PURPOSE: To provide a method and an apparatus for calculating the die processing size of extrusion molding capable of estimating the die shape near the object shape without depending upon the worker's experience and skill degree.

CONSTITUTION: When molding dimensions, sectional area, the type and extruding conditions of an extruder are input from an input unit 11 as information regarding an extrusion molding to be newly molded, the data similar to the information input from a data memory 10 is extracted by a data extractor 12 based on the input information and predetermined retrieving conditions, and displayed on a display unit 16. Then, when arbitrary data is selected from the displayed extracted data, the die processing dimensions are calculated by a calculator 13 based on the extracted data, and the calculated result is output to an output unit 14. In this case, the ratio of the molding dimension of the extracted data to the die dimension is calculated, and the die processing dimension is calculated by applying to the molding dimension input at the ratio.