For the first time we publish a taxonomically detailed and robust dataset of biomass conversion factors for macro-zoobenthos, often required in many studies. Georeferenced raw data for 497 taxa empower the user to make the best selections for combining them with their own data, and aggregation can help to quantify natural variability and uncertainty and refine current ecological theory. Standardised measurements were done on material collected for over 2 decades in the Baltic and the North seas.