A Rigid Unit Packing Fraction (RUPF) Model Was Used to Better Understand the Influence of Local Structural Units on the Young's Elastic Modulus (E) of Binary and Ternary Tin Phosphate Glasses. Quantitative Analyses of the Units that Constitute the Glass Structure, Obtained from X-Ray/neutron Diffraction and 31P MAS-NMR Spectroscopy, Were Used to Calculate Polyhedral Packing Fractions that, with Tabulated Bond Dissociation Energies, Were Used to Predict E based on a Modification of the Makashima-Mackenzie Relationship, Which Uses Ion Sizes to Calculate Packing Fractions. Predictions based on the RUPF Model Are Better Than Those based on Ion Sizes, and Extending the RUPF Model to All Cation-Polyhedra Accounts for the Compositional Dependence of the Sn-Coordination Number.