The heat transport capability of natural circulation loops is directly proportional to the flow rate it can generate. Therefore, reliable prediction of flow rate is essential for design and performance evaluation of natural circulation loops. The reported generalized flow equation applicable for single-phase natural circulation is only valid for cases where the entire loop follows a single friction law. Such a situation arises when the natural circulation loop is either fully laminar or fully turbulent. It is possible that a natural circulation loop can be partly laminar and partly in transition or turbulent flow. In such cases, a single friction law is not applicable throughout the loop. In the present study, a generalized flow equation is proposed for cases where a single friction law is not applicable for the entire loop. The proposed equation is tested with experimental data generated in a uniform diameter rectangular loop and is found to be in good agreement. Subsequently the equation is tested with data reported in the literature. Stability analysis reported in literature for single-phase loops are either for laminar or turbulent flows. In practice, in natural circulation loops, all the flow regimes like laminar, transition and turbulent are observed. Therefore, it is required to develop a stability map, which is valid for all the three regions. In the present paper, such a stability map is presented. (c) 2011 Elsevier Ltd. All rights reserved.