A method to determine the overall heat-transfer coefficient in a heat-how reaction calorimeter is presented. It can be used in situations in which calibration may not yield accurate results. For example, it does not require any holding period at or near the temperature of interest, which may interfere with the chemical process. Also it can provide a realistic temperature difference between the reactor and the jacket when dealing with viscous liquids. The proposed technique is based on the solution of the RC1 energy balance and only calorimeter data and parameters are utilized in the calculations. Standard calibration may still be necessary in conjunction with temperature ramps to evaluate specific heats. However, this procedure can be carried out at a lower temperature, away from the reaction conditions, An important concept presented in this work is the dimensionless group we shall call the heat-loss number. With it we can determine the overall heat-transfer coefficient with increased accuracy and have a realisitc account of heat losses.