We propose a measure of quality for OD trip tables estimated from link counts. This measure, called the total demand scale is based only on the estimated OD trip table and on the assignment matrix used to estimate it. It is independent of the OD estimation method and of the technique used to overcome the underdetermination of the OD estimation problem. The total demand scale measures the intrinsic underdetermination related to the OD estimation problem, considering only the network topology and the underlying route choice model. Because it identifies the level of arbitrariness introduced by the a priori matrix in the estimation process, our approach helps to evaluate if an important amount of resources must be invested for the a priori matrix, or if the available link counts contains sufficient information. The computation of the total demand scale enables to identify at zero additional cost those OD pairs such that no measurement is available on any path linking them. A MATLAB code for the computation of the total demand scale is also provided.