A method of calculating traffic values in a communications network (1), the communications network comprising a plurality of nodes (2,4), the nodes being connected to one another by links (24), the method comprising: (a) obtaining traffic data measurements (102) through said nodes and/or links in an initial scenario as input data; (b) deriving a traffic flow model for a modified scenario using a plurality of constraints describing the interdependency of said initial to said modified scenario (116); and (c) calculating values and/or upper and lower bounds of traffic values for said modified scenario from said traffic flow model using said input data (118).