An air conditioner and a method of controlling an air conditioner are provided. The air conditioner may include a compressor, an accumulator that recovers a liquid refrigerant contained in a refrigerant flowing into the compressor, an outdoor heat exchanger that performs heat exchange of air using the refrigerant, an outdoor unit fan that supplies outside air to the outdoor heat exchanger and discharges heat-exchanged air, a refrigerant charging pipe connected to a refrigerant pipe and allowing the refrigerant to the additionally introduced from the outside, a refrigerant charging valve installed in the refrigerant charging pipe to open and close the refrigerant charging pipe, and a controller that sets an operation mode so as to operate in a cooling operation or a heating operation upon setting of a refrigerant charging mode, sets an operation frequency of the compressor in accordance with a load of an indoor unit upon the setting of the refrigerant charging mode to allow the compressor to operate at the set operation frequency during the refrigerant charging, and calculates an amount of refrigerant in order to stop the refrigerant charging when a predetermined reference value is reached.