Reference evapotranspiration (ETo) is a key agrometeorological index for rational irrigation management. The standard method for ETo estimation, proposed by FAO, is based on a complicated Penman-Monteith equation, requires great number of meteorological inputs thus making it difficult for practical use by farmers. To the moment, there are many alternative simplified approaches for ETo estimation, most of them are directed to cutting the number of required meteorological inputs for calculation. Among them, special attention should be paid to various temperature-based methods of ETo assessment. One of the temperature-based models for ETo computation was realized in free mobile app ETo Calculator (Ukraine). The app gives Ukrainian farmers an opportunity to assess ETo values on daily or monthly scale using mean air temperature as the only input. The goal of the study was to test the app accuracy comparing to FAO-based calculations in five key regions of Ukraine, each of which representing a particular climatic zone of the country. It was established that the app provides relatively good accuracy of ETo estimation even in raw (not adjusted to windspeed and relative air humidity) run; the results of statistical comparison with the FAO-calculated values are: R2 within 0.82-0.87, RMSE within 0.74-0.81 mm, MAE within 0.60-0.70, MAPE within 18.07-25.50% depending on the region. ETo Calculator (Ukraine) is a good alternative for complicated Penman-Monteith method and could be recommended for Ukrainian farmers to be used for irrigation management.