Purpose As sustainability has become an important asset for production systems, it is important to conduct life cycle assessment (LCA) studies. When conducting an LCA of products derived from a multi-product process, the common challenge of impact partitioning arises. This is referred to as the allocation problem. Many allocation methods have been introduced in the literature, leading to a variety of results depending on the choice of allocation method. The purpose of this paper is to provide LCA practitioners with an operational approach that helps them choose the allocation method when evaluating co-products. Methods The extensive literature review carried out in the study allows identification of the main allocation methods available to practitioners. A cross-analysis also points out a strong link between allocation choices and decision-makers’ objectives. A decision-aid tool is introduced and tested on two case studies involving co-products. Results and discussion The resulting decision-aid tool helps guide the allocation choice in the two case studies through multi-level reasoning. Depending on the objective of the LCA study and the co-product being analyzed, a multi-level decision tree proposes a set of recommended allocation methods. The first level consists in identifying the purpose of the study, after which yes-or-no questions start to guide the path toward the right allocation method. In the most complex cases, the LCA practitioner is presented with a table containing all the allocation methods defended by other LCA practitioners for specific industries and contexts. This decision-aid tool complements the guidelines presented by ISO 14044 2006. Conclusion The presented tool creates a strong correlation between the aims of the study and the allocation choice, thus allowing practitioners to both justify their choice and to discuss the results with alternative scenarios for sensitivity analysis.