Speed of Sound Calculator (air temperature)

Understanding the speed of sound calculator — concept and principle

The speed of sound in air depends mainly on temperature. Enter the air temperature in °C and the calculator returns the speed of sound in m/s and km/h, and the m/s figure is also the distance sound covers in one second, which is handy for estimating how far away lightning struck. It is also used for ultrasonic distance sensors and acoustic measurements.

At 20 °C sound covers 343.2 m each second, so 3 s of thunder delay means about 1 km.

How it is calculated

c ≈ 331.3 × √(1 + T/273.15) m/s

T is the air temperature in °C; 1 + T/273.15 is the ratio of absolute temperature to 0 °C (273.15 K).
331.3 m/s is the speed of sound in dry air at 0 °C.
km/h = m/s × 3.6. The m/s value is also the distance sound travels in one second.

Near room temperature this behaves almost linearly: c ≈ 331.3 + 0.6·T.

The principle

Sound is a travelling pressure wave. Its speed in a gas is c = √(γ·R·T_K ÷ M), where γ is the ratio of specific heats (1.4 for air), R the molar gas constant, T_K the absolute temperature and M the molar mass of air (about 0.029 kg/mol). Newton first assumed the compressions were isothermal and got a value about 15% too low; Laplace corrected it by recognising that they are adiabatic, which adds the factor γ. Because everything except T_K is constant, c is proportional to √T_K. At constant temperature, pressure cancels out: higher pressure raises the stiffness and the density equally, so altitude affects the speed only through temperature.

Worked example

Input: T = 20 °C.
1 + 20/273.15 = 1.07322; √1.07322 = 1.03596.
c = 331.3 × 1.03596 = 343.21 m/s (shown as 343.2146227).
In km/h: 343.21 × 3.6 = 1,235.57 km/h.
Thunder: sound travels about 343 m per second, so 1 km takes about 2.9 s.

Each point is the calculator re-run with only “Air temperature (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Air temperature (°C)	Speed of sound (m/s)
5	334.3184661
10	337.3099222
15	340.2750805
20	343.2146227
25	346.1292014
30	349.0194419
40	354.7292833

How to read the result

Speed rises by roughly 0.6 m/s per °C: about 325 m/s at −10 °C and 349 m/s at 30 °C. For lightning, count the seconds between flash and thunder and divide by 3 for an estimate in kilometres. For ultrasonic sensors, a 10 °C error in assumed temperature changes the measured distance by about 1.7%. Humid air is very slightly faster than dry air, usually by a fraction of a percent.

Common mistakes

Using 340 or 343 m/s in all weather; in winter the real value can be several per cent lower.
Plugging °C directly into √T formulas that expect kelvin.
Thinking sound travels faster at high pressure; at the same temperature it does not.

Keep in mind: The formula is for dry air treated as an ideal gas. Humidity, wind, temperature layers and CO₂ content change real propagation, so for precision acoustics or metrology use a full model such as Cramer's equation.

Frequently asked questions

Why does helium make voices sound higher?

Sound travels nearly three times faster in helium, so the resonances of the vocal tract shift up, changing the voice's timbre.

What is the speed of sound in water?

About 1,480 m/s at 20 °C, more than four times faster than in air.

Is the speed of sound Mach 1?

Yes. Mach number is speed divided by the local speed of sound, so Mach 1 at sea level on a 15 °C day is about 340 m/s.

Sources: O. Cramer, The variation of the specific heat ratio and the speed of sound in air with temperature, pressure, humidity and CO₂ concentration, J. Acoust. Soc. Am. 93 (1993) 2510–2516 · L. E. Kinsler et al., Fundamentals of Acoustics, Wiley

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

Your numbers