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A method of designing tillage tool shapes based on mathematical expressions is needed. A mathematical equation for the macroshape of passive tillage tools is developed. The equation includes the main geometrical parameters and can be used for designing many different tillage tools. The mathematical description of tool geometry may determine how the design parameters influence the energy requirement and quality of operation. This description has the potential of achieving a quantitative analysis of the tillage process. A computer program has been developed to design and display selected passive tool shapes. The tool surface is represented by a multiplicity of quadrilateral faces limited by user-selected bounding curves. The (x,y,z) face coordinates are generated by a FORTRAN program and read into AutoCAD using an AutoLISP program. The method permits a complete study of the influence of the geometrical parameters upon the final soil condition and energy requirements, thus optimization of the tillage process may be possible.