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The paper describes a method for calculating margins to voltage instability. The method is based on sensitivity techniques. The measure to voltage instability can be given in terms of MW, MVAr or MVA depending on the components in the increase in load. The extensions to earlier publications of the method are the ability to increase the load and generation simultaneously on several buses without increasing the computation time. A single forward and backward substitution is sufficient to find the required sensitivities when the load is increased. The productions of the plant can be increased either by a specified profile or by using the turbine droop characteristics. A method for identification of contingencies needing an explicit reserve evaluation is briefly described.