Free · Atmospheric physics calculators ☁️

Flash-to-Thunder Delay and Distance Estimate

Estimate lightning sound-path distance from the measured flash-to-thunder delay and an editable positive sound speed.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

A 5-second delay at 343 m/s gives an estimated sound-path distance of 1,715 m, or 1.715 km.

d ≈ cₛt; t ≈ d/cₛ

Worked example

Enter these known values and leave the other values blank.

Speed of sound
343 m/s
Time between flash and thunder
5 sec
Estimated sound-path distance
1.715 km

Assumptions and limitations

  • Measure the delay from a visible flash to its associated thunder. This estimate neglects the much shorter light travel time and assumes a single constant sound speed along the acoustic path.
  • The initial sound-speed value is locked to preserve it as a known input. It remains editable, with unit selectors and an unlock control for inferring speed from positive delay and distance. Metric preference starts at 343 m/s; imperial preference starts at 1,125 ft/s. They are separate rounded approximations, not identical measurements.
  • Sound speed must be positive. Delay and distance may be zero, but zero delay with zero distance cannot determine a unique speed. An inferred nonfinite or impossible inverse does not become a usable result.
  • The estimate concerns the acoustic path to the associated discharge, not the distance to a storm center or a map location. Thunder can come from an extended channel, and temperature, wind, terrain and timing error affect the estimate.
  • This distance is not a safe-distance rule. The National Weather Service advises seeking shelter when thunder is heard; the calculation does not indicate that remaining outside is safe.

Common questions

Is five seconds exactly one mile?

No. Dividing seconds by five is a rough rule of thumb. This workflow multiplies the selected sound speed by the measured delay, then converts distance units.

Can I recover the delay from distance?

Yes. Leave delay empty and supply distance with a positive known sound speed; t=d/cₛ. Unlock sound speed to infer it from positive delay and distance instead.

Why can the speed differ from its initial value?

Sound speed varies with atmospheric conditions. The retained metric and imperial defaults are rounded estimates, and you may replace them with an appropriate positive speed.

Can this tell me when it is safe outside?

No. An estimated distance is not a storm safety threshold. Follow National Weather Service shelter guidance when thunder is heard.

References

Require a positive propagation speed; zero sound speed cannot describe this flash-to-thunder distance model. Zero delay and zero distance remain allowed, with no unique speed inverse from two zeros.

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