If you want to add a new duct run in your home, you need to calculate the size of the duct needed to supply that room. The duct size is dependent on the CFM that you will be supplying that room. CFM stands for cubic feet per minute. If a room is supplied with 100 CFM, that means there will be 100 cubic feet of air dispensed into that room each minute. Duct runs that are too small will not supply enough air and will have increased velocity, which leads to noise. Duct runs that are too big will have low velocity. This leads to a significant temperature drop before the air can make it to the room.
Calculate the square footage of the room to which you will be supplying air. Measure the width and length and multiply these together.
Plan on supplying 1 CFM per square foot. This amount is generally sufficient to keep a room comfortable. So if your room is 200 square feet, you will want 200 CFM.
Use a duct calculator to find the correct duct size. Use either an online calculator (see Resources) or a manual one that you can purchase from an HVAC distributor. Using 0.1 inches water column per 100 linear feet of duct as a friction rate, which is standard for supply air ducts, and using the example from before, you will see that for 200 CFM you need a 8-inch diameter duct.