The production and use of plastics have increased exponentially in the past decades, leading to a corresponding increment in plastics waste generation. Globally, a significant portion of the generated plastics waste ends up in landfills due to limited management schemes for plastics. These landfills have been recognized as a major source of plastics losses to the environment, where plastics can then be transported and distributed across different environmental domains and pose environmental problems. To gauge and address these problems, the monitoring and quantification of plastics losses and the pathways through which it happens is necessary. Yet, no study has quantified the plastics losses from landfills and open dumps in a consistent and comprehensive manner. Here, we propose a conceptual framework for quantitatively estimating plastics losses from landfills and open dumps through all possible pathways. These pathways cover (i) environmental processes, including the effects of wind, flooding, precipitation; (ii) the influence of biota, covering removal by animals; and (iii) relevant anthropogenic causes, representing influence of ragpickers. For each of these pathways, we review existing knowledge pertaining to plastics losses, and we provide recommendations for rigorous mathematical methods and models to estimate such losses. This framework is an important stepping stone in the evaluation of plastics pollution by enabling comprehensive assessments of plastics losses to the environment from landfills and open dumps.