The study of rainfall (P) and reference evapotranspiration (ETo) are essential in areas with poor rainfall distribution. The aim of this study was to determine the parameters of the best-fit probability density function and to establish the probable ETo and P for the regular Grid of 0.25º x 0.25º, for São Paulo State, Brazil. The adherence of the probability density functions (pdf) of the ten-day values was obtained with the Kolmogorov-Smirnov test at 5% significance level. With the pdfs (Exponential, Gamma, Normal, Uniform and Triangular) that best fit P and ETo series in each ten-year period, the probable ten-day values of P at 75% probability (P 75% ) and ETo at 25% probability (ETo25%) were determined for each locality in the regular Grid. The Gamma and Normal pdfs, respectively, adjusted better by approximately 60% and 55% of the total adherence tests. The P 75% is well defined in the São Paulo State, being higher in the summer and spring-summer. The highest probabilities of ETo25% are in the Northern and Northwestern regions. The difference between "P75%-ETo25%" shows that higher water restrictions are expected mainly in the Northern and Northwestern regions during throughout the year, with winter being the most critical season.