The gasdynamic performance equation for large-scale dry cooling towers is derived relating size, shape, fan power, and drag and heat-transfer performance of heat exchangers. Induced-, forced-, or natural-draft towers are special cases. The general result consists of a zero-order scaling formula involving a size function having a minimum for a particular combination of temperatures, together with a set of correction terms of first order in temperature rise. Thus systems that are nearly equivalent gasdynamically can be compared. Technical requirements for small tower size or fan power are expressed in terms of heat-exchanger parameters.