This paper describes a method for estimating the geometric errors of an aerostatic planar XY stage. The method calculates two-dimensional (2D) position errors and flatness based on measured guideway profiles. Profile measurements, estimates of motion error, and geometric error models are considered to estimate the 2D position and flatness errors along the X-and Y-axes. A three-probe system is used to measure the guideway profiles for the X- and Y-axes. The motion errors, which are used as input data for geometric error models, are calculated from the force equilibrium of aerostatic bearings along the measured guideway profiles and compared with laser interferometer measurements. Geometric error models were derived to calculate the 2D position and flatness errors of the stage. The measured results and estimates for the 2D position error and flatness differed by 0.5 and 0.6 mu m, respectively. Therefore, the proposed estimation method for 2D position and flatness errors of an aerostatic planar XY stage will be a useful tool during the machining and assembly of guideways.