Abstract Freshwater ecosystems provide essential services for human well-being, with their conservation success reliant on the precise quantification of biodiversity. Diplostraca (= Cladocera) and Copepoda are diverse groups of microcrustaceans in aquatic ecosystems, important for a multitude of these services. We examined biodiversity trends of these groups, utilizing a comprehensive dataset, approximately 2341 studies, spanning from the early 19th Century to the present day, aiming at predicting the total number of microcrustacean species that will have been described by 2100. Descriptions increased until the 1900s, surged, and then declined after 1975. The predictive models indicate that by the year 2100, an additional 16–68% of cladoceran species and 37–126% of copepod species are likely to be identified, representing a significant portion of currently unrealised biodiversity. Eighteen prolific authors contributed to a quarter of these descriptions, with the rest spread among numerous authorities. Based on our extrapolation, the total number of microcrustacean species could potentially exceed 6114 by 2050 and 9046 by 2100, notwithstanding extinctions. These findings, which point to a significant amount of unrealised biodiversity, underscore the need to refine biodiversity estimates beyond conventional expert opinion. Such accuracy is crucial for addressing the underappreciated scale of the current biodiversity crisis.