A theoretical analysis and experimental study of natural convective heat transfer from an isothermal downward-facing horizontal round plate has been carried out. The results are presented in the form of correlations between the Nusselt and Rayleigh numbers. The correctness of the theoretical model, proposed to describe convective heat transfer from the downward-facing round plate, and the simplifying assumptions introduced, were verified by comparison with contours of static temperature and velocity distributions determined from numerical simulations using the FLUENT/UNS code and estimated by flow visualisations using interferometry, images of particle tracks, and a liquid crystal foil. The experimental measurements were performed in air and water using a plate of diameter D=0.07 m. The theoretical, experimental, and numerical results are all in excellent agreement.