An air conditioner comprising: a power supply to supply power; a fan motor; a driving mechanism to convert the power, which is supplied by the power supply, into a motor driving voltage depending on a duty value and to transmit the motor driving voltage to the fan motor; a current detector to detect a current value input into the fan motor; a storage to store static pressure information and air volume information corresponding to the duty value and the current value; and a motor controller configured to control the fan motor by transmitting the duty value to the driving mechanism based on a target revolutions per minute, RPM, characterized in that the motor controller is configured to perform a system resistance calculating operation for calculating a setting RPM; and adjusting at least one of the duty value or the current value such that a present RPM reaches the setting RPM, wherein the system resistance is information of factors to interrupt setting air volume from being constantly supplied due to the environment condition or use condition, and wherein the motor controller is configured to perform the following steps when performing the system resistance calculation operation: controlling the fan motor by setting a target revolutions per minute, RPM, to a predetermined reference RPM, acquiring present static pressure and a present air volume corresponding to the duty value and the current value from the storage when operating in a state that the target RPM is set to the reference RPM, and calculating the setting RPM allowing the present RPM to reach the target RPM based on the present static pressure and the present air volume. a power supply to supply power; a fan motor; a driving mechanism to convert the power, which is supplied by the power supply, into a motor driving voltage depending on a duty value and to transmit the motor driving voltage to the fan motor; a current detector to detect a current value input into the fan motor; a storage to store static pressure information and air volume information corresponding to the duty value and the current value; and a motor controller configured to control the fan motor by transmitting the duty value to the driving mechanism based on a target revolutions per minute, RPM, characterized in that the motor controller is configured to perform a system resistance calculating operation for calculating a setting RPM; and adjusting at least one of the duty value or the current value such that a present RPM reaches the setting RPM, wherein the system resistance is information of factors to interrupt setting air volume from being constantly supplied due to the environment condition or use condition, and wherein the motor controller is configured to perform the following steps when performing the system resistance calculation operation: controlling the fan motor by setting a target revolutions per minute, RPM, to a predetermined reference RPM, acquiring present static pressure and a present air volume corresponding to the duty value and the current value from the storage when operating in a state that the target RPM is set to the reference RPM, and calculating the setting RPM allowing the present RPM to reach the target RPM based on the present static pressure and the present air volume. controlling the fan motor by setting a target revolutions per minute, RPM, to a predetermined reference RPM, acquiring present static pressure and a present air volume corresponding to the duty value and the current value from the storage when operating in a state that the target RPM is set to the reference RPM, and calculating the setting RPM allowing the present RPM to reach the target RPM based on the present static pressure and the present air volume.