A general form of the mathematical function in the crack-propagation law for specimens of both infinite and finite widths was determined by means of dimensional analysis. The exact function of the latter type was then determined by experiments. It was found that the general equation of the power law holds for specimens of finite width for three metals: Cold-rolled types 301 and 310 stainless-steel sheet and 2014-T6 aluminum-alloy sheet tested at 78° F and low cyclic load rates. The indices n of the power-law equation for the two stainless steels were found to be different from that of the aluminum alloy. The valuable application of the power-law equation to design work is mentioned.