Lightning Distance Calculator
Tap at the flash, tap at the thunder: get the strike distance, whether the storm is coming closer, and when to head indoors.
By Manzoor Ahmad, pharmacist · Updated How do you calculate how far away lightning is?
Distance = seconds between flash and thunder × speed of sound (about 343 m/s). Quick rule: divide the seconds by 3 for kilometers or by 5 for miles. A 10-second gap means the strike was about 3.4 km (2.1 miles) away, well within striking distance, so go indoors.
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Seconds to distance chart
At 20 °C (68 °F), sound travels 343 m/s: about 1 km every 2.9 seconds or 1 mile every 4.7 seconds.
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Lightning safety essentials (NWS)
- When thunder roars, go indoors. If you can hear thunder you are close enough to be struck.
- 30-30 rule: shelter if the count is 30 seconds or less; wait 30 minutes after the last thunder before going out again.
- Safe shelter: an enclosed building with plumbing and wiring, or a hard-topped metal vehicle with windows closed.
- Not safe: open fields, hilltops, under isolated trees, near water, tents, picnic shelters and open-sided structures.
- Indoors: avoid corded phones, plumbing and windows; unplug sensitive electronics ahead of the storm.
The method works because light arrives almost instantly while sound crawls along at about 0.34 km/s; see how wave speed shapes wavelength in the wavelength calculator.
Lightning distance FAQs
Count the seconds from the flash to the thunder and multiply by the speed of sound, about 343 m/s at 20 °C. In practice: divide the seconds by 3 for kilometers or by 5 for miles.
About 3.4 km (2.1 miles) at 20 °C. That is close enough to be struck: go indoors now.
Light travels at about 300,000 km per second and arrives almost instantly, while sound travels at roughly 0.34 km per second, nearly a million times slower.
If the time between flash and thunder is 30 seconds or less (about 10 km or 6 miles), seek shelter. Then wait 30 minutes after the last thunder before going back outside.
Lightning can strike 10 miles (16 km) or more from the storm, sometimes from a clear-looking sky. The NWS advice is simple: 'When thunder roars, go indoors.'
Slightly. Sound travels at about 331 m/s at 0 °C and 349 m/s at 30 °C, so the same 9-second count means 2.98 km in freezing air and 3.14 km on a hot day.
Take two counts a few minutes apart. If the seconds get shorter, the storm is approaching; the change in distance divided by the time between readings gives its speed.
Usually up to about 10 miles (16 km), depending on terrain, wind and background noise. If you can hear thunder, you are within striking distance.
A substantial building with wiring and plumbing, or a hard-topped metal vehicle with the windows up. Porches, picnic shelters, tents and open-sided structures are not safe.
Stay off corded phones, avoid plumbing (showers, washing dishes) and keep away from windows, doors and concrete walls. Unplug electronics before the storm if you can.
Lightning from a storm too far away for its thunder to be heard. It is ordinary lightning, and the storm producing it may be heading your way.
Within about 10% for most people, mainly limited by reaction time and by working out which thunderclap belongs to which flash. Long rumbles come from different parts of the lightning channel.
Sources
A distance estimate is not a safety guarantee: lightning can strike far from the storm and without warning. If you hear thunder, go indoors. In an emergency call your local emergency number.