A part of the charter of Panel M-9 is to "develop a Guide to the Selection of Backing Power." An essential basis for such a guide is a method for calculating stopping ability. This Research Bulletin is an interim report on that phase of the panel's work. The report is not a final, finished product, but is presented herewith in abbreviated form with the hope of stimulating interest in applying the proposed method to the solution of practical operating or design problems, and in checking the method by comparing calculated results with available trial data. Information forwarded to the authors on the results of such calculations will be of great assistance to them in their continuing work on simplifying, extending, and improving the method. The results of this further work will be published with a complete derivation of all equations and a discussion of the basic theory, use, limitations, and accuracy of the method.