Air properties table
Density and viscosity of dry air versus temperature at one atmosphere. Gases behave opposite to liquids here: as air heats, it thins out and its viscosity rises rather than falls.
Dry air at 1 atm
| Temp (°C) | Density (kg/m³) | Dyn. visc. (µPa·s) | Kin. visc. (mm²/s) |
|---|---|---|---|
| 0 | 1.293 | 17.2 | 13.3 |
| 20 | 1.204 | 18.2 | 15.1 |
| 40 | 1.127 | 19.1 | 17.0 |
| 60 | 1.060 | 20.1 | 18.9 |
| 80 | 1.000 | 21.0 | 21.0 |
| 100 | 0.946 | 21.8 | 23.1 |
| 150 | 0.835 | 23.8 | 28.5 |
| 200 | 0.746 | 25.7 | 34.5 |
Air density falls as it warms because the same mass spreads into a larger volume. Its dynamic viscosity, unlike a liquid, rises with temperature, because faster-moving gas molecules transfer momentum more readily. Kinematic viscosity, the dynamic value divided by density, climbs steeply since both effects push it the same way.
Calculating airflow Reynolds number?
Use the kinematic viscosity here with the Reynolds Number Calculator or the Reynolds Number Flow Regimes chart.
Why gases thicken when heated
In a liquid, viscosity comes from molecules clinging to each other, so heating loosens them and the liquid thins. In a gas, viscosity comes from molecules carrying momentum between layers as they fly about, and heating makes them move faster, so they transfer momentum more effectively and the gas grows more viscous. It is a genuine reversal from liquid behavior.
Density and pressure
These densities are at one atmosphere. Air density is proportional to absolute pressure and inversely proportional to absolute temperature, following the ideal gas law, so a compressor at several atmospheres holds proportionally denser air. Always note the pressure as well as the temperature when using air properties.
FAQ
What is the density of air?
About 1.2 kg/m3 at 20 degrees C and one atmosphere, falling as the air warms or the pressure drops.
Does air get more or less viscous when heated?
More viscous. Unlike liquids, a gas viscosity rises with temperature because its molecules move faster and transfer momentum more readily.
How does pressure affect air density?
Density is proportional to absolute pressure by the ideal gas law, so doubling the pressure at constant temperature doubles the density.