Sound does not have one speed — it has a speed per medium per temperature, and the differences are not trivia: 343 m/s in mild air, 1,481 in water, nearly 6,000 in steel. This speed of sound calculator computes the air value at your temperature from the standard relation, gives the canonical values for common media, and converts any travelling speed into its Mach number against your chosen sound speed.
Sound in Air: The Temperature Rule
For dry air the speed is set by temperature alone: c ≈ 331.3 + 0.606 × T m/s with T in °C (the linear form of c = 331.3√(1 + T/273.15)). The benchmarks every student should own: 0 °C → 331 m/s; 20 °C → 343.4 m/s (this page’s default, the textbook 343); 35 °C → 352.5. Pressure, counterintuitively, does not matter for an ideal gas — squeeze air and its density rises exactly as its stiffness does, cancelling out — while humidity nudges the speed up slightly (moist air is lighter), by a few tenths of a percent on a muggy day. Altitude changes sound speed only through the temperature it brings with it, which is why jets at −55 °C cruise meet Mach 1 at just 295 m/s up there.
Sound in Liquids and Solids
The table in the widget carries the canonical longitudinal values: fresh water 1,481 m/s (about 4.3 times air — the reason a distant splash and its underwater echo feel simultaneous to a swimmer), seawater about 1,560 (salt and pressure push it up, the basis of sonar ranging and its refraction headaches), steel about 5,960 (why the rail sings before the train arrives, and why pipeline leak detectors time arrivals in microseconds), aluminium rod about 3,240. Solids are mode-sensitive — rod, bulk and shear waves travel at different speeds in the same bar — so treat solid entries as the standard reference figures and read a materials table when the mode matters.
Mach Number: Speed in Units of Sound
Mach = your speed ÷ the local sound speed — local being the operative word. A fighter at 686 m/s is Mach 2.0 in mild sea-level air (2 × 343) but Mach 2.3 in the cold stratosphere, at the same true speed. The calculator’s optional speed field performs exactly this division against whichever medium you chose. The regimes the number names: subsonic below ~0.8, transonic 0.8–1.2 (where drag misbehaves), supersonic to Mach 5, hypersonic beyond. Because the denominator rides on temperature, every honest Mach quote is secretly a thermometer reading — pilots convert through indicated airspeed precisely because the raw numbers refuse to sit still.
Timing Sound: The Lightning Rule and Its Cousins
The same speed prices distance from delay: sound needs about 2.9 seconds per kilometre in mild air — the childhood rule of counting three seconds per kilometre (five per mile) between lightning and thunder is this page’s 343 m/s wearing folklore dress. Sonar and medical ultrasound are the rule industrialised: range = c × t/2, with the medium’s c doing all the calibration work, and an ultrasound scanner assumes 1,540 m/s in soft tissue so completely that a fat layer’s slower speed visibly bends its images. Echo sounding, seismic surveying and the concert hall’s delay towers are the same trick at different scales.
Why Temperature Rules Air
Kinetic theory gives the deep reason: sound rides molecular motion, and molecular speeds scale with √T — heat the gas and its molecules both carry the wave faster and hand it on faster. The full equation c = √(γRT/M) shows the cast: γ (1.4 for air), the gas constant, absolute temperature, and molar mass — lighter gases run faster, which is why helium (M = 4 against air’s 29) triples the speed and raises voices, and why the temperature must be absolute (kelvin) anywhere the square root appears, even though the linear rule on this page takes Celsius for convenience.
Common Mistakes
Quoting 343 m/s at −20 °C (the true value is about 319 — winter thunder arrives late). Using air’s speed for underwater timing. Asking pressure to do temperature’s job. Treating “the speed of sound” as a constant of nature like c: it is a property of stuff, varying by medium, mode and weather, and every precise use names all three. And Mach confusion: Mach 2 is twice the local sound speed, which may be well below 686 m/s where the aircraft actually flies.
Related Tools
The waves bench continues in the Physics Calculators hub — the Centrifugal Force Calculator covers rotational mechanics, and the Voltage Divider Calculator starts the electronics shelf.
Frequently Asked Questions
What is the speed of sound in air?
About 343 m/s at 20 °C. The temperature rule c = 331.3 + 0.606 × T (°C) gives 331 m/s at freezing and 352.5 at 35 °C. Pressure does not change it (for an ideal gas); humidity raises it slightly.
What is the speed of sound in water?
About 1,481 m/s in fresh water near 20 °C, and roughly 1,560 m/s in seawater — over four times faster than in air, because water’s stiffness vastly outweighs its density in the wave-speed balance.
How fast is Mach 1 in mph?
At 20 °C at sea level, 343 m/s is about 767 mph. But Mach 1 is local: in the −55 °C stratosphere it falls to about 660 mph, which is why Mach numbers and ground speeds are never converted without a temperature.
Does altitude change the speed of sound?
Only through temperature. Thin air at the same temperature carries sound at the same speed as sea-level air — but the stratosphere is cold, so the practical answer up high is a slower sound speed, about 295 m/s.
Why is sound faster in steel than in air?
Wave speed grows with stiffness and falls with density (c = √(elastic modulus ÷ density) in spirit). Steel is far denser than air but enormously stiffer, and the stiffness wins by a wide margin — about 5,960 m/s longitudinally.
How does the lightning counting rule work?
Count the seconds between flash and thunder and divide by about 3 for kilometres (5 for miles). At 343 m/s, sound takes 2.9 s per kilometre — the rule is the speed of sound rounded for field use.