There have been many experimental and theoretical investigations of external free convection from horizontal circular cylinders; however, relatively few studies have investigated the horizontal cylindrical bodies with noncircular cross-sections. This is partly due to the theoretical difficulties presented by the sharp comers. Nevertheless, the wide usage of rectangular ducts merits their study. The objective of this study was to correlate the convective coefficients of rectangular cross-sectional cylinders of various aspect ratios into one empirical relationship.