The kinetics and thermodynamics of the nucleation of magnetite crystals from primary particles are shown to be described by colloidal assembly theory, allowing for predictions of crystal sizes to be made. The nucleation of crystals has long been thought to occur through the stochastic association of ions, atoms or molecules to form critical nuclei, which will later grow out to crystals(1). Only in the past decade has the awareness grown that crystallization can also proceed through the assembly of different types of building blocks(2,3), including amorphous precursors(4), primary particles(5), prenucleation species(6,7), dense liquid droplets(8,9) or nanocrystals(10). However, the forces that control these alternative pathways are still poorly understood. Here, we investigate the crystallization of magnetite (Fe3O4) through the formation and aggregation of primary particles and show that both the thermodynamics and the kinetics of the process can be described in terms of colloidal assembly. This model allows predicting the average crystal size at a given initial Fe concentration, thereby opening the way to the design of crystals with predefined sizes and properties.