Architectural energetics is a methodology that translates architectural objects into a quantitative time-labor equivalent, from which information about past societies, labor organizations, or political relations can be inferred. Preceding such study, the volume of every architectural structure must be determined. This is usually done by in situ measurements and computing of volume by mathematical formulae or using UAV-based photogrammetry processed into digital surface model. However, both of these methods are impracticable in the case of buried or semi-buried monuments where the only remaining option is direct excavation. Hence, we introduce a new method for the determination of volumetric information based on the electrical resistivity tomography (ERT) geophysical method. We conducted our study at defensive lines (ramparts/ditches) within two hillforts of different ages, constructed from different building materials, in the Czech Republic. ERT surveys performed in 3D can differentiate ramparts/ditches in detail from the surrounding environment based on resistivity. Compared to previous excavations, the 3D inversion results show that ERT can obtain meaningful volumes based on the chosen resistivity threshold. The best results were achieved on homogeneous semi-buried ramparts and the ditch. ERT can be performed at a fraction of the cost of direct excavation. This method also leaves an intact site for future generations.