Supply Register Sizing Calculator
How many registers and what size for a room.
Enter your values
ⓘ Live results
Results
- Registers needed
- 2
- CFM per register
- 90
- Free area each
- 22 sq in
- Typical size
- 4 x 10 in nominal
- Throw guidance
- 9 ft approx
Quick answer
Supply registers are sized on face velocity, typically 500–700 FPM, with each register handling 100–150 CFM.
face area = CFM / face velocity
How to use the Supply Register Sizing Calculator
- 1Enter the airflow the room needs.
- 2Keep face velocity around 600 FPM to avoid whistling.
- 3Split into more registers when one would exceed about 150 CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Room airflow180 CFM
- Face velocity600 FPM
- Max CFM per register120 CFM
Results
- Registers needed2
- CFM per register90
- Free area each22 sq in
- Typical size4 x 10 in nominal
- Throw guidance9 ft approx
Frequently asked questions
Why does one vent whistle?
Too much CFM through too little free area — split the flow or fit a larger register.
Where should registers go?
Under windows or along exterior walls so the throw washes the coldest surfaces.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.