A multi-criteria decision-making analysis linked to a Geographical Information System was developed to solve the spatial siting for offshore wind farms taking into account appropriate conflicting factors/constraints. A new approach is presented to solve the conflicting factors by determining the Importance Index (I) for offshore wind farms. This is based on a newly defined parameter Representative Cost Ratio (RCR) facilitating the comparison process. The method compares factor pairs and overcomes the issue where the evaluation of "alternatives" and "criteria", conducted by a number of experts result in reduced accuracy, coherence, and making the process time-consuming. The approach is tested through two case studies (i) UK deployed projects and (ii) determining the offshore wind energy potential around the Arabian Peninsula at scale. The presented method circumvents the literature-highlighted shortcomings with the advantage of considering all restrictions/constraints together at the start of the analysis, arriving at a signally combined Boolean Mask. RCR compares factor pairs to interpret the relationship between Importance Index scale (1-9) and its descriptors. Results from both case studies provided excellent outcomes, confirming the robustness of the RCR approach and its global applicability in addressing the spatial planning of offshore wind farms. (C) 2020 The Authors. Published by Elsevier Ltd.