To get the optimal amount of air from your heating and air conditioning system, you need to have the right size of ducts. There are three different ways to calculate duct size, known as the velocity method, constant pressure loss method, and static pressure recovery method. These are all very complex methods and usually should be conducted by a professional.
The velocity method uses equations to figure out how much air flows through the ducts at a certain speed to figure out what the best size would be for the ducts. Constant pressure loss focuses on the pressure loss of the duct when air flows through it. Static pressure recovery focuses on the pressure recovery of the duct’s vents and inlets.
A rule of thumb says ducts should carry 400 cubic feet per minute (CFM) of air for every one ton of cooling. So if you have a two-ton system, you need ductwork that will carry 800 CFM. One six-inch round duct is good for about 120 CFM, a six-inch duct carries about 150 CFM, and eight-inch carries about 200 CFM of cool air. Also, you want to make sure that the return ducts are the same size as the supply ducts. In adjusting your system, try not to damper down the system any more than needed because reduced airflow could cause the indoor coil to freeze.