A general correlation is proposed for the laminar Graetz problem associated with convection in round pipes and parallel plate ducts. The general correlation for the average Nusselt number can be expressed under conditions of either constant wall temperature or constant wall heat flux. Additionally, closed form expressions for the local Nusselt number can be derived from the general correlation. New correlations are found to be within 1.5% of exact solutions to the Graetz problem for round pipes and parallel plates, and are contrasted with existing correlations in the literature. (C) 2019 Elsevier Ltd. All rights reserved.