A control method for a compressor system includes a compressor connected to a pressure vessel and a frequency converter controlling an electric motor of the compressor. In the method, the present operating state is estimated on the basis of a monitored/estimated electrical quantity of the compressor system. The operating state may represent the pressure in the pressure vessel. The pressure in the pressure vessel causes a counter-torque to the motor. The counter-torque is proportional to the pressure and may be used for estimating the pressure inside the pressure vessel. An estimate of a counter-torque may be calculated on the basis of the monitored electrical quantity or quantities.