Calculation processing is a new concept for man-computer interaction. Its main objective is to improve the overall productivity when producing design calculations. Its basic capabilities can be described as a word processor with calculation ability. Calculation processing allows the user to use the computer in an easy and flexible way. It can handle text and arithmetic and there is no need to use special commands to distinguish between them, since calculation processing does this automatically. From this concept a more detailed description of a calculation processing system was developed. This grants the user a friendly environment giving him full control of the system. All engineering decisions are made by the user, thus ensuring total flexibility. Design calculations are easily prepared, and their storage and indexing are performed automatically. Design calculations are stored in a number of calculation units and they are presented in different forms depending on the degree of their formatting. Calculation units are organized in a hierarchical structure. Different types of variables are handled by the system and arithmetic functions are included. A prototype of the system was implemented on a Unix-based workstation using the C programming language and yacc. The main objective was to test the concept of calculation processing. A programme of evaluation was executed using the prototype. The evaluation revealed calculation processing is a good concept for man-computer interaction, the resulting calculations are similar to traditional design calculations, and the overall productivity is increased since preparing calculations is faster than that by hand. © 1995.