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.

A quick way to find the suitable grille size is by taking the CFM of the HVAC unit and divide it by 350 which will get you the grille area in square feet. Multiply it by 144 to get the grille size in square inches and choose your preferred grille size based on that.

For instance, 400 cfm ÷ 350 x 144 = 164.57 So, the possible grille sizes are 20×8 (160, 16×10 (160 or 14×12 (168 depending on your duct size.

The above way is based on the grille sizing formula which:

Grille Area ( = Airflow (cfm) ÷ [ Face Velocity (fpm) x Free Area (%) ] x 144

Where, the face velocity and the free area of the grille are assumed to be 500 fpm and 70% respectively (and therefore, 500 x 0.7 = 350).

If you’re not good at manual calculation, I have a simple grille size calculator:

Grille Size Calculator
Free Area

The face velocity and free area are set by default. However, you can change them under certain circumstances. In the following, I’ll explain what are they and what value you should use.

Return Air Grille CFM Charts

Return air grilles are typically sized based on a face velocity of 500 fpm and a free area of 70%. However, small grilles tend to have less free area and large grilles tend to have more free area due to nature of the grille construction (more on this later).

  • Face velocity is the amount of air volume passing through a grille. An optimal balance between airflow and noise is 500 FPM.
  • Free area is the amount of space on a grille that is available for the air to pass through. It is about 70% on average.

However, the free area of a grille decreases along with the grille size. So, small grilles must account for the drop in the free area or else, they’ll be undersized.

Following are charts that shows the airflow capacity for different grille sizes based on a face velocity of 500 fpm with variable free area for different airflow rates:

Airflow (cfm)Grille Area ( Size Examples (in)
50 cfm48 sq.in12×4, 8×6
100 cfm72 sq.in18×4, 12×6
150 cfm96 sq.in16×6, 12×8, 10×10
200 cfm105 sq.in18×6, 14×8, 10×10
250 cfm120 sq.in16×8, 12×10
300 cfm144 sq.in18×8, 14×10, 12×12
400 cfm165 sq.in20×8, 16×10, 14×12
500 cfm206 sq.in26×8, 20×10, 18×12, 14×14
600 cfm247 sq.in24×10, 20×12, 18×14, 16×16
800 cfm307 sq.in30×10, 26×12, 22×14, 20×16
1,000 cfm360 sq.in36×10, 30×12, 26×14, 22×16
1,200 cfm432 sq.in36×12, 30×14, 28×16, 24×18
1,400 cfm504 sq.in32×16, 28×18, 24×20
1,600 cfm576 sq.in42×14, 36×16, 32×18, 24×24
1,800 cfm610 sq.in34×18, 30×20, 28×22, 26×24
2,000 cfm678 sq.in38×18, 34×20, 28×24, 26×26

The above grille will result in a low-noise 500 FPM of return air velocity. If you have specific requirements such as the orientation of the grille, below charts may help you find the right grille size.

Vertical-Oriented Grilles (Portrait)

Based on the same quiet 500 FPM return air velocity, the following is a series of vertically-oriented return air grilles that you can match with your airflow:

Grille Area ( Size (in)Airflow (cfm)
4812 x 450
7212 x 6100
12012 x 10250
8414 x 6131
11214 x 8229
14014 x 10292
16020 x 8361
20020 x 10486
24020 x 12625
28020 x 14729
9624 x 4167
14424 x 6325
19224 x 8467
28824 x 12750
33624 x 14934
22428 x 8544
64428 x 231901
18030 x 6437
24030 x 8645
12832 x 4267
25632 x 8667
38432 x 121067
64032 x 201889
27234 x 8708
Grille Area ( Size (in)Airflow (cfm)
9616 x 6167
12816 x 8267
19216 x 12467
18018 x 10437
21618 x 12525
25218 x 14656
32020 x 16889
36020 x 181000
17622 x 8428
22022 x 10535
38424 x 161067
43224 x 181200
36426 x 141011
41626 x 161156
46826 x 181300
30030 x 10781
36030 x 121000
42030 x 141167
48030 x 161333
36036 x 101000
43236 x 121200
50436 x 141400
60838 x 161795
80040 x 202361

