Because mass and can neither be created nor destroyed, only transformed, the physical inputs entering a system should be equal to the physical outputs from the system, taking into account any inventory changes within the system.
Thus, in order to ensure that all of the materials subject to the MFCA analysis are accounted for material balance should be performed, comparing the quantities of material inputs to outputs (i.e. products and material losses) and changes in inventory to identify any significant "missing" materials or other data gaps. Quantification of material flows and the assurance of a balance between material inputs and outputs products and material losses) are both essential requirements for MFCA