Abstract The importance of maintaining acceptably low salt levels in irrigated soils is well recognised. Current guidelines for the calculation of the leaching requirement (LR) for the leaching of salts imported with irrigation water are demonstrably inadequate and have been generally shown to result in an over-estimate of the LR. This results in the application of excessive amounts of water as irrigation and increased salt loads in the drainage system. In this paper the methods available for the estimation of LR are reviewed and their inadequacies discussed. The basic concept of leaching is reanalysed and a simple physically based equation is developed which involves only the concentration of the applied water C i and a desired soil-water salinity C s ∗ : C s ∗ C i = ln(LR) LR−1 As this equation is independent of the pattern of plant water use, it is of general application and it is concluded that the equation provides a better method of estimating the leaching requirements than the current guidelines.