This document specifies a calculation method for the determination of the biobased carbon content in monomers, polymers, and plastic materials and products, based on the 14C content measurement. This document is applicable to plastic products and plastic materials (e.g. plasticisers or modifiers), polymer resins, monomers, or additives, which are made from biobased or fossil-based constituents. Knowing the biobased content of plastic products is useful when evaluating their environmental impact.