The ratio of chlorophyll a per cell and carbon per cell is considered to be a key quantity in phytoplankton growth. The quantity varies with nutrient and light availability. The aim of this paper is to predict how the chlorophyll a to carbon ratio varies in relation to environmental conditions. A cell-based model is presented, that allows the description of carbon and chlorophyll a celt quota, and thus the chlorophyll a to carbon ratio, in nutrient-limited as well as light-limited growth. The model predictions are in line with experimental data. Variations in Chl:C result, among others, from photoacclimation. The analysis in this paper shows that the impact photoacclimation on phytoplankton growth is rather small, despite large variations in Chl:C. This is mainly due to the package effect, a phenomenon that can be easily accounted for especially in cell-based models.