It is described a method for balancing a rotor mounted on a hub of a wind turbine. The method includes measuring a parameter value of a parameter being indicative of the revolution frequency components of the rotor and/or of a generator of the wind turbine during operation of the wind turbine, calculating a change of the spatial mass distribution of the rotor based on the parameter value of the parameter, which change is needed for balancing the rotor, and balancing the spatial mass distribution of the rotor by using at least one balancing weight element being attachable to at least one blade of the rotor based on the calculated change of the spatial mass distribution. It is further described a system for balancing a rotor, a wind turbine, a computer program and a computer-readable medium, which are all adapted for carrying out the above described balancing method.