The object of the paper is to outline a simple and comprehensive procedure for the practical application of the principles determining the economic selection of transformers. If some degree of uniformity could be achieved in these calculations it would be of great advantage to all concerned. Two aspects of the problem are considered: the relatively simple one relating to the purchase of transformers, and the much more complex one dealing with the designing of transformers to satisfy specific economic requirements in individual cases. The formulae developed are supplemented by practical information on the selection of the correct values for the requisite coefficients. A statistical survey is made of methods used hitherto by different buyers of transformers for the capitalization of the transformer losses. Finally, approximate values of the coefficients in the formulae for the capitalization of losses are suggested, which may be used in the absence of more precise information.