In this study, we define the relationship between the FAS values of mixtures and their energy consumption in the composting process. Composting is the process of decomposing organic wastes by microorganisms under aerobic conditions. The resulting compost can be used in agriculture as a soil conditioner or manure to prevent erosion, and in forests and landscaping. The composting process requires external energy to provide adequate aerobic conditions. The largest use of energy in the composting process is for fans that blow air to the piles. The FAS value indicates the air-filled porosity in materials to be composted. A predetermined optimal FAS value should be ensured for oxygen supply to the composting mixtures during the composting process and to decrease the amount of the energy consumed by the fans. An increase in the FAS value decreases the air resistance in compost masses. While very high FAS values cause cooling of the masses by increasing heat transfer, very low FAS values increase the air transition resistance and causes a failure in the ventilation. Determining the optimal FAS value for the composting process increases its success and decreases energy consumption, and thus contributes to its environmental and economic sustainability.