Based on the authors＇ previous numerical results of a reverse osmosis desalination system for seawater or brackish water utilizing the static pressure head of seawater in a deep-sea region or brackish water in a vertical pit, a dimensionless expression is formulated to predict the transmitted fresh water flow rate from the geometry and the submerged depth of the device, the pure water permeability and the solute permeability of membrane, and the physical properties of seawater or brackish water. The derived expression is compared with experimental results obtained by field experiments carried out in the Sea of Japan and the East China Sea with fair agreement and thus confirmed the applicability and usefulness of the expression.