About the Speed of Sound Calculator
This speed of sound calculator works out how fast a sound wave travels. For air it uses the ideal-gas relation v = √(γRT/M), so you simply enter the temperature in °C or °F; you can also enter the heat-capacity ratio and molar mass of any other gas (helium, carbon dioxide, hydrogen…), or pick a typical value for water, seawater, steel, aluminium, copper or glass. Results are shown in m/s, km/h, mph and ft/s, along with how long the sound takes to cover a distance you choose.
It is handy for physics homework, acoustics and audio work (speaker delay and room timing), estimating how far away lightning or fireworks are, and for drone, aviation and ballistics hobbyists who need the local Mach 1 speed. The chart shows how the speed in air climbs by about 0.6 m/s for every degree Celsius.
Air is treated as dry and ideal, which is accurate to well under 1% at normal conditions; humidity raises the speed slightly (by roughly 0.1–0.6% at room temperature) and pressure on its own has almost no effect. Liquid and solid values are typical handbook figures and vary with composition, temperature and, for seawater, salinity and depth.
With the default inputs, the speed of sound is 343.24 m/s. Change any value above to recalculate instantly.
How to use the speed of sound calculator
- 1Pick air, a custom gas, or a liquid or solid.
- 2For a gas, enter the temperature and choose °C, °F or K.
- 3For a custom gas, enter γ and the molar mass in g/mol.
- 4Optionally enter a distance to see how long sound takes to cover it.
- 5Read the speed in m/s, km/h, mph and ft/s.
Formula and method
In an ideal gas, sound is an adiabatic pressure wave, so its speed depends on the heat-capacity ratio γ, the absolute temperature T and the molar mass M: v = √(γRT/M) with R = 8.314462618 J/(mol·K). Pressure cancels out, which is why altitude affects the speed of sound only through temperature. For dry air this gives 331.3 m/s at 0 °C and 343.2 m/s at 20 °C.
Light gases such as helium carry sound much faster because M is small. In liquids and solids the speed is set by stiffness and density (v = √(K/ρ) or √(E/ρ)) instead, so the calculator uses typical measured values for those media rather than a formula.
- v
- Speed of sound (m/s)
- γ
- Ratio of specific heats cp/cv
- R
- Molar gas constant, 8.314462618 J/(mol·K)
- T
- Absolute temperature (K)
- M
- Molar mass (kg/mol)
Worked examples
Air at 20 °C (room temperature)
v = √(1.4 × 8.3145 × 293.15 ÷ 0.0289645) = 343.2 m/s, or about 1,236 km/h (768 mph). Sound needs 2.91 s to travel 1 km — the basis of the “count three seconds per kilometre” lightning rule.
Cold air at 0 °F
0 °F is 255.37 K, so v = √(1.4 × 8.3145 × 255.37 ÷ 0.0289645) = 320.4 m/s (717 mph) — about 23 m/s slower than at room temperature, which is why Mach 1 is lower at cruising altitude.
Helium at 20 °C
Helium is monatomic (γ = 5/3) and only 4.0 g/mol, so v = √(1.667 × 8.3145 × 293.15 ÷ 0.0040026) ≈ 1,007 m/s, almost three times the speed in air — which is why helium raises the resonant pitch of your voice.
Fresh water
Sound travels about 1,481 m/s in fresh water near 20 °C, roughly 4.3 times faster than in air, covering 1 km in 0.675 s.
Frequently asked questions
What is the speed of sound in air?+
At 20 °C in dry air the speed of sound is about 343 m/s (1,235 km/h or 767 mph). At 0 °C it is 331 m/s. It rises by roughly 0.6 m/s for every 1 °C increase in temperature.
Does altitude change the speed of sound?+
Only through temperature. Air pressure and density fall together with altitude and their effects cancel, but the air is colder higher up, so the speed of sound drops to about 295 m/s at airliner cruising altitudes near −56 °C.
Why does sound travel faster in water and steel?+
Sound speed depends on how stiff a material is relative to its density. Water and steel are vastly stiffer than air, so although they are denser, sound moves at about 1,480 m/s in water and nearly 6,000 m/s in steel.
Does humidity affect the speed of sound?+
Yes, slightly. Water vapour is lighter than nitrogen and oxygen, so humid air carries sound a little faster — typically 0.1% to 0.6% faster at room temperature. This calculator assumes dry air.
How do I estimate how far away lightning is?+
Count the seconds between the flash and the thunder and divide by about 3 for kilometres (or 5 for miles), because sound travels roughly 343 m per second in air at room temperature.