One of the parameters of the hydrological cycle that has great influence in water management and agricultural production is evapotranspiration. Finding ways to restore this amount of lost water to the ground is essential, whether from rain or irrigation techniques. In general, indirect methods are used to determine evapotranspiration, especially Penman-Monteith. However, the method requires a large number of variables, requiring the use of other indirect methods, less demanding in relation to the required data. Due to a great number of methods to be compared it is interesting to make use of computational tools that allow to automate these calculations. In the face of the problem, the development of a web application that calculates the reference evapotranspiration (ETo) in an automated way, from the several mathematical methods found in the literature, was glimpsed. The system provides the user with daily ETo results (mm / month) for a given period of time, as well as the daily average (mm / month), for the month, and monthly (mm / month) for a given month during the period, using up-to-date data from INMET-operated weather stations. The application facilitates the calculation process minimizing the estimation time and contributing to the analysis of the results by the user.