The objective of this toolkit is for the UN member States to identify sources of plastic pollution and quantify the amount which will eventually enter the aquatic environment. The toolkit is not intended to harmonize existing models or methodologies but intended to use the results of application of such models/methodologies to obtain simple estimation formulas to be converted into empirical model (possibly supported by Plastic Leakage Factors/Coefficients), allowing the estimation of plastics entering the aquatic environment.