Airflow per Room

Notes

• Split by load, not by area. Two rooms of 180 ft² do not need the same air if one faces west behind glass and the other sits in the middle of the house. A kitchen runs at 1.4 CFM per square foot against 0.9 for a bedroom — dividing the system evenly across rooms is the single most common reason one room is always uncomfortable.
• 3 registers, not one big one. Around 120 CFM is where a residential supply register starts to whistle. Pushing 252 CFM through a single boot buys noise and a draught in one part of the room while the far corner stays still.
• A closed door turns a bedroom into a pressurised box. Air pushed in has to get out. Without a return, a jumper duct or an undercut door, the room pressurises, the supply air slows, and the room ends up short of the 252 CFM the duct is delivering to the boot.
• 9″ branch at 600 fpm. Smaller is cheaper and noisier; bigger is quieter and eats ceiling. If this run ends in flex rather than metal, size it as flex — the corrugated wall costs real static pressure.
• This is a check, not a design. The right number comes from a room-by-room Manual J, which accounts for window area, orientation, insulation and infiltration rather than a factor per square foot. Use this to sanity-check a drawing or a quote — a contractor who cannot show a room-by-room load is guessing too, just less transparently.