International shipping plays a vital role in the world’s transport system and provide socio-economic benefits for the society. However, shipping is also an activity responsible for a range of emissions, e.g., greenhouse gases, air pollutants, and chemical substances, impacting air quality, human health, and the marine environment. In particular, the marine growth (biofouling) on underwater surfaces of a ship hull affects the ship’s energy efficiency, leading to propulsion powering penalties which can be translated to increased fuel consumption, air emissions and costs for the shipowner. Measures to control biofouling, using biocidal antifouling coatings and in-water hull cleaning, are however associated with impacts on the marine environment. Determining the quantities and types of emissions from a vessel is highly complex. These will depend on the selected hull coating and its associated maintenance, as well as engine specifics, fuel choice and types of exhaust abatement systems. The emissions will also be contingent on the vessel route, as the intensity of hull fouling varies geo-graphically (impacting air emissions and fuel consumption) while biocide emissions from antifouling coatings depend on salinity (impacting water emissions). As a result, optimum hull maintenance strategies to minimize environmental emissions need to be determined for specific vessels and routes. In this project report, we present a standalone cost calculation tool, HullMASTER (Hull MAintenance STrategies for Emission Reduction), specifically developed for this intricate task. In HullMASTER, developed models simulate the emissions to water and air associated with user-defined hull maintenance scenarios for ships and routes in the Baltic Sea region. A region-specific fouling growth model, based on field data from the Baltic Sea, has been implemented to predict the powering penalty of modelled vessels due to hull fouling. The model has been validated against inservice performance using data from 9 ships (40 vessel-years) operating in the Baltic Sea region with very good agreement. Based on the simulated emissions, HullMASTER calculates the differences in health- and environmental damage costs between scenarios. These are presented alongside the economic cost for operators (i.e., paint application and maintenance costs, increased energyconsumption costs due to biofouling and in-water cleaning costs) in order for shipowners to select the most sustainable hull maintenance strategies.