An air conditioner and a fluid filling method for an air conditioner are provided. The method may include operating, by the air conditioner, in a fluid supply mode, to fill a group of pipes with a fluid, such as water, determining whether an amount of the fluid in the group of pipes is appropriate while continuously filling the group of pipes with the fluid, and outputting, by an output device, information indicating that filling of the fluid has been completed when it is determined that the amount of the fluid is appropriate. The fluid supply mode may include a first fluid supply mode in which fluid is supplied while the outdoor unit is stopped, and a second fluid supply mode in which the fluid is supplied while the outdoor unit is operated.