The accuracy of downhole weight on bit (DWOB) is one of the most important factors in the improvement of drilling efficiency, especially in horizontal well drilling. This paper introduced an analytical model and developed a corresponding program to calculate DWOB using drilling data on the surface. A well with field data measured was utilized to verify the model. The results indicate that the calculated friction factors are reasonable, and the DWOB calculated using the analytical model is consistent with those measured by the measurement tool. To satisfy the requirement of real-time drilling monitoring, a least square fitting (LSF) model for the relationship between the DWOB and surface weight on bit (SWOB) measured by deadline sensor on the surface was established to improve the calculation speed. Compared to the analytical method, the LSF model features much faster calculation speed, and the DWOB predicted by LSF model shows much better agreement with the measured DWOB. In addition, the influence of friction factors and standpipe pressure on the coefficients of LSF model was analyzed. The method and calculation technique proposed have applications potential, such as integration with present autodriller systems, post-drilling analysis, drilling operation monitoring, drilling optimization and automation.