A simple interpolation formula for the evaluation of physical properties of polypeptides as a function of the number of the amino acid residues is suggested. The formula is applied to a model calculation of the band width and total π-electron energy of the exciton states of a finite helical polypeptide as well as to the interpolation of the binding energy and dipole moment known from the peptide quantum chemical analysis. The number of the amino acids necessary to achieve 10%, respectively 1% convergence of the interpolated quantities to their limit values is also calculated. Finally, the formula is used to discuss the increasing experimental tendency to take on the α-helical conformation in a series of the sequential peptides (L-lysyl-L-alanyl-L-alanine) n with their i