Display omitted •The thermal lag causes the heating-rate-dependent onset of melting temperature.•At larger samples, the onset temperature increases linearly with the heating rate.•At smaller sample mass, the onset temperature behaves nonlinearly.•This behavior is explained by using a heat flow model and an equation is derived.•The heating rate dependence for small samples and fast heating is corrected. The thermal lag of DSC is a heating rate independent constant and influences the measured onset temperature of melting peaks and must be taken into account during the temperature calibration. In the literature, however, non-linear behavior of the measured onset temperature is reported for the case of high scanning rates and the use of small samples. Based on a conventional heat transfer model of a DSC a general relation of the heating rate and sample mass dependence of the onset temperature is derived. The reason of this unusual behavior is that the time for the melting event is less than the equilibration time of the DSC. The approach presented can be used to correct the onset temperatures in the case of non-linear behavior, which may occur in conventional DSC and Fast DSC (using chip sensors).