Determining the degree of degradation is an important means of assessing the efficiency of biological processes. However, one should consider the fact that during degradation, the reference value, such as volume or the mass of total solids, is also subject to change. The assumption that the incoming and outgoing mass flows are identical is only possible for substrates with a high water content and hence, a low energy density. For substrates with a higher energy density, a correction by the gaseous mass flow is required, but usually its quantification is difficult, especially when examining full-scale plants or open systems. Based on the assumption that the mass of inorganic solids is constant during the process, a universally applicable equation has been developed, requiring only the input and output volatile solids concentrations for calculation. (C) 2015 Elsevier Ltd. All rights reserved.