Scaling-up a chemical process from lab to manufacturing only gives useful results with accurate heat transfer coefficients. If thermal resistances and reaction rates are only approximate, large safety margins must be applied, resulting in larger investments or longer batch times, lowering the productivity of the process. In stirred tank reactors, the mechanism of heat transfer is forced convection, and it is of particular interest when scaling a process from the lab to the plant. The overall heat transfer coefficient in the reactor consists of three partial resistances (reactor film, reactor wall, oil film), which are determined by reaction calorimeters. Measuring the jacket and reactor temperature during the release of a well-defined amount of heat allows researchers to accurately compute the thermal resistance, which is used to model the heat transfer and to make critical predictions for reactors at larger scale.