The glazed surface throughout the building envelope influences the energy demand of a building. In this paper, we propose the window-to-volume ratio (WVR) indicator, in contrast with the window-to-wall ratio (WWR) indicator, to evaluate the energy impact of glazing on the building envelope. The WVR indicator relates the glazed surface on facades to the total surface area of the built envelope. We consider the total built envelope as the sum of the facades, roof, surfaces in contact with other buildings, and surfaces in contact with the ground. We compared the WVR and WWR indicators for twelve existing buildings in different parts of Barcelona, a city with a Mediterranean climate. Case studies of residential buildings have volumes less than 5,000 m3, and office buildings have volumes between 30,000 m3 and 150,000 m3. The ARCHISUN software was used to simulate the annual energy demand (kWh/year) of each case study. We analyzed the indicator’s values in relation to the simulated annual energy demands. The results showed a higher correlation between WVR values and the simulated energy demand of almost all buildings, especially those with larger volumes. The WVR indicator could be used as a tool to pre-size the energy demand of buildings.