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 low 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.

Return Air Grille Sizing Guide CFM ... x
Return Air Grille Sizing Guide CFM Chart & Calculation

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 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

Honestly, I did spent a lot of unproductive time searching for the best grille size when I was working as a junior project engineer. Why I never thought of creating a whole table for myself? Anyway, let's see how to calculate return air grille size in the manual way.

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.

Although calculating return air grille size manually is quite easy, it can become very time-consuming when you're dealing with hundreds of grilles. So, I create an excel calculator in my Design Engineer Starter Pack alongside other useful HVAC calculators. Check it out!

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.

Different grilles have different applications. If you want to learn about the different types of grille used in HVAC, feel free to check out my blog post 4 Types of Grilles in HVAC (Common Applications).


Featured Products by aircondlounge

Design Engineer Starter Pack

aircondlounge's best seller

Kick start your HVAC design journey with nine (9) calculators, five (5) diagrams and three (3) charts

HVAC Basics (eBook)

best for fresh graduates

Learn the basics of HVAC from working principle to system, components and applications for

Chilled Water System (eBook)

for engineers, operators and managers

Understand HVAC chilled water system from working principle to performance and efficiency for

Latest Product Recommendations

DAIKIN (Atmosphera) Mini Split Heat Pump

Highly efficient and reliable wall-mounted mini split heat pump with the latest environmental-friendly R32 refrigerant gas that runs incredibly quietly.

MIDEA (U-Shaped) Window Air Conditioner

Beautifully-designed trendy window air conditioner with the latest environmental-friendly R32 refrigerant gas that runs more quietly than most other window ACs.

Yu Chang Zhen

I enjoy working on air conditioners and ventilation fans because Malaysia is a very hot country.

Recent Posts