This paper introduces a method for measuring odometry errors in mobile robots and for expressing these errors quantitatively. When measuring odometry errors, one must distinguish between (1) systematic errors, which are caused by kinematic imperfections of the mobile robot (for example, unequal wheel-diameters), and (2) non-systematic errors, which may be caused by wheel slippage or irregularities of the floor. Systematic errors are a property of the robot itself, and they stay almost constant over prolonged periods of time, while non- systematic errors are a function of the properties of the floor. Our method, called the University of Michigan benchmark test (UMBmark), is especially designed to uncover certain systematic errors that are likely to compensate for each other (and thus, remain undetected) in less rigorous tests. This paper explains the rationale for the UMBmark procedure and explains the procedure in detail. Experimental results from different mobile robots are also presented and discussed. Furthermore, the paper proposes a method for measuring non-systematic errors, called extended UMBmark. Although the measurement of non-systematic errors is less useful because it depends strongly on the floor characteristics, one can use the extended UMBmark test for comparison of different robots under similar conditions.