Methods, systems and apparatus for determining stowability based on item-size categories are described. Embodiments may select a storage module for a stow operation based on accessing stowability information. Stowability information, in various embodiments may include stowability information for each storage module in a materials handling system. Stowability information may indicate a capacity to hold additional items in a storage module. The capacity may be determined according an item-size category descriptor, describing the quantity of items in each category of a storage module, and a storage module capacity module characterizing item descriptors of fully-stowed storage modules. In some embodiments the storage module capacity model is a function fitted to data points representing the values of item-size descriptors of fully-stowed storage modules. Embodiments may update the stowability information for a storage module in response to the removal of an item from the storage module.