The description of users with motor limitations is a significant dilemma for accessibility researchers and system designers alike. Current practice is to use descriptors such as medical diagnoses to represent a person's physical capabilities. This solution is not adequate due to similarities in functional capabilities between diagnoses as well as differences in capabilities within a diagnosis. An alternative is user self-reporting or observation by another person. These solutions are also problematic because they rely on individual interpretation of capabilities. The current research focuses on defining an objective, quantitative and repeatable methodology for assessing a person's physical capabilities in relation to use of computer technology. Results from this initial study are encouraging, including the development of a model which accounts for up to 85% of the variance in user capabilities.