The total airflow a system moves is set by the equipment. How that air is divided between rooms is set by the ductwork, and it should follow each room's share of the load.
The calculation
room CFM = total system CFM × (room load ÷ total house load)
A room representing 12% of the cooling load gets 12% of the air.
Total system CFM comes from the equipment — roughly 400 CFM per ton for cooling, adjusted for climate — and the room loads come from a room-by-room Manual J.
Why floor area is the wrong divisor
Two rooms of identical size can have very different loads.
A south-west corner bedroom with two large windows, a room over a garage, and an interior hallway all have different heat gains per square foot. Dividing air by area gives the corner bedroom the same as the hallway, and the corner bedroom is then the room everybody complains about.
This is why a house can have a correctly sized system and one room that is never right.
Heating and cooling do not want the same split
A room's share of the cooling load is driven by solar gain and by west and south glass.
Its share of the heating load is driven by exterior surface area and infiltration, with no solar term.
Those produce different percentages. Manual D resolves it by sizing the duct for the larger of the two requirements and balancing with dampers, which is why a system with balancing dampers can be adjusted seasonally and one without cannot.
Rules of thumb, and what they are worth
Around 1 CFM per square foot is the figure most often quoted for a typical room in a mixed climate.
It is a sanity check rather than a design. A bedroom of 150 square feet getting 40 CFM is plausible; the same room getting 15 is not, and that is worth knowing before measuring anything.
Return air
Supply air has to leave the room. If it cannot, the room pressurises and the supply air stops entering.
A closed bedroom door with a supply register and no return path is the classic case: the room goes positive, air is forced out through the envelope, and the rest of the house goes negative and draws air in from outside.
The remedies are a return in the room, a transfer grille, a jump duct through the ceiling, or a substantial undercut on the door. The undercut alone is usually not enough for a room with meaningful airflow.
Measuring what a room actually gets
A flow hood over the register reads CFM directly and is the definitive method.
An anemometer across the register face, multiplied by the free area, is an approximation and good enough to find a room that is far short.
The useful comparison is the measured figure against the calculated requirement, room by room. That table is what a balancing report should contain, and it almost never exists in residential work.
Balancing
Dampers at the takeoffs, adjusted so each room gets its calculated share. Set them, mark them, and record the positions.
Closing the registers in a room is not balancing. It raises system static pressure, reduces total airflow, and moves the problem somewhere else. Dampers at the branch do the same job without the penalty, because they are part of the design rather than a restriction at the end of it.
Ventilation is a separate requirement
The airflow calculated here is for heating and cooling. Fresh air ventilation is a different quantity, set by standards based on floor area and occupancy, and it is delivered by its own provision — an ERV, an HRV, or a controlled outdoor air inlet.
Assuming the heating and cooling airflow provides ventilation is a common confusion. A recirculating system moves house air around; it does not bring any in.