Natural ventilation is an alternative to create comfortable and healthy indoor conditions. This work presents the development of a methodology to evaluate the thermal comfort produced by indoor natural ventilation in hot climates. This methodology includes the definition of the Heat Balance Index, HBI. It is based on models of the heat transfer of the human body to the surroundings. To estimate the comfort evaporation term a correlation between the Heat Balance Index (HBI) and the Predicted Mean Value (PMV) was made. The HBI gives the comfort air velocity range, which is useful to calculate the well-ventilated percentage of an indoor space for an specific climate condition in hot climates. A numerical simulation of a cross ventilated building is used as an application example. The numerical simulation is solved by using Computational Fluid Dynamics and is validated with experimental results.