A downhole tool is placed in a borehole of a subsurface formation. The downhole tool comprises a plurality of calipers arranged around a circumference of the downhole tool. A plurality of standoff measurements is received for different rotation angles of the downhole tool, wherein each standoff measurement is indicative of a distance between one of the plurality of calipers and a wall of the borehole. A plurality of apparent diameters of the borehole is determined for the different rotation angles of the downhole tool based on the plurality of standoff measurements and a radius of the downhole tool. A probability function is determined based on the plurality of apparent diameters and at least one of a semiminor axis, semimajor axis, and ellipticity of the borehole is identified based on the probability function.