Cities are complex systems of accumulated matter. The continuous process of matter accumulation in urban systems differs in intensity across the globe according to specific urban features, such as location and age of the urban tissue, and as a physical manifestation of metabolic lineaments, material accumulation should differ amongst cities. In this paper, a new indicator to measure this process of material accumulation is proposed, namely, the Technomass. Emulating ecology, which measures biomass in natural ecosystems, a sample of different urban tissues in a given city - Bogotá - was measured in terms of volume and rates of matter accumulation. Technomass is able to indicate overall asymptotic behaviour, specific spatial profiles and intensification of rates in time. In metabolic terms, the indicator looks into the black box, providing the possibility to link metabolic behaviours with urban form and attempting to fill the gap between urban planning, urban metabolism (UM) and Material Flow Analysis (MFA). This new indicator offers a broad scope of applications. Further possibilities and links to urban research and policy making are explored in the discussion section.