A method of balancing a physical rotor includes: determining a first balance state of the physical rotor at one or more rotational frequencies; identifying one or more balance zones on a surface of the rotor; providing a computerised simulation of the rotor having the first balance state; providing a first test mass in a first test location within one of the balance zones on the rotor simulation; determining a second balance state of the rotor simulation; providing at least one subsequent test mass in at least one subsequent test location within a balance zone on the rotor simulation and determining at least one subsequent balance state of the rotor simulation; selecting a mass and location from one of the first and subsequent test masses and test locations; and performing a material deposition process to add the selected mass of material to the selected location on the physical rotor.