Horizontal-Oriented Grilles (Landscape)

If you need a horizontally-oriented return air grille, you can find one that matches your airflow in the following chart:

Grille Area ( Size (in)Airflow (cfm)
244 x 625
484 x 1250
606 x 1063
1806 x 30437
20010 x 20486
22010 x 22535
24010 x 24625
30010 x 30781
23814 x 17579
28014 x 20729
35014 x 25972
36414 x 261011
43218 x 241200
46818 x 261300
54018 x 301500
57618 x 321600
Grille Area ( Size (in)Airflow (cfm)
1288 x 16267
1448 x 18325
1928 x 24467
2408 x 30625
2568 x 32667
21612 x 18525
24012 x 20625
28812 x 24750
36012 x 301000
28816 x 18750
32016 x 20889
40016 x 251111
41616 x 261156
48020 x 241333
50020 x 251389
60020 x 301771

Square-Shaped Grilles

If you prefer a square-shaped return air grille, one of the following sizes should match your airflow:

Grille Area ( Size (in)Airflow (cfm)
366 x 638
648 x 878
10010 x 10191
14412 x 12325
19614 x 14476
25616 x 16667
Grille Area ( Size (in)Airflow (cfm)
32418 x 18900
40020 x 201111
48422 x 221344
57624 x 241700
67626 x 261995
90030 x 302656

Standard Return Air Grille Sizes

Return air grilles are typically made in standardized sizes. However, upon request, many grille suppliers do provide custom-made sizes at a higher price.

Return air grilles are standardized based on 2″ per size increase. The smallest return air grille is usually starts at 4 inches by 4 inches. So, the next corresponding return air grille size includes 4×6, 6×6, 6×4, 8×6, 4×8 and so on. The largest return air grille is typically stops at 48 inches by 24 inches.

Here are some examples of a standard return air grille:

  • 4×4
  • 6×6
  • 8×8
  • 10×4
  • 12×6
  • 14×8
  • 20×10
  • 24×12
  • 28×14
  • 32×16
  • 40×20
  • 48×24

Standard return air grilles typically stop at around 48 inches by 24 inches as subsequent sizes are too large for applications in residential and commercial buildings. Factories and production plants may call for larger grille sizes but they are mostly custom-made.

How to Measure Return Air Grilles?

When a return air grille is due for a replacement, you must replace it with an equally-sized one, assuming the previous one doesn’t give you any problem (too noisy, not enough return, etc.).

To appropriately measure a return air grille, always measure the duct opening size and look for a grille that matches it. So, naturally, you’ll need to know what return duct size you need. If you don’t, I have a post about how to size duct.

Generally, return air grilles are measured as follows:

Return Air Grille Measurement

Sellers and manufacturers publish their return air grilles based on the opening size. It is the “Width” and “Height” shown in the above diagram.

Do not use the face width and face height of your old return air grilles to buy new return air grilles. Always use the opening size. As a general rule, the face width/height is about 1 inch.

What Face Velocity and Free Area to Use?

Before I go on, if you think there’s too much texts, I have a video that explain how to size return grilles:

The size of a grille is determined by how much airflow it allows to pass without creating too much noise and pressure drop (air resistance). If you use an undersized grille, you’ll notice the HVAC system is noisier and potentially consuming more power.

Most of the time, sellers don’t publish the airflow capacity of their return air grilles. This is fine in many cases as consumers can buy based on their old grille size or duct opening size.

However, it troubles the first timers or someone who just had their HVAC system upgraded.

Fortunately, based on my experience, most return air grilles have a free area of about 60-80%. Very small ones like 4×4 and 6×6 tend to have only about 30-40% free area.

