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A method is presented to estimate the values of monthly rainfall amount at different probability levels using only average monthly rainfall. First, a relationship is derived between average monthly rainfall and monthly rainfall at 50% probability for the stations with available data. The rainfall estimated at 50% probability and the average rainfall is then used to calculate the shape parameter of the Gamma distribution through an iterative search procedure for stations for which only average values are available. After the shape parameter is calculated, then the rainfall at any other probability level can be estimated. The method is tested for countries of Brazil, India, Cameroon and Central African Republic. It was concluded that when past records are not available, this method can be used to estimate the rainfall probability distribution from average values with reasonable accuracy.