a method of forming a wellbore with a drill string and that includes continuously and automatically measuring a tare value of the drill string . the tare value of the drill string is measured while the drill string is rotating , fluid is circulating in the drill string , and after the drill string has been axially stationary for a set period of time . the tare value is designated as an average of the measured hook load over the latter half of the set period of time . knowing the measured tare value and a designated weight on bit of the drill string , a hook load for supporting the drill string is calculated . matching the force applied that supports the drill string to the calculated hook load results in an actual wob that matches the designated wob .