Sound Travel Time Calculator
Example: 3 seconds = 1 km away
Sound travels at about 343 m/s in dry air at 20°C, so a one-kilometre trip takes roughly 2.9 seconds. This calculator uses a temperature approximation for air and representative reference values for water and solids. Humidity, wind, altitude, salinity, pressure, material composition and wave direction can change real travel times.
Calculation results updated
Elapsed time as sound travels
Travel Time
Speed of Sound
Echo Time (round trip)
Related Calculators
You might also find these calculators helpful: Relativistic Doppler Shift Calculator, Lightning Strike Odds Calculator, and compare travel time near the speed of light.
How to use this calculator
- Enter the distance the sound has to cover. For a lightning strike, this is how far away the storm is; for an echo, it is the distance to the wall or cliff.
- Pick the medium. Air is the default. Water, steel, concrete and wood use representative reference values.
- Set the air temperature if you are using air. Sound moves about 0.6 m/s faster for every 1°C, so a hot afternoon and a freezing night give noticeably different answers.
- Read the one-way time for a sound reaching you directly, or the echo time for a sound that travels out and back.
- To work backwards from a thunder delay instead, count the seconds between the flash and the bang and divide by 3 for kilometres, or by 5 for miles.
How We Calculate This
For dry air near ordinary atmospheric conditions, v ≈ 331.3 + 0.606T m/s over the supported −40°C to 50°C range. One-way time is distance ÷ speed; echo time is twice that distance. Water, steel, concrete and wood use representative values only because temperature, composition, pressure, moisture, grain and wave type matter.
Methodology last reviewed: July 2026. How SparkCalc works
Sources: NASA Glenn Research Center: Speed of Sound · NOAA National Weather Service: Understanding Lightning: Thunder · NOAA National Ocean Service: How Far Does Sound Travel in the Ocean?
How to tell how far away lightning struck
Light covers a kilometre in about three millionths of a second, so for any storm you can see, the flash arrives instantly. Thunder does not. It travels at the speed of sound, roughly 343 m/s, which means it needs about 2.9 seconds per kilometre or 4.7 seconds per mile. That gives the rule most people learn as children: count the seconds between the flash and the bang, then divide by 3 for kilometres or by 5 for miles. A 9 second gap puts the strike about 3 km away. A 2 second gap puts it close enough that the next one could reach you. One important caveat. The NOAA National Weather Service advises going indoors as soon as you can hear thunder at all, because lightning regularly strikes well outside the rain area of a storm. The flash-to-bang count tells you where the last strike landed. It does not tell you where the next one will.
Why temperature changes the answer
Sound moves through air by knocking molecules into one another. Warmer air has faster-moving molecules, so each collision passes the disturbance along sooner and the wave travels quicker. The relationship is close enough to linear across normal weather that a simple formula works: v ≈ 331.3 + 0.606T, with T in degrees Celsius. The effect is not trivial. At −20°C sound manages about 319 m/s; at 35°C it reaches about 353 m/s. Over a 5 km distance that is a difference of roughly 1.5 seconds, which matters if you are timing something precisely. Altitude, by contrast, has almost no direct effect. Thinner air is less dense but also less stiff, and the two changes very nearly cancel. Sound is slower high in the atmosphere mainly because it is colder up there, not because the air is thin. Humidity has a small effect, raising the speed slightly, because water molecules are lighter than the nitrogen and oxygen they displace.
Sound in water, steel and other materials
Sound travels about 4.4 times faster in water than in air, and roughly 15 times faster in steel. The instinct is to blame density, but that gets it backwards: water and steel are far denser than air, and density alone would slow a wave down. What actually governs the speed is the ratio of a material's stiffness to its density. Steel resists compression enormously more than air does, and that stiffness advantage far outweighs its greater density. This is why you can hear a train through a rail long before you hear it through the air, and why whales can communicate across ocean basins. The figures used here are representative values, not precise constants. Sound speed in seawater varies with temperature, salinity and pressure, which is why sonar systems carry live measurements rather than a fixed number. In solids it depends on composition, grain direction and whether the wave is compressional or shear. Treat the non-air results as sound order-of-magnitude comparisons rather than engineering figures.
How long does sound take in air?
Approximate one-way times below use dry air at 20°C (343.42 m/s). Wind and atmospheric conditions are ignored.
| Distance | Travel time |
|---|---|
| 1 metre | 2.9 milliseconds |
| 1 kilometre | 2.91 seconds |
| 1 mile | 4.69 seconds |
Thunder delay as an approximate distance
Light arrival is effectively immediate at these distances, so multiply the delay by the speed of sound.
| Lightning-to-thunder delay | Approximate distance |
|---|---|
| 3 seconds | 1.03 km (0.64 mi) |
| 5 seconds | 1.72 km (1.07 mi) |
| 10 seconds | 3.43 km (2.13 mi) |
NOAA advises going indoors when thunder is heard; delay is not a safety radius.
Key terms
- Speed of sound
- How fast a pressure wave moves through a material. In dry air at 20°C it is about 343 m/s (1,235 km/h or 767 mph). It is a property of the material, not of the sound itself, so a whisper and a thunderclap travel at the same speed.
- Flash-to-bang
- The gap between seeing lightning and hearing thunder. Light arrives effectively instantly, so the whole delay is sound travel time. Divide the seconds by 3 for the distance in kilometres, or by 5 for miles.
- Echo time
- The time for a sound to reach a surface and return. It is double the one-way time, because the wave covers the distance twice. A cliff 170 m away returns an echo after about one second.
- Temperature coefficient
- The rate at which sound speed changes with air temperature, roughly 0.6 m/s per °C. This is why the simple formula v ≈ 331.3 + 0.606T works well across ordinary weather conditions.
Frequently Asked Questions
How do I calculate lightning distance?
Count the seconds between seeing lightning and hearing thunder. Each 3 seconds equals roughly 1 kilometre (or 5 seconds per mile). Light travels almost instantly, so the delay is entirely due to sound travel time.
Why does sound travel faster in water than air?
Sound speed depends on a medium’s elastic stiffness as well as density. Water and steel are much stiffer than air, so their ratio of stiffness to density produces a higher wave speed. Density alone does not explain it.
How does temperature affect sound speed?
Warmer air has faster-moving molecules that transfer sound vibrations more quickly. Sound speed in air increases by about 0.6 m/s for each 1°C rise in temperature.
Why do I see fireworks before hearing them?
Light travels at 300,000 km/s while sound travels at only 0.34 km/s in air. For fireworks 1 km away, you see them instantly but hear them about 3 seconds later.
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