Under standard atmospheric conditions (25 °C and pressure of 1 bar), the dynamic viscosity of air is 18.5 μPa·s, roughly 50 times smaller than the viscosity of water at the same temperature. Except at very high pressure, the viscosity of air depends mostly on the temperature. Among the many possible approximate formulas for the temperature dependence (see Temperature dependence of viscosity), one is:
which is accurate in the range −20 °C to 400 °C. For this formula to be valid, the temperature must be given in kelvins;  then corresponds to the viscosity in Pa·s.