Evapotranspiration has two main components: evaporation and plant transpiration. Evaporation occurs when water evaporates into the air from the soil and plant canopy. Plant transpiration is the flow of water within a plant and its loss by vapor through its leaves. The amount of water that evaporates from the soil and the quantity of water that sweats from the plant to the atmosphere is called evapotranspiration (ET). It is the process of transferring moisture from the earth into the atmosphere. Evaporation and transpiration occur simultaneously at the same time. Evapotranspiration corresponds to the amount of water required to replace the water quantity that transpired and evaporated over a given period. In the farming sector, ET is used to determine when and how much irrigation water is needed for crops. Evapotranspiration is also helpful to study plant physiology and for research in water requirements and drainage. This smartphone app calculates the average reference ET per day for a selected month and location. The user can then apply the proper crop coefficient to estimate the average daily crop ET (ETc). The app uses the FAO56 method to compute evapotranspiration provided by the United Nations Food and Agriculture Organization in their Irrigation and Drainage paper 56 (FAO56).