AbstractThis paper presents a method to estimate the optimal pairing and ordering method for cascade‐type state‐space digital filters, each having a second‐order section realized in 2D form or optimal form.The optimality criterion is based on minimizing the normalized round‐off noise. The scaling method used is theL2norm orL∞norm. To show the effectiveness of the proposed method, this method is compared with four conventional methods. As numerical examples, the estimation method is shown for three filters having different orders.The result of these examples allows a more optimal filter to be selected in terms of the optimality criterion of normalized round‐off noise when compared to the other conventional methods. Further, the proposed method also is capable of providing an estimation method for filters having section optimal form, of which the conventional methods are not capable. Finally, if the coefficient matrix is transformed using an equivalent transformation, it is also possible to extend the method to realizations other than 2D or the section o