How many CFM does a 300 sq ft bathroom need?

300 ft² bathroom

300CFM

3 supply registers at 100 CFM each, fed by 10″ duct

Airflow needed300 ft² × 1 CFM per ft² for a bathroom	300CFM
Room factorsupply air is separate from the exhaust fan — both are needed	1CFM/ft²
Supply registersabout 120 CFM each before they start to whistle	3registers
Branch ductround, at 600 fpm — the quiet band for a branch	10″
Share of a 3-ton systema 3-ton system moves about 1,200 CFM in total	25%
Bedroom comparisonthe same 300 ft² as a bedroom	270CFM

Notes

**Split by load, not by area.** Two rooms of 300 ft² do not need the same air if one faces west behind glass and the other sits in the middle of the house. A bathroom runs at 1 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 300 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 300 CFM the duct is delivering to the boot.
**10″ 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.

A 300 sq ft bathroom

300 CFM, delivered through 3 supply register3 on a 10″ branch duct.

Split by load, not by area

Two rooms of 300 sq ft do not need the same air if one faces west behind glass and the other sits in the middle of the house.

A bathroom runs at 1 CFM per square foot against 0.9 for a bedroom. Dividing a system evenly across rooms is the single most common reason one room is always uncomfortable.

Why more than one register

Around 120 CFM is where a residential supply register starts to whistle.

Pushing all 300 CFM through one boot buys noise and a draught in one part of the room while the far corner stays still.

A closed door turns a room 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 slows, and it ends up short of the air the duct is actually delivering to the boot. This is why bedrooms are the rooms people complain about.

The branch

10″ at 600 fpm — the quiet band. Smaller is cheaper and noisier; bigger is quieter and eats ceiling.

If the run ends in flex rather than metal, size it as flex. The corrugated wall costs real static pressure, and flex that is not pulled tight costs more again.

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.