To determine the size ducting you need for your worksite, you need to consider your HVAC system’s airflow (in cubic feet per minute/CFM), square footage and friction loss rate. To calculate the CFM, multiply the HVAC unit’s size (in tons) by 400, and then divide that number by your location’s square footage. Discuss this information with a United Rentals representative to determine what size duct you will need for your worksite.