The aim of this work was to evaluate, for an olive orchard in the Aljarafe county, the method developed by Orgaz et al. (2005) for determining the crop evapotranspiration (ETc). We compared the calculated ETc (ETc (Orgaz)) values with those determined by the crop coefficient approach (ETc (crop) (coef)), as described by Fernandez et al. (2006), who used coefficient values previously calibrated for our orchard conditions. In addition, we compared the tree transpiration (E-p) values estimated with the mentioned Excel application (E-p (Orgaz)) with those simulated by a transpiration model (E-p (sim)) based on Penman-Monteith, validated for our orchard conditions. Results showed that the Excel application is a user-friendly tool valid for calculating reasonably accurate values of ETc from very few easy-to-measure inputs. The crop coefficient approach does not have this limitation, but years with unusual leaf area density may lead to errors on the calculated ETc. E-p (sim) is highly affected by variables difficult to measure in commercial orchards, such as the leaf area and the available soil water. In addition, processes related to leaf ageing, soil temperature and recovery after drought are not included yet in the model, which affects the reliability of the E-p (sim) values at the end of the irrigation.