On most onshore rigs, weight on bit (WOB) is usually obtained from the deadline hook load measured by deadline the sensor installed on the deadline of the block-and-tackle system. To obtain a more accurate WOB, a measurement tool, called torque and tension sub (TTS), has been developed to measure the load below the quill of a top-drive system. Theoretically, the load from the TTS, called the TTS hook load, is more accurate than the deadline hookload because the TTS measurement is not affected by the friction between the drilling lines and sheaves and the top drive rails. The downhole weight on bit (DWOB) is a key parameter in optimizing rate of penetration (ROP) and drilling optimization because accurate DWOB plays a very important role in improving the overall performance of a bit run. The objective of the work presented herein is to acquire rig parameters such as sheave efficiency, static weight of travelling block and friction between top drive and the rail by using theoretical analysis and regression analysis of acual drilling data. These parameters can further be used for DWOB automation calculation, which will greatly improve the overall drilling efficiency and save drilling cost. A set of drilling data was collected from a well in western Canada, including bit depth, hole depth, deadline hookload, TTS hookload, surface measured WOB and standpipe pressure. Force analysis to the upper part of a top drive system was conducted, and a theoretical equation derived which reflects the relationship between the deadline hook load and the TTS hook load. An empirical relationship is also obtained by regression analysis of the drilling data. The results show that there is a good agreement between the theoretical equation and the empirically-derived relationship. Therefore, the empirical relationship can be used to get the sheave efficiency, static weight of travelling block and friction between the top drive and the rail. The relationship between the deadline hook load and TTS hook load established in this paper brings about a new method to acquire rig parameters which are usually obtained by experience or a rule of thumb. This will potentially improve the calculation of DWOB automation. The proposed new correlation method will find a wide range of potential applications, such as on new types of automation drilling systems being developed in the drilling industry, which is especially suitable for horizontal drilling.