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Optimizing the structural reliability of bulk metallic glass (BMG) components demands a detailed understanding of the atomic structure of the glass, particularly the defects that control plastic flow. These defects are thought to be associated with regions of low atomic density, which facilitate the required diffusion-like atomic rearrangement processes. In the present article, the distribution of low-density regions in a simulated Cu-Zr glass is studied with two different techniques. Using a hard-sphere model, the interstitial volume distribution was obtained by constructing Voronoi polyhedra around each atom and inserting spheres into the unoccupied regions at the vertices. The volumes of touching spheres were summed and corrected for any overlap to obtain the size distribution of the unoccupied sites. The resulting distribution is in good agreement with Cohen and Turnbull's free volume model and provides insight into how a single free volume site may be described. However, this model depends significantly on the somewhat arbitrary selection of the hard-sphere atomic radii and may not give a realistic indication of the shape or connectivity of the low atomic-density regions. Recent experimental studies of the open volume distribution using positron annihilation spectroscopy probe the electron and not the atomic density. We therefore propose a novel method to identify lowdensity regions from ab initio calculated radially averaged electron-density distributions, which allows a more physical and less ambiguous identification of low-density areas and, at the same time, connects atomic and electron distributions. Our results show that the qualitative volume distribution from the electron-density model agrees well with the hard-sphere model, while allowing a more physical quantitative analysis.