An approach to achieve absolute planarity characterization of an x-ray mirror through an interferometric method is presented. With three measurements and two nominally flat auxiliary mirrors, the radius of curvature and the slope profile of a nominally flat x-ray mirror are retrieved. The height profile is then calculated through integration of the slope profile and merging this information with the radius of curvature knowledge. The method is explained in detail and a measurement example is provided. A comparison with a state-of-the-art deflectometric method is shown, with an agreement level of about 0.14 nm rms.