# 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 sq.in. So, the possible grille sizes are 20×8 (160 sq.in), 16×10 (160 sq.in) or 14×12 (168 sq.in) depending on your duct size.

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

Grille Area (sq.in) = 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
Airflow
CFM
Velocity
FPM
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:

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:

### 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:

### Square-Shaped Grilles

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

## 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:

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.

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 (sq.in) = 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 sq.in

From the formula, we need a grille size that has a grille area of 432 sq.in. So, any grille sizes that have 432 sq.in 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 sq.in, 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 (sq.in) = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144
Grille Area (sq.in) = 1050 ÷ 500 ÷ 0.8 x 144
Grille Area (sq.in) = 378 sq.in

With 80% free area, the resulted grille area is now 378 sq.in and a smaller grille size like 16×25 (400 sq.in) 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 (sq.in) ] 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 sq.in. Given that the total area of the 10×4 grille is 40 sq.in, 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:

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.