Abstract This paper describes a new method for predicting wellbore position uncertainty which responds to the current needs of the Industry. An error model applicable to a basic directional MWD service is presented and used for illustration. As far as possible within the limitations of space, the paper is a self-contained reference work, including all the necessary information to develop and test a software implementation of the method. The paper is the product of a collaboration between the many companies and individuals cited in the text.