This paper proposes a computational method to obtain simultaneous scale-up of mixing and heat transfer in single screw extruders by several parametric 3D non-isothermal numerical simulations. The numerical experiments of flow and heat transfer modeling studies are conducted using the Mackey and Ofoli [Cereal Chem. 67 (1990) 221] viscosity model for low to intermediate moisture wheat doughs in the metering section of a single screw extruder. The non-isothermal flow model includes viscous dissipation and the complete three-dimensional flow geometry including leakage flows without any simplifications such as unwinding the screw. Based on Rauwendaal [Polymer Extrusion, Hanser Publishers, New York, 1986] residence time distribution (RTD) and specific mechanical energy (SME) were chosen as the design parameters for the scale-up of mixing and heat transfer respectively. Parametric numerical simulations were conducted by varying screw geometric variables such as helix angle, channel depth, screw diameter to channel depth ratio, screw length to screw diameter ratio, and the clearance between the screw flights and barrel. SME and RTD curves vs. screw parameters were developed from the numerical simulations. From the design charts two differently sized extruders were obtained which had a scale-up of about 10 times based on throughput rates but had the same RTD and SME. (C) 2003 Elsevier Ltd. All rights reserved.