This work aims to provide oyster farmers a tool to estimate the stock biomass to hold in their nursery. Herein, a steady-state single-compartment mass balance model, which includes the feeding activity of oyster spat, was developed. This model applies to nurseries such as floating upweller systems or land-based tanks and estimates (1) the optimal stock as a function of external food concentration or (2) the food concentration required for a given stock. The model was implemented for the Pacific oyster (Crassostrea gigas Thunberg) and is available online:  http://seaplusplus4.com/oysterspatbud.html. The model was evaluated using published data and further tested by simulating a general rule of thumb regarding the spat-holding capacity for a given nursery. According to a general rule of thumb, 1 hectare of a shallow pond can hold between 1 and 3 tons of spat. The model allows to further specify the rule depending on the spat grade to stock. If the spat is around 0.38 g, the model estimates a holding stock within the range of the rule of thumb; however, if the spat is around 0.04 g, the biomass stock sustained is lower (between 0.7 and 2 tons).