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.

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)
24	4 x 6	58
48	4 x 12	117
60	6 x 10	146
180	6 x 30	438
128	8 x 16	311
144	8 x 18	350
192	8 x 24	467
240	8 x 30	583
256	8 x 32	622
200	10 x 20	486
220	10 x 22	535
240	10 x 24	583
300	10 x 30	729
216	12 x 18	525
240	12 x 20	583
288	12 x 24	700
360	12 x 30	875
238	14 x 17	578
280	14 x 20	681
350	14 x 25	851
364	14 x 26	885
288	16 x 18	700
320	16 x 20	778
400	16 x 25	972
416	16 x 26	1011
432	18 x 24	1050
468	18 x 26	1138
540	18 x 30	1313
576	18 x 32	1400
480	20 x 24	1167
500	20 x 25	1215
600	20 x 30	1458

CFM Chart for SQUARE Return Air Grilles

Grille Area (sq.in)	Grille Size (in)	Airflow (cfm)
36	6 x 6	88
64	8 x 8	156
100	10 x 10	243
144	12 x 12	350
196	14 x 14	476
256	16 x 16	622
324	18 x 18	788
400	20 x 20	972
484	22 x 22	1176
576	24 x 24	1400
676	26 x 26	1643
900	30 x 30	2188

CFM Chart for VERTICAL Return Air Grilles

Grille Area (sq.in)	Grille Size (in)	Airflow (cfm)
48	12 x 4	117
72	12 x 6	175
120	12 x 10	292
84	14 x 6	204
112	14 x 8	272
140	14 x 10	340
96	16 x 6	233
128	16 x 8	311
192	16 x 12	467
180	18 x 10	438
216	18 x 12	525
252	18 x 14	613
160	20 x 8	389
200	20 x 10	486
240	20 x 12	583
280	20 x 14	681
320	20 x 16	778
360	20 x 18	875
176	22 x 8	428
220	22 x 10	535
96	24 x 4	233
144	24 x 6	350
192	24 x 8	467
288	24 x 12	700
336	24 x 14	817
384	24 x 16	922
432	24 x 18	1050
364	26 x 14	885
416	26 x 16	1011
468	26 x 18	1138
224	28 x 8	544
644	28 x 23	1565
180	30 x 6	438
240	30 x 8	583
300	30 x 10	729
360	30 x 12	875
420	30 x 14	1021
480	30 x 16	1167
128	32 x 4	311
256	32 x 8	622
384	32 x 12	933
640	32 x 20	1556
272	34 x 8	661
360	36 x 10	875
432	36 x 12	1050
504	36 x 14	1225
608	38 x 16	1478
800	40 x 20	1944

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).

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