The most common approach for the estimation of crop water requirements is to pair a crop factor with the evaporation from a reference surface. In this study, a user-friendly computer tool was developed to facilitate the calculation of daily FAO (Food and Agricultural Organization of the United Nations, Rome, Italy) Penman-Monteith reference crop evaporation (ET0), and to estimate errors that can arise if solar radiation, wind and vapour pressure data are not available. The ET0 calculator imports comma, tab or space-delimited daily weather data files in any user-specified format. It displays graphically and processes statistically, ET0 values calculated from full and incomplete weather data sets. The program is written in Delphi with a Paradox database and includes a comprehensive, context-sensitive help file. Sensitivity analyses were carried out for three locations as examples. The error in predicting ET0 using estimated weather parameters was reduced by using 5-day averages of ET0 rather than daily values. Although some error is incurred by estimating weather parameters, this is somewhat compensated for by the absence of any error that may have been associated with the measurements.