4'-Methyl-biphenyl-4-carbonitrile and the 4'-Methoxy-biphenyl-4-carbonitrile homologous series are known to exhibit calamitic (nematic and/or smectic) liquid crystal phases. In this work, aspect ratios x (length/diameter) for these molecules are computed from electronic structure calculations using ab initio HF/3-21G//PM3 level of theory/numerical precision. Aspect ratios were calculated from ovality O-v, defined as O-v =(A/4 pi)/(3V/4 pi)(2/3) , where V is the molecular volume and A the surface area. Derivations of thermotropic liquid crystal aspect ratios as a function of O-v, by this model are in agreement with observed values and permit an adequate description of the liquid crystal molecular geometry.