Reference evapotranspiration (ETo) is defined as the rate of evapotranspiration from a hypothetical grass reference crop closely resembling an extensive surface of 8 to 15cm tall, green grass cover of uniform height, actively growing, completely shading the ground and not short of water. The main function of the ETo calculator software is to calculate ETo from meteorological data according to FAO standards (FAO Penman-Monteith equation). The program can handle daily, ten-day and monthly climatic data. Variables can be expressed in a wide variety of units. When data for some weather variables are missing, procedures are used for estimating missing climatic data. The ETo Calculator is meant as a practical tool to help agro-meteorologists, agronomists, and irrigation engineers to carry out standard calculations of ETo, to be used in crop water use estimates.