A water purifier comprising: a filter unit comprising at least one filter filtering water flowing into the filter unit to generate purified water, wherein the filter comprises any one of a reverse osmosis membrane filter and a hollow fiber membrane filter; a flow rate sensor adapted to measure a flow rate of the water flowing into the filter unit; a concentrated water channel adapted to discharge concentrated water, containing solid matter filtered by the filter; an extraction unit comprising at least one purified water extraction aperture extracting the purified water; a flow rate control valve adapted to control a flow rate of purified water flowing into the purified water extraction aperture; an opening and closing degree measurement unit, adapted to measure a degree of opening and closing of the flow rate control valve; a concentrated water ratio calculation unit, adapted to calculate a ratio between purified water and concentrated water, having been generated by the filter, according to the degree of opening and closing of the flow rate control valve, wherein the concentrated water ratio calculation unit calculates the ratio between purified water and concentrated water by using a look-up table for the ratio between purified water and concentrated water, according to the degree of opening and closing of the flow rate control valve; and a controller, using the flow rate of water measured by the flow rate sensor and the calculated ratio between purified water and concentrated water, to calculate amounts of water used, integrating the calculated amounts of water used during a usage period of the water purifier, and using the integrated amounts of water used to calculate a lifespan of the filter. a filter unit comprising at least one filter filtering water flowing into the filter unit to generate purified water, wherein the filter comprises any one of a reverse osmosis membrane filter and a hollow fiber membrane filter; a flow rate sensor adapted to measure a flow rate of the water flowing into the filter unit; a concentrated water channel adapted to discharge concentrated water, containing solid matter filtered by the filter; an extraction unit comprising at least one purified water extraction aperture extracting the purified water; a flow rate control valve adapted to control a flow rate of purified water flowing into the purified water extraction aperture; an opening and closing degree measurement unit, adapted to measure a degree of opening and closing of the flow rate control valve; a concentrated water ratio calculation unit, adapted to calculate a ratio between purified water and concentrated water, having been generated by the filter, according to the degree of opening and closing of the flow rate control valve, wherein the concentrated water ratio calculation unit calculates the ratio between purified water and concentrated water by using a look-up table for the ratio between purified water and concentrated water, according to the degree of opening and closing of the flow rate control valve; and a controller, using the flow rate of water measured by the flow rate sensor and the calculated ratio between purified water and concentrated water, to calculate amounts of water used, integrating the calculated amounts of water used during a usage period of the water purifier, and using the integrated amounts of water used to calculate a lifespan of the filter.