Functional Unit (FU) ensures the consideration of comparable product quantities to provide reliable Life Cycle Assessment (LCA) results. Although the definition of this FU is essential, it receives only a few attention in the normative texts. A high part of subjectivity is let to the LCA practitioner. In this paper variability sources of the FU definition are identified to propose a more structured and adapted approach. Literature references and data collected among 8 LCA experts on 5 case studies allow us to draw first recommendations towards a more structured FU definition framework.