This paper describes three computer algorithms that have been developed to perform rating calculations of three evaporatively cooled heat exchanger types -cooling towers, fluid coolers and condensers. They are particularly useful for rating commercially available heat exchangers at off-design conditions. The heat and mass transfer "characteristic equation" of a particular heat exchanger is derived from the manufacturer's rating data at the design point. The algorithms are capable of predicting the heat exchange capacity at any weather or process condition. They also allow one to assess the effect of changes in the air flow on fan power, for specified operating conditions. Simulations using the algorithms showed that they were able to predict the heat duty within 3 percent of the manufacturer's rating data. This paper contains listings of the computer programs and user instructions.