Precision control of the slab width is crucial for product quality and production economy in roughing mill. In this paper, we present a new model for the prediction of the width spread of a slab during rolling in the roughing train of a hot strip mill. The approach is based on the extremum principle for rigid plastic materials, and applicable to horizontal rolling of a slab with either a dog-bone shaped cross section or a rectangular cross section. Also, the upper bound theorem is used for calculating the width spread of a slab. The prediction accuracy of the proposed model is examined through comparison with the predictions from the 3D finite element (FE) process simulations.