We have determined the dependence of the measured overall heat transfer coefficient, U, on the agitation speed, the volume of fluid inside the reactor vessel and the calibration time by designing 36 experiments. The matrix of these experiments was prepared using software for the design of the experiments. The overall heat transfer coefficient, U, for the above 36 experiments was calculated based on both the heat transfer area at zero agitation and the observed area after the liquid level climbed up the wall due to the fluid being stirred. The analysis of the results shows the dependence of U, based on either initial area or observed area, on the agitation speed and fluid volume. Calibration time was found not to influence the value of U.