Speed of Sound in Dry Air at 20 °C
| Value | 343.2 m/s |
| Status | Measured: ± 0.2 m/s (0.00058 relative) |
| Source | CRC Handbook of Chemistry and Physics / ANSI S1.26 |
| Categories | Material PropertiesEngineering & Trade |
| millimeter per year | 10,830,568,000,000 mm/yr |
| millimeter per day | 29,652,480,000 mm/day |
| millimeter per hour | 1,235,520,000 mm/h |
| inch per day | 1,167,420,500 in/day |
| foot per day | 97,285,039 ft/d |
| inch per hour | 48,642,520 in/h |
| meter per day | 29,652,480 m/d |
| millimeter per minute | 20,592,000 mm/min |
| meter per hour | 1,235,520 m/h |
| inch per minute | 810,708.66 in/min |
| millimeter per second | 343,200 mm/s |
| foot per minute | 67,559.055 ft/min |
| centimeter per second | 34,320 cm/s |
| meter per minute | 20,592 m/min |
| inch per second | 13,511.811 in/s |
| kilometer per hour | 1,235.52 km/h |
| foot per second | 1,125.9843 ft/s |
| mile per hour | 767.71654 mph |
| knot | 667.12743 kn |
| meter per second | 343.2 m/s |
| mach (sea level) | 1.0085397 Ma |
| kilometer per second | 0.3432 km/s |
| speed of light | 0.000001144792 c |
Dry air at any temperature
-50 to 300 °CThe value above is one point on this curve. Dry air at 1 atm from −50 to 300 °C — the ideal-gas density and Sutherland viscosity behind every duct, fan and drag calculation.
Learning zone
Sound speed in an ideal gas is c = √(γRT/M): for dry air with γ = 1.40, R = 287.05 J/(kg·K) and T = 293.15 K, that is 343.2 m/s. The striking feature is what is missing — pressure. Raising pressure raises both stiffness and density in the same proportion, so they cancel, and sound travels at the same speed at sea level and at 10 000 m for the same temperature. Altitude changes the speed only because it is colder up there, which is why the sound barrier is a lower true airspeed at altitude.
Temperature is everything: c ≈ 331.3 + 0.606 × T(°C) m/s, so a 0 °C winter morning gives 331 m/s and a 35 °C afternoon 352 m/s. Humidity adds a little (a percent or so at saturation) because water vapour lowers the mean molar mass. Use it for the 340 m/s round trip in ultrasonic level and distance sensors — which must be temperature-compensated or they read 6 % long across a seasonal swing — for duct acoustics, and for the classic five-seconds-per-mile lightning estimate.