THIS – Tool for Heat Island Simulation, is the result of the development of a simulation model, which was incorporated into a Geographic Information System (GIS), in order to calculate the maximum intensity of urban heat islands (UHI_max) based on urban geometry data. The urban heat island phenomenon is defined by the temperature rise in dense city centers compared with the surrounding countryside. The methodology of this study is based on a theoretical-numerical basis (Oke’s model), followed by the development of a calculation algorithm incorporated into the GIS platform, which is then submitted to an adjustment and applied as exemplification. This adjustment was made by calibrating Oke’s model in a case study, based on two Brazilian cities, and the different trends found for different roughness length ranges. As a consequence, this work has resulted in the automation of an algorithm to obtain maximum intensity values of heat islands based on a simplified model. After finishing the subroutine, the application of the THIS tool in a simulation of different urban scenarios showed different trends in the UHI_max value for the H/W ratio and the roughness length. The UHI_max increases when the H/W ratio increases, but the urban canyons with greater roughness (larger areas of facades and more heterogeneous heights, Z₀ ≥ 2.0) result in UHI_max values of approximately two times smaller than canyons with less roughness (homogeneous with highest average areas occupied by buildings, Z₀ < 2.0) for the same value of H/W ratio. Overall, the developed tool has a single goal: to simulate the effect of the isolated variable of urban geometry on the maximum intensity of nocturnal heat islands, considering different urban scenarios.