A simple, inexpensive, and expedient method is presented for determining the initial infiltration capacity (fo), the decay rate of infiltration (k), and the constant infiltration capacity (fc) to be used in Horton’s equation for a certain soil and for any antecedent precipitation. The method consists of performing a series of infiltration tests using flooding-type test tube infiltrometers for different antecedent precipitation conditions. Horton’s equation is fitted to the available data, thus obtaining values of fo, fc and k for each test. Daily rainfall records are used, starting at least 2 months prior to the infiltration tests, to determine the antecedent precipitation index values corresponding to each test. Curves of antecedent precipitation index plotted against Horton’s constants provide the designer with the proper values of fo and k for the soil on which infiltration tests were performed for any specified antecedent condition.