Godfrey's Island Rule is rederived in terms of the barotrapic streamfunction for flow in a stratified ocean with a rigid lid. Modifications to include bottom topography and frictional effects along eastern boundaries are derived. The “Island Rule” has an important application in describing the net transport through the Indonesian seas, that is the Indonesian Throughflow. The original rule, derived from a Sverdrup model, yielded an annual mean throughflow of 16±4 Sv (Sv ≡ 106 m3) s−1). Observations of depth-integrated steric height differences indicate that frictional effects within the lndonesian seas will reduce this value by the order of 2 Sv. The reduction estimated from a frictional channel model depends on the parameterization and boundary conditions adopted, and ranges from 5%–20%. Topographic effects could give an increase in transport. For example, if the archipelago is represented as a simple sill, then warmer water on the Pacific slope than on the Indian slope would produce an increase ... Abstract Godfrey's Island Rule is rederived in terms of the barotrapic streamfunction for flow in a stratified ocean with a rigid lid. Modifications to include bottom topography and frictional effects along eastern boundaries are derived. The “Island Rule” has an important application in describing the net transport through the Indonesian seas, that is the Indonesian Throughflow. The original rule, derived from a Sverdrup model, yielded an annual mean throughflow of 16±4 Sv (Sv ≡ 106 m3) s−1). Observations of depth-integrated steric height differences indicate that frictional effects within the lndonesian seas will reduce this value by the order of 2 Sv. The reduction estimated from a frictional channel model depends on the parameterization and boundary conditions adopted, and ranges from 5%–20%. Topographic effects could give an increase in transport. For example, if the archipelago is represented as a simple sill, then warmer water on the Pacific slope than on the Indian slope would produce an increase ...