The article deals with the technique of friction stir spot welding. The significant element of this technique is to determine the temperature of the material in the zone of stirring which can be determined by calculating the value of the heat input in the welding zone. To determine the value, the authors have carried out experimental studies of the dependency of the heat input on the rotational speed of the experimental tube simulating the welded aluminum alloy. As a result of the calculations, the distribution of heat input along the concentric rings of the tool have been obtained taking into account changes in the friction torque and heat losses by heat conduction and convection as well as the dependency of the total heat input as a function on the diameter of the tool.