Air-duct calculators provide a quick and accurate way to size ducts based on CFM or FPM, ensuring that the ductwork is properly sized for the HVAC system. This helps to prevent issues such as inadequate airflow, increased pressure drops, and reduced system efficiency. By using air-duct calculators, designers and engineers can also easily convert between round and square duct shapes, making it easier to design and install ductwork that meets the specific requirements of a building or application.