Abstract: An air conditioner and a method for controlling an air conditioner are provided. The air conditioner may include a fan that suctions in indoor air from an indoor space and exhausts the suctioned air to the indoor space, the fan being driven selectively at one of a plurality of wind speeds; at least one filter provided adjacent to the fan; at least one contamination sensor that senses a contamination level of the indoor air; and a controller electrically connected to the fan and the at least one contamination sensor. An accumulated driving time of the fan may be calculated by the controller considering the wind speed, and an exchanging timing of the at least one filter may be determined by the controller on the basis of the accumulated driving time of the fan. The plurality of wind speeds may include low speed, medium speed, and high speed.