This paper deals with the assignment of bearing lengths for the control of material flow in the flat die hot extrusion. The design process makes the use of the three-dimensional non-steady analysis using the thermo-rigid–viscoplastic finite element method that includes an automatic remeshing module. The exit velocity distribution of the workpiece obtained from the analysis results was used to find appropriate values for the factors used in the proposed bearing length design equation. This equation for designing bearing lengths is a function of the cross-sectional thickness and distance from the die center of die exit section. A geometric factor was included in formulation of the design equation to consider the end region of the die exit. The appropriate values of factors were determined from three-dimensional analyses of flat-die hot extrusion processes with single and double channel-sections. The analysis of a flat-die hot extrusion process with a L-section was used to verify the proposed design equation. It was found that the design equation determined bearing lengths that resulted in a fairly uniform exit velocity distribution throughout the extruded section. From the results of this study, it was found that the proposed design equation can be effectively used to estimate appropriate bearing lengths.