This is applicable to standard louver type grille. Those specially-designed grilles may have way less or more free area depending on the manufacturer. Different types of grilles also have different free areas.

Louver Grille Example

To ensure the noise created by the grille is acceptable, it is recommend to use a face velocity of 500 fpm when sizing a return air grille. If the grille can’t fit due to limited space, you can allow the face velocity to go higher but the maximum face velocity I would recommend is 800 fpm.

It is common that the air velocity inside the duct is higher than the recommended face velocity of return grilles. In that case, you need a plenum box to reduce the velocity.

Although it is recommended to use a face velocity of 500 fpm when sizing a return air grille, you can use a 600-800 fpm as well but take note that the noise created by the grille is expected to be higher. Also, if you use a lower face velocity like 300-400 fpm, the grille size will be bigger.

Grille Sizing Formula

Earlier, I mentioned about the grille sizing formula. This part is dedicated to enthusiasts, technicians and engineers who want to go deeper into the rabbit hole.

First, let’s revisit the formula:

Grille Area ( = Airflow (cfm) ÷ [ Face Velocity (fpm) x Grille Free Area (%) ] x 144

Say we have an HVAC unit with 1050 CFM. Using an optimal face velocity of 500 fpm and assuming the grille has a free area of 70%, the required grille size is:

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

From the formula, we need a grille size that has a grille area of 432 So, any grille sizes that have 432 of grille area or above can be used.

However, for an airflow of 1050 CFM, we may be able to use a smaller grille size like a 16 inches by 25 inches grille which only has a grille area of 400, given that we account for the actual free area of the grille.

Accounting for the Free Area of the Grille

Earlier, we calculated the grille based for 1050 CFM based on a free area of 70%. Now, what if the actual free area of the grille is 80%?

Grille Area ( = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144
Grille Area ( = 1050 ÷ 500 ÷ 0.8 x 144
Grille Area ( = 378

With 80% free area, the resulted grille area is now 378 and a smaller grille size like 16×25 (400 can be used.

So, the free area of a grille significantly affects how much airflow it can take without causing too much noise. However, we can’t find the free area data from the grille spec. sheet. But, we can reverse calculate to find out.

Grille Free Area Calculation

The free area of a grille is the net space available for the air to pass through the grille. The free area of a grille can be calculated by rearranging the grille area formula as follows:

Grille Free Area (%) = [ Airflow (cfm) x 144 ] ÷ [ Face Velocity (fpm) x Grille Area ( ] x 100

For example:

Grille Free Area = (1050 x 144) ÷ (500 x 378) x 100
Grille Free Area = 80%

Alternatively, you can refer to the AK factor of the grille to calculate its free area. The AK factor can be found in the performance datasheet published by grille manufacturers. The AK factor of a grille is expressed in terms of square inch or square feet.

For example, a 10×4 grille has an AK factor of 0.14. Thus, the effective area of the grille is 0.14 sqft or 20.16 Given that the total area of the 10×4 grille is 40, the resulted free area in terms of percentage is 50.4%.

Not many sellers publish their the free area and airflow capacity of their grilles. But, we know that smaller grilles have lesser free area. So, I plotted a graph showing the drop in the free area against the airflow as follows:

Grille Free Area Against Airflow

From the above graph, we can see that grille size is directly proportional to the airflow which airflow is basically grille size given that the face velocity is fixed.

Nonetheless, the free area of a grille varies depending on the manufacturer and the model of the grille. The reason why small grilles are usually less in the free area is because their blades occupy a larger percentage of their total mass.

External weatherproof grilles usually have a free area of 50% and below because their blades are packed much closer together to prevent rainwater from going in.

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.

If you still not sure or not very confident in sizing your grilles or even designing your house ductwork, you can consider my Email Consultation service where I can help you design, check and verify.

After return grille sizing, you might also need to decide the supply diffuser size. If so, I recommended you read my post about diffuser sizing with CFM chart and selection guide.

If you have anything to add (or ask) about this topic, leave a comment down below!

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