Airflow per Room

How much air a room needs, and how to check what it is getting.

How many CFM does a 180 sq ft kitchen need?

180 ft² kitchen

252CFM

3 supply registers at 84 CFM each, fed by 9″ duct

Airflow needed180 ft² × 1.4 CFM per ft² for a kitchen	252CFM
Room factorappliances add heat the floor area cannot see	1.4CFM/ft²
Supply registersabout 120 CFM each before they start to whistle	3registers
Branch ductround, at 600 fpm — the quiet band for a branch	9″
Share of a 3-ton systema 3-ton system moves about 1,200 CFM in total	21%
Bedroom comparisonthe same 180 ft² as a bedroom	162CFM

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.

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