Plastic pollution originates from various sources. While many industries are taking steps to reduce their dependence on plastic, there is currently no reliable methodology to forecast specifically the extent of the marine plastic leakage from an industry or a country and map potential leakage hotspots throughout the value chain. This report offers, for the first time, a comprehensive framework to measure the inventory of marine plastic leakage, step-by-step and using a life-cycle perspective. It also offers generic data that can be used to calculate marine plastic leakage for a defined list of identified sources, including plastic waste, textile fibres, tyre dust, micro beads in cosmetics, and fishing nets. The approach discussed in this report focuses on inventorying plastic flows into the marine environment only. It does not look at plastic releases into the terrestrial environment, or any ecosystem or human health impacts that may result from marine plastic leakage.