# Lightning distance: how far away?

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.

- Page: https://www.acalculator.org/physics/lightning-distance-calculator
- JSON spec: https://www.acalculator.org/physics/lightning-distance-calculator.json
- Version: 0272d3bb0436

## Default answer

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).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| s | Seconds from flash to thunder | The time between seeing the lightning and hearing its thunder, in seconds. |
| t | Air temperature | The outside air temperature, which sets the speed of sound. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| distance | Distance to the lightning | The speed of sound times the seconds from flash to thunder. |
| km | Distance (km) | The same distance in kilometres. |
| feet | Distance (ft) | The same distance in feet. |
| ruleMiles | Rule of thumb (miles) | Seconds ÷ 5, the National Weather Service rule. |
| ruleKm | Rule of thumb (km) | Seconds ÷ 3, the NOAA rule for kilometres. |
| speed | Speed of sound (m/s) | 331 × √(1 + T ÷ 273), with T in °C. |
| speedFt | Speed of sound (ft/s) | The same speed in feet per second. |

## 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.

## Assumptions

- 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).

## Worked examples

1. s = 5, t = 293.15 gives ruleMiles = 1, speed = 342.910261, distance = 1,714.551303, 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. s = 15, t = 293.15 gives ruleMiles = 3, ruleKm = 5, distance = 5,143.65391. 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. s = 10, t = 303.15 gives speed = 348.712877, distance = 3,487.128766. 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. s = 10, t = 273.15 gives speed = 331, distance = 3,310, feet = 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. s = 0, t = 293.15 gives distance = 0, ruleMiles = 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).

## FAQ

### 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.

## Sources

- National Weather Service, Understanding Lightning: Thunder: count the seconds between the flash and the thunder and divide by 5 for miles (5 seconds = 1 mile, 15 seconds = 3 miles, 0 seconds = very close); thunder can be heard about 10 miles away, retrieved 2026-10-18. https://www.weather.gov/safety/lightning-science-thunder
- NOAA JetStream, Lightning Safety: divide by 3 for kilometres (15 seconds is 3 miles, 5 km); lightning can strike more than 10 miles from the rainfall; stay indoors until 30 minutes after the last thunder, retrieved 2026-10-18. https://www.noaa.gov/jetstream/lightning/lightning-safety
- National Weather Service, Lightning Safety: when thunder roars, go indoors; if you hear thunder you are likely within striking distance, retrieved 2026-10-18. https://www.weather.gov/safety/lightning-safety
- 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, retrieved 2026-10-18. https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound
- NIST Special Publication 811 (2008), Appendix B.8: 1 ft = 0.3048 m, 1 mi = 1,609.344 m, retrieved 2026-10-18. https://www.nist.gov/pml/special-publication-811