Mechanical compliance is important for a robust and safe physical interaction of robots with humans and unstructured environments. Using adjustable stiffness, the advantages of compliant and stiff systems can be combined and thus the versatility of a robot increased. The realisation of adjustable stiffness in robot joints through compliant mechanisms shows several advantages over active control approaches, especially in terms of robustness. The compactness of adjustable stiffness mechanisms (ASMs) is important for their integration in robotic systems. An important aspect of compactness in ASMs is the storable work per volume, i.e. the work density. Therefore we propose a set of benchmarks to analyse the work density of elastic mechanisms on different design levels. The application of these benchmarks is demonstrated on a novel ASM, which is part of the adjustable impedance element AIE Uno, and on DLR's FSJ. The analysis of these ASMs demonstrates the application and the benefit of the proposed benchmarks. The benchmarks support the choice between alternative solutions and the identification of improvement potential in an existing design.