In this paper, three-dimensional forward extrusion of a square section from a round billet through a straight converging die is analysed using both physical modelling and numerical simulation (finite element and upper bound analysis). Theoretical fundamentals for each method are reviewed, and comparisons are made between the numerical predictions and experimental data obtained through the utilisation of physical modelling. Assessment is made in terms of flow pattern and strain distribution for two different cross-sections corresponding to the axial symmetry planes of the three-dimensional extrusion part. The experimental distribution of strain is determined from the shape change of quadrilateral grids previously printed on the surface of the axial cross-sections of the undeformed billets by means of large deformation square-grid analysis. Good agreement is obtained between physical and numerical modelling of the three-dimensional extrusion process.