We define two parameters, Pi(ice) and Pi(parent), estimated from hydrate and parent volumes, to be a measure of the efficiency of the packing of molecules in a hydrate compared on the one hand to that of idealized solid ice and on the other to the parent molecule from which the hydrate is derived. The packing efficiency ratio, Pi(ice)[M(p)center dot X(q)center dot nH(2)O], is defined for a general hydrate M(p)center dot X(q)center dot nH(2)O containing p (complex) cations M(q+) and q (complex) anions X(p-) by Pi(ice)[hydrate] = [V(at)(parent) - V(at)(hydrate)/V(at)(Parent) - 0.0082] where V(at)(parent) and V(at)(hydrate) are the average volumes occupied per atom in the parent and hydrate structures, respectively, and 0.0082 corresponds to the average volume per atom in ice. All volumes are measured in units of nm(3). Pi(par)[hydrate] +Pi(ice)[hydrate] = 1. Two simpler indices are studied, Pi and Pi*, and we explore the use of these as a means of categorizing hydrates and defining their domain of existence.