Properly sizing and installing your air conditioning system are important in making sure your air conditioner operates efficiently, according to the U.S. Department of Energy. One thing to remember is that you must have enough supply vents (vents blowing air out) and return vents (vent sucking warm air back in the air conditioner). Also, improper duct installation will impact your air conditioner's efficiency and your electric bill.
Look for the data tag on the outside of your condenser (outside unit) to determine the size of your air conditioner (example: 3 ton).
Decide if you'll be using round or rectangular duct.
Calculate the return duct size based on the air conditioner's size and the type of duct you are using. Use the following formulas:
For rectangular duct: Size of air conditioner (example: 4 tons) x 144 square inches = return air duct in square inches
For round duct: Diameter of duct x 3.14 = return air duct in square inches
Calculate the square root of the total. (Example: 4 (ton unit) x 144 square inches = 576 squared. The square root of 576 is 24. Therefore, your return air duct and grill size will be 24 by 24 inches.)