A computer program was written to calculate and display graphically the van der Waals volume (Vw) and surface area (Sw) of molecules. Using van der Waals radii of C, H, Cl, and Br, and geometries optimised by the force-field method, Vw and Sw were obtained for about a hundred simple molecules: alkanes, cycloalkanes, and chlorinated and brominated derivatives thereof. Sw is more sensitive than Vw to stereochemical and positional isomerism. It was found that Vw correlates with the molar refraction and with the constant b in the equation of state, and can be used to estimate the molar volume. For very similar species, Sw correlates with the free volume that encapsulates the molecule in the bulk, so that a combined usage of Vw and Sw leads to the difference in molar volume between conformers. Numbers obtained by the procedure bear upon the ‘conformational rule’ of organic stereochemistry, and suggest a novel interpretation of the torsional term in the molecular-mechanical force field.