Properties of Air vs Temperature
Density, viscosity, specific heat, conductivity, Prandtl number and speed of sound of dry air from -20 to 400 C.
Air is the working fluid in ducts, fans, compressors, cooling and wind calculations, and all its properties change with temperature. This table gives the properties used most often for dry air at atmospheric pressure: density, viscosity, specific heat, thermal conductivity, the Prandtl number (which sets heat transfer in convection) and the speed of sound.
The table
| Temperature (C) | Density (kg/m3) | Dynamic viscosity (micropascal seconds) | Kinematic viscosity (mm2/s) | Specific heat cp (J/kg K) | Thermal conductivity (mW/m K) | Prandtl number | Speed of sound (m/s) |
|---|---|---|---|---|---|---|---|
| -20 | 1.3944 | 16.153 | 11.584 | 1003 | 22.52 | 0.719 | 319 |
| 0 | 1.2923 | 17.16 | 13.279 | 1004 | 24.11 | 0.715 | 331.3 |
| 20 | 1.2041 | 18.133 | 15.059 | 1005 | 25.7 | 0.709 | 343.2 |
| 40 | 1.1272 | 19.075 | 16.922 | 1006 | 27.24 | 0.704 | 354.7 |
| 60 | 1.0595 | 19.987 | 18.864 | 1007 | 28.75 | 0.7 | 365.9 |
| 80 | 0.9995 | 20.873 | 20.882 | 1008 | 30.26 | 0.696 | 376.7 |
| 100 | 0.946 | 21.733 | 22.974 | 1010 | 31.71 | 0.693 | 387.2 |
| 150 | 0.8342 | 23.785 | 28.513 | 1016 | 35.23 | 0.686 | 412.4 |
| 200 | 0.746 | 25.713 | 34.467 | 1024 | 38.6 | 0.682 | 436.1 |
| 250 | 0.6747 | 27.536 | 40.81 | 1034 | 41.87 | 0.68 | 458.5 |
| 300 | 0.6159 | 29.266 | 47.52 | 1045 | 44.98 | 0.68 | 479.9 |
| 400 | 0.5244 | 32.497 | 61.971 | 1069 | 50.81 | 0.683 | 520.1 |
Source and notes: Dry air at 101.325 kPa. Density from the ideal gas law (R = 287.05 J/kg K); viscosity from Sutherland's law (reference 17.16 micropascal seconds at 273.15 K, S = 110.4 K); specific heat and conductivity interpolated from standard tables (Cengel and Ghajar, Appendix, air at 1 atm); Prandtl number = cp x viscosity / conductivity; speed of sound = sqrt(1.4 R T). Humid air is slightly lighter.
How to use this table
- Find the air temperature (interpolate between rows).
- Use density for mass flow and pressure, kinematic viscosity for the Reynolds number, and conductivity, specific heat and Prandtl number for heat transfer.
- For another pressure, scale the density in proportion; viscosity, conductivity and specific heat hardly change with pressure.
Worked example: air in a 100 mm duct at 10 m/s
- At 20 C the kinematic viscosity is 15.06 mm²/s, so Re = V D ÷ ν = 10 × 0.1 ÷ 1.506E-5 = 66 400 (turbulent).
- Heating the air to 100 C lowers the density from 1.204 to 0.946 kg/m³: the same volume flow carries 21 percent less mass, and a fan moves less heat per cubic meter of air than the room-temperature figure suggests.
Notes and cautions
- The table is for dry air. Humid air is slightly less dense (water vapor is lighter than air); use the psychrometric calculator for humid air.
- Viscosity increases with temperature for gases (the opposite of liquids).
- Standard air for fan ratings is 20 C, 101.325 kPa and 1.2 kg/m³ (about 0.075 lb/ft³).
Related tools and guides
- Psychrometric Calculator and Chart
- Fluid Properties and Fluid Statics (guide)
- Standard Atmosphere Table (ISA)
- Properties of Common Gases
- All reference tables
FAQ
What is the density of air at 20 C?
About 1.204 kg/m³ (0.0752 lb/ft³) at 101.325 kPa.
What is the speed of sound in air?
About 343 m/s (1 235 km/h, 767 mph) at 20 C. It depends only on temperature and rises about 0.6 m/s per degree C.
What is the Prandtl number of air?
About 0.71, nearly constant over a wide range of temperatures.
Sources
- Sutherland’s law for viscosity; thermal properties from Cengel and Ghajar, Heat and Mass Transfer, Table A-15; ideal gas law with R = 287.05 J/kg·K.
- Dry air at 101.325 kPa. Density from the ideal gas law (R = 287.05 J/kg K); viscosity from Sutherland's law (reference 17.16 micropascal seconds at 273.15 K, S = 110.4 K); specific heat and conductivity interpolated from standard tables (Cengel and Ghajar, Appendix, air at 1 atm); Prandtl number = cp x viscosity / conductivity; speed of sound = sqrt(1.4 R T). Humid air is slightly lighter.