The Graduation Thesis deals with design of reinforced high-strength concrete cross-sections, exposed to bending load. A theoretical basis and other necessary tools for appropriate design of cross-sections due to the limit states method are being presented. Limit states method is also presented in a Slovenian standard SIST EN 1992-1-1. Theoretical tools served as the basis for the Microsoft Excel software tool development. Based on specific input data the program calculates all the necessary quantities due to the design of limit states method. Emphasis is placed on algorithms for accurate calculation. Basic quantities are taken into account, while considering all the necessary conditions of the limit states method. When there are more conditions considered in the calculation, the program always follows the stricter one. The research problem of the design of concrete structures is being analyzed in the thesis. When cross-sections of the elements are exposed only to bending load, the program developed serves as an appropriate tool for faster calculation.