Relative humidity in an air-water mixture is the fraction of the partial pressure of vapor to the saturated vapor pressure at a specific temperature. The relative humidity of air depends on the overall pressure of the system. Relative humidity is a percentage. It is calculated by the following equation:

Φ = e_{w}/e*_{w} x 100%

The humidity of an air-water vapor mixture is determined through the use of psychrometirc charts if both the dry bulb temperature (T) and the wet bulb temperature (T_{w}) of the mixture are known. There are many several empirical relations that can be used to estimate the saturated vapor pressure of water vapor as a function of temperature. The Bucks correlation is the most commonly used equation to calculate the relative humidity. It is seen below

e*_{w }= (1.0007 + 3.46 x 10^{-6}P) x (6.1121)e^(17.502T/(240.97 + T))

where T is the dry bulb temperature, P is the absolute pressure, and e*_{w }is the saturated vapor pressure.