Fan size is expressed as CFM capacity and is determined by dividing the area of a room by the number of minutes per hour needed to replace the volume of air. CFM indicates cubic feet per minute and indicates the amount of circulation necessary to properly ventilate a room.
Proper ventilation is required to prevent the accumulation of humidity and moisture that leads to molds and fungi. Various rooms have different ventilation and fan size requirements. For example, a bathroom fan must generally replace the air eight times per hour, or every 7.5 minutes, while a greenhouse or growing chamber fan may need to replace the air every minute. Dividing the area of the room or growing area by the rate of air replacement yields the CFM capacity or size of the fan required to properly ventilate the area.
This is a simplified model that fails to consider several real-world considerations. In actual practice, factors such as the length and number of folds in duct work, the use of filters and the inflow of fresh air affect the actual CFM capacity and fan size.