The cooling effectiveness of air-cooled steam condenser units is impacted by the performance of the large diameter axial flow cooling fans, which ultimately affects the overall efficiency of the power plant. Because of the large diameters of these fans, performance tests are carried out at test facilities with smaller, standardized diameters and measuring equipment. The performance of the large scale fans can be predicted based on the small scale test results using the similarity laws and scale-up formulae. This article details the results of small scale experimental tests and numerical simulations that were performed on a pair of 1.25 m diameter axial flow fans. Full scale, 10.360 m, diameter simulations of the same axial flow fans were subsequently performed and compared with the experimental results that were scaled up using the fan scaling laws.