Geochemical measurements have been widely used for understanding geophysical dynamic systems. Fluid mechanics and box models are quantitative tools for testing the reliability of these interpretations. We present here the connection between these two methods, especially in the case of the Earth's mantle. We show that box models implicitly assume a chemical diffusivity inversely proportional to the number of boxes. From fluid dynamics considerations we suggest that at least 15 boxes should be used to model the mantle. Then, we compare the results of a simple convective geochemical model with box models to illustrate a way to incorporate dynamical constraints in them. ¿ 2000 American Geophysical Union