How To Calculate Return Air Grill Size

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How To Calculate Return Air Grill Size

How To Calculate Return Air Grill Size? A return air grille is available in a variety of sizes. It is critical to choose the proper return air grille size to ensure that the HVAC system has enough airflow while being quiet. As a result, I prepared a number of CFM charts as well as guidance on how to determine return air grille size. Calculating is not a difficult task but one should know how to cope up with it. 

To get the size of the return air grille, multiply the CFM of the HVAC unit by 350 to obtain the grille area in square feet. Then, increase the grille area by 144 to get the square inches. Finally, select your ideal grille size that corresponds to the needed grille area.

Alternatively, you may use the CFM charts below to rapidly determine the grille size based on the HVAC unit’s CFM or airflow. The following parameters can be used:

  • Face Velocity = 500 fpm (feet per minute) or 2.5 m/s (metres per second).
  • 70% of the area is free.

The quantity of air volume going through a grille is referred to as its face velocity. 500 fpm is often ideal and well-balanced between air volume and noise.

The free area of a grille is the amount of space available for air to travel through. It is dependent on the kind of grille and the manufacturer. However, many grilles have around 80% free space, whereas some exterior grilles have 50% free area. As a result, I use a 70 percent free area.

Many individuals pick a more modest barbecue than they need. This outcome in food varieties takes too long to even think about cooking.

Then again, the people who have a greater barbecue than required think that it is trying to keep up with. Additionally, it doesn’t cook as well.

Considering that, how do you have any idea what barbecue size to pick? You may also wonder how much grilling space do you need? Read ahead to find the answers to these questions. 

Standard BBQ Grill Size

With regards to a standard barbecue size, the vast majority who need to cook sausages and burgers go for a comparable size.

These barbecues are generally at least 360 square inches. The people who need to have the option to cook for a bigger group may increase to around 400 square inches.

How to Calculate Return Air Grille Size

The following formula is used to compute the size of the return air grille:

Airflow (cfm) Face Velocity (fpm) Grille Free Area (percent) x 144 = Grille Area (

Using the aforementioned calculation and 500 fpm with 70% free space, we get:

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

Assume we have a 1050 CFM airflow HVAC unit, then:

  • Grille Area (in square inches) = 1050 500 0.7 x 144
  • 432 sq. in. grille area

This is no doubt very easy to go through.

Using the CFM chart for horizontal grilles, the 18-inch-by-24-inch grille has a grille area of 432, which is sufficient for the HVAC unit with a capacity of 1050 CFM.

If you don’t want to utilize a horizontal grille, you can find the 24 inches by an 18-inch grille or the 36 inches by 12-inch grille using the CFM table for vertical grilles. Both grilles are suited for the HVAC unit with a capacity of 1050 CFM.

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

When measuring exterior grilles, I recommend using 50% free space because external grilles often have thin blade profiles to prevent rainfall from entering.

What Happens If A Smaller Return Air Grille Is Used

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

If you must utilize a smaller return air grille, I recommend that the face velocity not exceed 800 fpm. You may also look for return air grilles with a bigger free space. Otherwise, if feasible, utilize more than one grille.

How to Size Return Filter Grilles

Because furnace filters should be replaced at least every 90 days, going into an attic to change the filter may be impractical. The return air filter grille must be properly sized for the furnace to perform properly.

Step 1 To Calculate Return Air Grill Size

To obtain access to the service plate, open the cabinet of your furnace. Check the plate to find how many BTUs (British Thermal Units) your furnace produces.

2nd Step To Calculate Return Air Grill Size

Divide the BTU by 50 to determine the CFM, or cubic feet per minute, that your furnace produces. For example, a 40,000 BTU furnace would produce 800 CFM.

Step Three to Calculate Return Air Grill Size

Determine the area in square inches required for your filter grille. Allow 2 CFM per square inch of filter grille area for appropriate functioning. So, if your furnace produces 800 CFM, you’ll require 400 square inches of filter grille space.

Step 4 to calculate Return Air Grill Size

To compute the square inches of each, multiply the lengths and widths of standard filter grille sizes. You can use it as your filter grille size once you’ve found one that meets or surpasses the above-calculated required value. The most popular sizes are 20-by-14, 25-by-16, and 20-by-20. Using those figures, their square inches would be 280, 400, and 400, respectively. Because the furnace requires 400 square inches for your filter grille in this example, you can use either the 25-by-16 filter grille or the 20-by-20 filter grille.


Furnace filters are most commonly found in the unit or in the return air ducting or adjacent to the unit. There may be occasions when you want or need to put the filter at the return air grille. This is useful if your furnace is in an attic or other difficult-to-reach location.

As this furnace filters should be replaced at least every 90 days, going into an attic to change the filter may be impractical. The return air filter grille must be properly sized for the furnace to perform properly. Hence it is very important to calculate return air grill size  without any difficulty.