The biology literature is rife with misleading information on how to quantify catabolic reaction energetics. The principal misconception is that the sign and value of the standard Gibbs energy (ΔGr0) define the direction and energy yield of a reaction; they do not. ΔGr0 is one part of the actual Gibbs energy of a reaction (ΔGr), with a second part accounting for deviations from the standard composition. It is also frequently assumed that ΔGr0 applies only to 25 °C and 1 bar; it does not. ΔGr0 is a function of temperature and pressure. Here, we review how to determine ΔGr as a function of temperature, pressure and chemical composition for microbial catabolic reactions, including a discussion of the effects of ionic strength on ΔGr and highlighting the large effects when multi‐valent ions are part of the reaction. We also calculate ΔGr for five example catabolisms at specific environmental conditions: aerobic respiration of glucose in freshwater, anaerobic respiration of acetate in marine sediment, hydrogenotrophic methanogenesis in a laboratory batch reactor, anaerobic ammonia oxidation in a wastewater reactor and aerobic pyrite oxidation in acid mine drainage. These examples serve as templates to determine the energy yields of other catabolic reactions at environmentally relevant conditions.