In the heat industry, design of the best-fitting heat exchanger in energy-converting systems has gained vital importance. In addition to fitness, the time needed to design an exchanger is also of great importance. In order to obtain the optimum heat exchanger, the geometry, the number of heat transfer units (NTU), and the pressure drop (Delta P) must be in harmony such that the requirements will be satisfied. The most important feature of this study, which makes it distinguishable from others, is the determination of optimum geometry, given performance limits, using a genetic algorithm. Therefore, this study offers an alternative way to calculate the needed geometry with a set of given initial values.