The influence of natural convection due to buoyancy has been studied both experimentally and analytically for many cases, but the existing correlations are not accurate for horizontal, isothermal tube flow when the heated length is relatively short. The research reported in this paper was undertaken to alleviate this shortcoming by deriving a correlation to include new experimental data on a tube with L/D = 28.4. The experimental data from this and other investigations represents a variety of fluids, heating and cooling, and various apparatus and conditions. The equation which correlates the present and previous data to within ±40 percent is