The ''Guide to the Expression of Uncertainty in Measurement'' (GUM) is used to determine the uncertainty of heat capacity measurements by means of differential scanning calorimetry (DSC). The approach to the expression of uncertainty according to this guide gives a uniform basis for the comparison of experimental results, methods and instruments. On the basis of measurement results obtained with a power-compensated differential scanning calorimeter on a glassy ceramic and using common evaluation methods, an uncertainty analysis based on the ''ISO Guide'' is presented by an example. The relative expanded (k = 2) uncertainty of the heat capacity determination is U(c(P))/c(P) = 1.5%.