Controlling the grain size in electrodeposited coatings for the prevention of corrosion is highly important. To understand the relationship with grain size and electrochemical performance many experiments need to be undertaken to vary the grain size of the deposit. In the present work the (crystallite) grain size of electrodeposited Ni coatings formed in the presence of metalloids such as boron (B), sulphur (S) and phosphorus (P) was estimated from analysing mass transfer at the cathode-electrolyte interface. A mathematical model has been proposed which indicates that the grain size of the deposit is directly proportional to current efficiency and the deposition rate while being inversely proportional to the current density and metalloid (B, S, P) content in the coatings. A simple relationship is developed which is in agreement with experimental data and data that is reported in the literature. The development of such a model should significantly decrease the amount of experimentation required to achieve the desired grain size in such systems (Ni-B, Ni-S, Ni-P coatings) obtained by electrodeposition.