Measuring for air conditioning ductwork is important if you want your system to operate properly. An undersized duct opening will cause the system to work too hard to push through the air, and an oversize duct will create waste. The proper size is going to depend on several factors, including the size of the home and the type of air conditioner you have.
Step 1- Find the CFM
CFM, or cubic feet per minute, can be found by measuring the square footage in each room. In most cases you can figure on 1 CFM per each square foot. If you have very large windows or are measuring a sunroom you will want to use 2 CFMs per square foot.
Step 2- Use a Duct Calculator
Once you have the number of CFMs needed for each room you can find a duct calculator. You will simply enter the CFMs per room in to this tool and it will give you the best size for a duct opening in each room. You can find these calculators by searching online.
Step 3- Add Them Up for the Trunk Size
Once you have the measurements for all the rooms, you can work on finding the size of the main trunk duct. To do this you need to add all the CFMs needed for every room, and enter this number in to the calculator to come up with the right size.