The goal of the modern farming systems is to economize energy consumption and to reduce farming costs. Proper selection and use of agricultural machines are important factors to achieve this end. Rotary tillers are tillage tools that are used for accomplishment of both the primary and secondary tillage practices. Considering the widespread application of rotary tillers, optimal design of these machines is mandatory. Selection of appropriate tractor power and equipment is more complex and tedious due to computational work involved in solving the equations for optimal working width and optimal diameter of the rotary tiller's rotor proportionate with the tractor power. Therefore in this study, a computer programme was developed in visual basic language to calculate optimal working width and optimal diameter of the rotary tiller's rotor proportionate with the power of the tractor was delineated based on the energy method. This programme is based on the specific works of tractor and rotary tiller. Results obtained by using this software were validated by comparing the simulated dimensions with some of the commercially available rotary tillers. The variation between simulated and available data for different horsepower range 12, 45 and 105 hp were found to be within 5 per cent limit. Hence, the developed software can be used as a tool for designing rotavator for different horsepower ranges of tractor and soil conditions.