In continuous casting, slab weight is normally calculated just after torch cutting of slab using the process parameters and measured length. Because the weight is dependent on the slab temperature as well as slab width, the conventional method has the possibility of inaccuracy in calculated slab weight where contraction ratio of hot-slab was used in weight calculation. In this study, the slab width and surface temperature were measured using pyrometer simultaneously and used for determining the slab dimension and high temperature density. Slab density was formulated as a function of slab surface temperature and slab dimension. The accuracy of slab weight could be improved using this new measured method, and the slab could be cut with good matching weight with ordered-weight.