × √(1 + 20 ÷ 273) = 331 × 1.035983 = 342.910 m/s; distance = 342.910 × 5 = 1,714.6 m = 1.07 miles (1.71 km). The rule of thumb gives 5 ÷ 5 = 1 mile.

15 seconds at 20 °C. 342.910 × 15 = 5,143.7 m = 3.20 miles; the rules give 3 miles and 5 km.

10 seconds at 30 °C. v = 331 × √(303 ÷ 273) = 348.713 m/s; distance = 3,487.1 m (2.17 miles).

10 seconds at 0 °C. v = 331 m/s; distance = 3,310 m = 10,859.6 ft (2.06 miles).

0 seconds. The distance is 0: the lightning was very close.

Other questions people ask

How do you calculate how far away lightning is?

Count the seconds from the flash to the thunder and divide by 5 for miles, or by 3 for kilometres. Thunder 15 seconds after the flash means the lightning was about 3 miles (5 km) away. Light arrives almost at once, so the delay is the time the sound takes to reach you.

Why divide by 5?

Sound travels about 343 m/s at 20 °C, which is about 1 mile every 4.7 seconds. The National Weather Service rounds that to 5 seconds a mile so it is easy to work out in your head.

Does the temperature change the distance?

A little. The speed of sound is 331 m/s at 0 °C and rises with temperature: 331 × √(1 + T ÷ 273) m/s with T in °C, so 343 m/s at 20 °C and 349 m/s at 30 °C. Over 10 seconds that is 3.31 km in freezing air and 3.49 km on a hot day.

How close is too close?

If you can hear thunder, you are likely within striking distance, so go indoors. Lightning can strike more than 10 miles from the storm, and thunder can be heard only about 10 miles away (National Weather Service and NOAA).

How long should I wait after the last thunder?

Stay in a safe shelter until 30 minutes after the last thunder you hear (National Weather Service).

What does 0 seconds mean?

The flash and the thunder came together, so the lightning was very close.