AbstractFor many islands around the globe freshwater lenses (FWLs) are an important source of drinking water. Therefore, it is important to be able to estimate the amount of potable water below an island. This study provides a new approach on estimating FWL volumes from the islands' shape using a circularity parameter. FWLs of islands having several shapes, either shapes of real islands or idealized shapes, were modeled using a numerical steady‐state approach and the Ghyben‐Herzberg relation. Results were then compared in order to estimate possible FWL volumes of islands of various shapes from FWL volumes of islands with idealized shapes. Approximate lower and upper boundaries for the FWL volume were defined depending on the lens volumes of an elliptical island having the same circularity and that of a circular island, respectively, and on the circularity. For the maximum depth of a FWL it is not possible to define such an interval from the subset of islands used in this study. The presented findings can help to estimate the FWL volume on islands for which no data are available. The method may also be applied to give a first indication on potential FWL volume changes following climate change.