A method and a system for determining a running resistance Fres for a vehicle. A model-based running resistance Fres,mod is estimated. Thereafter an energy change difference ΔWdiff between a model-based estimated energy change ΔWpred and an actual energy change ΔWreal is determined for the vehicle from a starting point to an end point of a distance S which is traveled by the vehicle. A braking force Funknown which is unknown to the model and which acts upon the vehicle over the distance S is also determined on the basis of the energy change difference ΔWdiff. Thereafter the running resistance Fres is determined to an adjusted value of the model-based estimate of the running resistance Fres,mod, the adjustment being based on the braking force Funknown which is unknown to the model.