The new version of NBR 15527 (ABNT 2019) determines that any methodology used for the design of rainwater reservoirs must be performed considering the demand for rainwater resources. Thus, the objective of this study was to develop mathematical models for the design of rainwater reservoirs in residential buildings for all Brazilian states. Different use scenarios were simulated, which included drinking water demands of 100, 150, 200, and 300 litres per capita and rainwater replacement for drinking water at rates of 25, 35, 45, and 55%, according to rainfall data from the last decade. The optimal and maximal volumes of each scenario were calculated using the yield after spill (YAS) behavioural method. Based on the values of the volumes obtained, the studied municipalities were grouped, and for each group, the practical models were adjusted and evaluated using the coefficient of determination (R2) and the root mean square error (RMSE). A good adherence of the models to the observed data was observed, which simplified the estimation of the volume for rainwater reservoirs; practical applications demand that designs use discreet daily rainfall values. HIGHLIGHTS Rainfall-related variables were used as variables to estimate the reservoir volumes.; Practical models were obtained for behavioural simulations of the reservoir operations.; The local climatic conditions influence the feasibility of using rainwater reservoirs.; The maximal volume of reservoirs tends to increase until reaching a certain maximal water demand, at which point it begins to decrease.;
