The use of cable yarding systems constitutes an interesting solution for steep-slope harvesting in mountain forests. However, it requires many specific skills for both forest managers and operators. The purpose of this study is to provide a numerical tool to help managers in planning cable yarding projects and to facilitate the set-up of a cable line by operators. A model was developed and coded in Python language which allows fast calculation of the load path and tensile forces along the cable for different 3D configurations. This model uses theoretical equations initially developed for the mechanical design of cable structures. Its applicability in forestry was validated by field experiments. An additional module using a high-resolution digital terrain model optimizes the cable layout and automatically places required intermediate supports. If geographic information about timber volume is available, it also allows combining technical possibility with harvesting potential. From the managerial point of view, this tool presents significant advantages for forest operations planning. Moreover, operators’ productivity can be improved with the technical outputs of this model, which help to shorten the time spent in setting up a cable line and thereby reduce the total cost of a cable project.