The EPANET software to simulate the hydraulic behavior of water distribution systems allows the user to select three different formulas for calculating the head losses in pipes due to friction, and one among ten flow units grouped into two systems: Conventional (US) and Metric (SI). When selecting a new formula for head losses and / or a new system of units, Epanet does not make any change on the affected data, causing faults in the simulation or giving rise to incorrect results. This tool allows to identify the properties of the elements that are affected by the changes in the system of units. Concerning the head the loss formula, the user can choose between two complementary methodologies to calculate the equivalent roughness coefficients, being the method based on the results of a previous simulation those that shows the best results, compared to the direct conversion of the roughness coefficients using potential-logarithmic equations. Both methods have been applied to different real water supply and irrigation networks obtaining satisfactory results. The new calculated roughness coefficients calculated, on the cases of networks studied, report a maximum error rate of 1.5% with respect to the pressures obtained with the original model, and an average error rate of 1.7% with respect to the flow rates.