The present paper deals with a computationally efficient procedure to calculate the psychrometric properties of moist air. Taking into account the simplified approximations by ASHRAE and the accurate correlations for the dry air (Lemmon et al., 1999) and saturated steam (Wagner & Kretzschmar, 2008) properties, a simple model is built to predict the thermodynamic behaviour of any moist air working system. All properties are calculated using polynomial correlations of higher grade or of lower grade, according to their accuracy: thus, the computational efficiency is achieved, while the values of each property is satisfactorily accurate (average errors are lower than 0.05%). Apart from improved correlations for extensive properties (volume, enthalpy and entropy) and intensive properties (partial pressure, dew point temperature) of moist air, a new approximation is presented to calculate the wet-bulb temperature without solving any iteration based complex equation. The results of the presented method are compared to the ASHRAE approximation and the accurate correlations by Lemmon and Kretzschmar, in terms of error, average error and standard deviation of error in a range of dry – bulb temperature between 0οC and 80oC.