Return Air Grille Sizing Guide: CFM Chart & Calculation


There are many different sizes available for a return air grille. Using the correct return air grille size is important to ensure that the HVAC system has sufficient airflow as well as not creating a loud noise. So, I created several CFM charts and a guide on how to calculate return air grille size.

To calculate return air grille size, take the CFM of the HVAC unit and divide it by 350 to get the grille area in square feet. Next, multiply the grille area by 144 to convert it into square inches. Finally, choose your preferred grille size that matches the required grille area.

Alternatively, you can use the following CFM charts to quickly find the grille size based on the CFM or airflow of the HVAC unit. Parameters used for the following charts are as below:

  • Face Velocity = 500 feet per minute (fpm) or 2.5 meter per second (m/s)
  • Free Area = 70%

Face velocity is the amount of air volume passing through a grille. Typically, 500 fpm is optimal and balanced between air volume and noise.

Free area is the amount of space on a grille available for the air to pass through. It’s depending on the type of grille and the manufacturer of the grille. However, many grilles have about 80% free area while some external grilles have 50% free area. Hence, I use a 70% free area.

CFM Chart for HORIZONTAL Return Air Grilles

Grille Area (sq.in)Grille Size (in)Airflow (cfm)
244 x 658
484 x 12117
606 x 10146
1806 x 30438
1288 x 16311
1448 x 18350
1928 x 24467
2408 x 30583
2568 x 32622
20010 x 20486
22010 x 22535
24010 x 24583
30010 x 30729
21612 x 18525
24012 x 20583
28812 x 24700
36012 x 30875
23814 x 17578
28014 x 20681
35014 x 25851
36414 x 26885
28816 x 18700
32016 x 20778
40016 x 25972
41616 x 261011
43218 x 241050
46818 x 261138
54018 x 301313
57618 x 321400
48020 x 241167
50020 x 251215
60020 x 301458
Horizontal Grille Selection Table

CFM Chart for SQUARE Return Air Grilles

Grille Area (sq.in)Grille Size (in)Airflow (cfm)
366 x 688
648 x 8156
10010 x 10243
14412 x 12350
19614 x 14476
25616 x 16622
32418 x 18788
40020 x 20972
48422 x 221176
57624 x 241400
67626 x 261643
90030 x 302188
Square Grille Selection Table

CFM Chart for VERTICAL Return Air Grilles

Grille Area (sq.in)Grille Size (in)Airflow (cfm)
4812 x 4117
7212 x 6175
12012 x 10292
8414 x 6204
11214 x 8272
14014 x 10340
9616 x 6233
12816 x 8311
19216 x 12467
18018 x 10438
21618 x 12525
25218 x 14613
16020 x 8389
20020 x 10486
24020 x 12583
28020 x 14681
32020 x 16778
36020 x 18875
17622 x 8428
22022 x 10535
9624 x 4233
14424 x 6350
19224 x 8467
28824 x 12700
33624 x 14817
38424 x 16922
43224 x 181050
36426 x 14885
41626 x 161011
46826 x 181138
22428 x 8544
64428 x 231565
18030 x 6438
24030 x 8583
30030 x 10729
36030 x 12875
42030 x 141021
48030 x 161167
12832 x 4311
25632 x 8622
38432 x 12933
64032 x 201556
27234 x 8661
36036 x 10875
43236 x 121050
50436 x 141225
60838 x 161478
80040 x 201944
Vertical Grille Selection Table

How to Calculate Return Air Grille Size?

Return air grille size is calculated using the following formula:

Grille Area (sq.in) = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144

From the above formula, using 500 fpm with 70% free area, we get:

Grille Area (sq.in) = Airflow (cfm) ÷ 500 ÷ 0.7 x 144

Let’s say we have an HVAC unit with 1050 CFM of airflow, then:

Grille Area (sq.in) = 1050 ÷ 500 ÷ 0.7 x 144
Grille Area = 432 sq.in

Using the CFM chart for horizontal grilles, the 18 inches by 24 inches grille has a grille area of 432 which is suitable for the HVAC unit with 1050 CFM of airflow.

If you don’t prefer to use a horizontal grille, you can use the CFM chart for vertical grilles to find the 24 inches by 18 inches grille or the 36 inches by 12 inches grille. Both grilles are suitable for the HVAC unit with 1050 CFM of airflow.

Alternatively, you can simply match the grille size to the airflow (cfm) of your HVAC unit.

If you are sizing external grilles, I suggest you use 50% free area because external grilles usually have narrow blades profile to prevent rainwater from entering.

What Happen If You Use a Smaller Return Air Grille?

Using a smaller return air grille will result in a higher face velocity with the same type of grille (thus, same free area). With a higher face velocity, the grille may create audible noise. The smaller the grille, the greater the noise.

If you really need to use a smaller return air grille, I recommend you don’t exceed 800 fpm of face velocity. Alternatively, you can seek return air grilles with a larger free area. Otherwise, use more than one grille if possible.

My Recommended Return Air Grilles

If you are looking to buy some return air grilles, I got some recommendations.

While working on grille-related posts, I found some good return air grilles that you might be interested in buying. These return air grilles received good feedback from many customers and they are made of durable steel with a decent amount of free area.

Here are my recommended return air grilles, each of them is available in multi different sizes. So, follow the link to see other available grille sizes:

Design Engineer Starter Pack

Many junior engineers often don’t get enough support from their seniors, managers and bosses. Back when I was a fresh graduate, I had no idea what I was doing. I wish someone had given me guides especially on design work.

Hence, I started to work on some gsheet/excel calculators, diagrams and charts and then, I packaged them together to create the Design Engineer Starter Pack (click to view details) to help Junior Design Engineers with their HVAC design work.

I think it will be great if you have it. So, I encourage you to check it out.

Yu Chang Zhen

Yu was working in the air conditioning industry for the past 7 years, covering from design to installation and maintenance. Yu wish to share all the things he learned with the people around the world.

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