Measuring the volume of a material compressed quasi-statically to a high-pressure typically involves the determination of a primitive crystal cell volume using x-ray diffraction (XRD) techniques. The accumulation of pressure dependent volumes leads to an understanding of the material's equation of state, (EOS); however, in the case of low-symmetry (e.g. triclinic) materials with twining features, and large primitive cells, conventional XRD approaches can be technically problematic and lead to ambiguous or mathematically under-determined lattice constants. To resolve this long-standing issue, we examined the utility of a "direct" approach toward determining a material's volume by measuring surface area and sample height using optical microscopy and interferometry respectively. To test the validity of our proposed approach, we first compared our results from pressurized Triamino-Trinitrobenzene (TATB, SG P-1) with a published EOS determined by synchrotron XRD measurements of Stevens et al. A near-perfect match between the two sets of V(P, 300K) data is observed; and thus, we proceeded to determine the EOS of the energetic material 5-nitro-2,4-dihydro-1,2,4,-triazol-3-one (α-NTO), which under ambient conditions crystallizes as a fourcomponent twinned system with a triclinic symmetry (SG P-1). No high-pressure XRD EOS data have been published on α-NTO, probably due to its unambiguously complex crystal structure. The results of this study reveal that our proposed approach applied to quasi-statically compressed anisotropic materials is a reliable alternative toward determining material EOSs,-especially when conventional methodologies are not feasible.