Steady, three-dimensional natural convection in a rectangular parallelepiped filled with a saturated porous medium is analyzed numerically. Convergence of the current three-dimensional model is enhanced using a mutiigrid method. Three new cases are studied in addition to a known case (used to validate the solution) A new approach for calculating heat flow rate at walls is developed by applying the principle of an overall heat balance. The geometry encompasses different aspect ratios, while the boundary conditions combine the influence of the internal and external heat flows in terms ofBiot number. The current analysis shows that the heat flow rate is a strong function ofRayleigh number, the geometry, and the boundary conditions