A product life cycle inventory (LCI) is done by modelling the reality in a flow diagram or map of processes. The map contains simple tree-structures and eventually networks with sophisticated recycling loops. The unit processes are scaled to 1 unit of a selected input or output for better understanding. The map determines the demand of intermediate products of the various unit processes in the whole system. When performing the balance of the map, the unit processes are scaled in such a way that the map complies with the rules and conventions of mapping, e.g. the delivered product quantity of one process should be equal to the amount received by the other process. The final map balance is the vector sum of all scaled unit processes.