Accurate knowledge of the lattice parameters comprised in the four factor formula k∞=η∈Pi are required for optimum designing of reactors. In the special case of cluster fuel elements conventional handbook formulas are of little use. It is the intent of this paper to report the results of some recent improvements in the calculation of these parameters. By the addition of certain refinements in the calculation of these parameters. By the addition of certain refinements to the conventional calculational methods considerable success has been obtained in the correlation of theory with experiment.