How many CFM does a 100 sq ft basement need?

100 ft² basement

70CFM

1 supply register at 70 CFM each, fed by 5″ duct

Airflow needed100 ft² × 0.7 CFM per ft² for a basement	70CFM
Room factorground contact moderates the load, so it needs less than the area suggests	0.7CFM/ft²
Supply registersabout 120 CFM each before they start to whistle	1registers
Branch ductround, at 600 fpm — the quiet band for a branch	5″
Share of a 3-ton systema 3-ton system moves about 1,200 CFM in total	6%
Bedroom comparisonthe same 100 ft² as a bedroom	90CFM

Notes

**Split by load, not by area.** Two rooms of 100 ft² do not need the same air if one faces west behind glass and the other sits in the middle of the house. A basement runs at 0.7 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.
**1 register, not one big one.** Around 120 CFM is where a residential supply register starts to whistle. Pushing 70 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 70 CFM the duct is delivering to the boot.
**5″ 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 100 sq ft basement

70 CFM, delivered through 1 supply register1 on a 5″ branch duct.

Split by load, not by area

Two rooms of 100 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 basement runs at 0.7 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 70 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

5″ 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.