Grille Size Calculator
Enter the airflow in cfm, or the system size in tons, and choose a return, quiet or supply grille. The calculator gives the free area needed and the standard grille size to buy.
- Grille size
- Free area
- Face velocity
About 400 cfm of air moves for every ton of cooling.
Free area = airflow ÷ face velocity. A 3 ton system moves about 1,200 cfm. At 400 fpm a return grille needs 432 sq in of free area. With 70% free area that is a 617 sq in face, a 30 × 24 in grille.
Return grille size chart
400 fpm, 70% free area. Search for airflow.
| Airflow (cfm) | Tons | Free area (sq in) | Grille (in) |
|---|---|---|---|
| 200 | 0.5 | 72 | 12 × 12 |
| 400 | 1 | 144 | 16 × 16 |
| 600 | 1.5 | 216 | 20 × 20 |
| 800 | 2 | 288 | 24 × 18 |
| 1000 | 2.5 | 360 | 24 × 24 |
| 1200 | 3 | 432 | 30 × 24 |
| 1600 | 4 | 576 | 30 × 30 |
| 2000 | 5 | 720 | 36 × 30 |
Inches.
Airflow and grille
Return grille. Slide the airflow.
The grille grows with the air.
How to size an air grille
Airflow to size. Step through it.
Airflow
cfm.
Undersized return grilles whistle and make the system work harder.
Return, supply or transfer
Grille types. Compare them.
Back to system.
Into the room.
Room to room.
Return grilles move air back to the system and are bigger than supply grilles.
Before you buy
Grille checks. Tick each item.
Tick each item as you go.
Check the maker's free area figure for the exact grille.
What you get
Grille sizing. Each tile explains one result.
Inches.
Square inches.
cfm.
fpm.
Real velocity.
The grille's nominal size is the opening it fits, not its outer frame.
Frequently Asked Questions
No question matches that search.
What size return grille do I need?
Divide the airflow by four hundred, multiply by one hundred and forty four and divide by point seven.
What size return grille for a three ton system?
About thirty by twenty four inches.
What is free area on a grille?
The open part air passes through, often about seventy per cent of the face.
Why does my return grille whistle?
It is too small, so air rushes through too fast.
How fast should air move through a return grille?
About three hundred to four hundred feet a minute.
Are supply grilles smaller than returns?
Yes. Supply air can move faster without noise.
How much air does a ton of cooling move?
About four hundred cubic feet a minute.
Quiet air, easy breathing
Size the grille to the airflow and the system runs quietly and efficiently.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.