Cooling tower approach temperature is the difference in leaving water temperature and entering air wet-bulb temperature. Cooling tower manufacturers describe the design capacity with water volumetric flow, entering water temperature, leaving water temperature, and entering air wet-bulb temperature. Cooling tower performance can be measured by comparing the design criteria to live operation, but only at design conditions. It is necessary to describe cooling tower approach temperatures when tonnage, flow, and wet bulb temperatures are not at design conditions. Gradient descent regression analysis was performed on data including 8760 hours from seven cooling tower installations to produce a generalized description of approach temperature. Fault detection diagnostics can be performed on such expected approach temperatures against live data.