The rainwater (rw) of harvesting systems used in households and commercial buildings comes from independent system, i.e. disconnected of drinking water system. So, rw is not directly measured by conventional drinking water meters which are installed in pipes from public system. Therefore, the used rw volumes are not properly measured. This paper presents a methodology to estimate rainwater loading discharged in sewage systems by means of harvesting practices in households. The estimation was indexed to the drinking water meter. The toilet flush was adopted as object analysis. Results pointed out that the efficiency of rw system varies from 24 to 87%, whose correspond to values varying from 8 a 370 L.inhab.(-1).dia(-1) keeping fixed reservoir volume of 0,5 m(3)inhab.(-1) for all ranges of per capita consumption. The rw in sewage represented values lower than 30% of the total demand considering specific roof from 4 to 25 m(2).inhab.(-1). On the other hand, for reservoir of rw harvesting systems bigger than 3 m(3).inhab.(-1) 15 m(2) of roof.inhab.(-1) was enough to plenty supply the toilet flush., based on 120 L.inhab.(-1) consumption. In general terms, the correction factor proposed for estimating wastewater discharge on the new scenario (considering rw harvesting), varies from 1 to 1.42 (1 <=(f(corrigido)/f)<= 1.42). So, the new ratio, given by wastewater and inlet drinking water, varied from 0.8 to 1.14 (0.8 <= f(corrigido)<= 1.14).