In order to examine the influence of the degree of convergence for converging geometry on lag time, a comparison of surface water lag times for converging and plane surfaces is made using the Overton lag time concept. Assuming that four physical characteristics, watershed length, slope, roughness and rainfall excess are the same, an expression of the ratio of the lag time for converging surface to the lag time for plane surface is derived. The expression is solved numerically, and evaluated graphically for practical purposes. And the findings are compared with those of a previous study.