The objective of this paper is to introduce a method that quantifies the degree of visibility of an offshore wind farm from an observer located along the coast. Due to the relative vicinity with living areas, the installation of an offshore wind farm leads in many cases to public opposition. Therefore, there is still a need to provide planners and decision makers appropriate methods and objective criteria to actually evaluate the visual impact of offshore wind farms. The aim is to provide an intelligible method for visual assessment that can be applied and used within a full decision-aid system oriented towards offshore wind turbines. A visual operator that takes into account the visual distance of an observer located on the ground to the farms layout is introduced. Such operator is based on the aggregation of three different indexes. The horizon occupation is the main index, it evaluates the horizon surface impact. This parameter is modulated by a second index that computes the number of distinguishable turbines and a third index considered as an aesthetic one based on turbine alignments. The whole method is implemented on top of a Geographical Information System (GIS). Overall, the approach provides an estimation of the visual impact without further need for local surveys, it is conceived as a decision-aided mechanism oriented to decision-makers and that can be used at the preliminary steps of the study of an offshore wind farm. The whole approach is experimented in the context of a wind farm in North West France.