A mass-balance calculation is used to modelt he release rate of biocide from antifouling paints in order to provide nominal values fort he cumulative release of biocide over the first 14 days of the coatings's specified lifetime (in-service-period), the averages rate of biocide release over the remainder of the coating lifetime, and the cumulative release of biocide over the entire lifetime. The method is applicable to any antifouling coating that contains a biocidal component and, where an antifouling paint contains more than one biocide, it allows the release rate of each biocide to be independently modelled. There are no minimum or maximum limiting values of release rate for which the use of this method can be used.