DuctRule: HVAC Duct Sizing

The ductulator in your van is two cardboard discs and a rivet. This is that tool, on your phone, with the arithmetic done properly. Turn the outer ring to the airflow you have. Turn the blue wheel to the friction rate you are designing to. The answer sits in the middle, under the hairline: round diameter, air velocity, and the next stock size up. Up, never down, because a duct one size over costs a little sheet metal and a duct one size under costs the job. The velocity is not decoration. The ring around the answer runs green while the air stays inside the range for the run you picked, and turns amber the moment it leaves. Supply trunk, branch and return have different ranges, and you choose which one you are sizing. A duct that is slightly too small is quiet on paper and loud in the room. Under the dial are the rectangular sizes of equal friction, each with the velocity it produces, so…