Natural convection is a suitable method for thermal control because of its low cost, longevity, and convenience over a wide range of applications. It is suitable for the stances of low heat dissipation. The phenomenon of natural convection from horizontal heated surfaces is associated with various industries and equipment. The present study focuses on explaining the heat transfer by natural convection to a pool of water from an upward-facing heater surface. Stainless steel (SS 304) plates of different aspect ratios maintained under uniform heat flux conditions are considered for this study. An infrared (IR) camera is used to measure the surface temperature of the heater plates. The temperature distribution along the plate during the heat transfer process is analysed. Convection currents emerging during the process are visualized using the shadowgraph technique. This research puts light on the effects of aspect ratio, length, and width of the heater on heat transfer during the natural convection process. It is observed that the Nusselt number is independent of the aspect ratio. However, the length and width of the heater individually govern the heat transfer process. Independently increasing the length and width of the heater results in an increase in the Nusselt number. The experimental data of the present study are compared statistically with the existing correlations. An empirical correlation is suggested for analysing the natural convection heat transfer process. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.