Mean friction factors for three types of laminar stratified flow have been determined by interpretation of precise results obtained from boundary element calculations. The three cases considered are gravity-driven (open channel) laminar stratified flow, turbulent-gas laminar-liquid stratified flow, and laminar-laminar liquid-liquid stratified flow. The issue of incorporating the interface length into the definition of the hydraulic diameter IS lso addressed. It is found that the wall friction factors for all three types of flow can differ substantially from the commonly used expression, f = 16/Re. Simple correlations are presented for the gravity-driven and laminar-turbulent cases. Owing to the interactions of the two phases in laminar laminar flow, the wall and interface friction factors are quite variable and not easily approximated. In general, the more viscous fluid behaves like the laminar phase in a turbulent-laminar flow, while the less Viscous phase can be approximated as a confined flow.