Abstract: To control precisely the inclination and the azimuth over the assumed well portion as required by the design is key to directional drilling. The existing calculation method is not conducive to give a quick prediction of the situation of the well bottom. With the least square principle, the inclination and azimuth calculation formula are fitted out through analysis of inclination, azimuth, build-rate, length of well portion, and toolface data. The field cases have demonstrated that compared with the conventional formula, the methods give the maximum inclination error of 0.036° and maximum azimuth error of +0.02°over 12 m well length. The formula involves only the starting angle, length of well portion, build-rate, and toolface, thus, providing high precision, easy use, quick prediction of well bottom trajectory, less directional drilling failure, and important guidance to quick site prediction for directional drilling.