Laboratory experiments with iron offer important insight into the physiology of marine phytoplankton and the biogeochemical cycles they influence. These experiments often rely on chelators to buffer the concentration of available iron, but the buffer can fail when cell density increases, causing the concentration of that iron to drop rapidly. To more easily determine the point when the iron concentration falls, we developed an online calculator to estimate the maximum phytoplankton density that a growth medium can support. The results of the calculator were compared to the numerical simulations of a Fe-limited culture of the diatom Thalassiosira weissflogii (Grunow) Fryxell and Hasle. Modeling reveals that the assumptions behind thermodynamic estimates of unchelated Fe concentration can fail before easily perceptible changes in growth rate, potentially causing physiological changes that could alter the conclusions of culture experiments. The calculator is available at http://www.marsci.uga.edu/fidoplankter.