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 the 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.**

Apart from the airflow (CFM), grille sizes are depending on the face velocity and the free area of the grille. In addition, the grille should not create too much noise or causing too greater of a pressure drop.

What’s Covered

- Return Air Grille Sizes
- Return Air Grille CFM Charts
- How to Measure Return Air Grilles?
- How to Size a Return Air Grille?
- How to Calculate the Free Area of a Grille?
- What Happen If You Use a Smaller Return Air Grille?

## 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 of the standard return air grille sizes:

- 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.

## 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. Typically, 500 fpm is an optimal balance between airflow and noise. **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:

### Vertical-Oriented Return Air Grilles

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

48 | 12 x 4 | 50 |

72 | 12 x 6 | 100 |

120 | 12 x 10 | 250 |

84 | 14 x 6 | 131 |

112 | 14 x 8 | 229 |

140 | 14 x 10 | 292 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

96 | 16 x 6 | 167 |

128 | 16 x 8 | 267 |

192 | 16 x 12 | 467 |

180 | 18 x 10 | 437 |

216 | 18 x 12 | 525 |

252 | 18 x 14 | 656 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

160 | 20 x 8 | 361 |

200 | 20 x 10 | 486 |

240 | 20 x 12 | 625 |

280 | 20 x 14 | 729 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

320 | 20 x 16 | 889 |

360 | 20 x 18 | 1000 |

176 | 22 x 8 | 428 |

220 | 22 x 10 | 535 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

96 | 24 x 4 | 167 |

144 | 24 x 6 | 325 |

192 | 24 x 8 | 467 |

288 | 24 x 12 | 750 |

336 | 24 x 14 | 934 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

384 | 24 x 16 | 1067 |

432 | 24 x 18 | 1200 |

364 | 26 x 14 | 1011 |

416 | 26 x 16 | 1156 |

468 | 26 x 18 | 1300 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

224 | 28 x 8 | 544 |

644 | 28 x 23 | 1901 |

180 | 30 x 6 | 437 |

240 | 30 x 8 | 645 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

300 | 30 x 10 | 781 |

360 | 30 x 12 | 1000 |

420 | 30 x 14 | 1167 |

480 | 30 x 16 | 1333 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

128 | 32 x 4 | 267 |

256 | 32 x 8 | 667 |

384 | 32 x 12 | 1067 |

640 | 32 x 20 | 1889 |

272 | 34 x 8 | 708 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

360 | 36 x 10 | 1000 |

432 | 36 x 12 | 1200 |

504 | 36 x 14 | 1400 |

608 | 38 x 16 | 1795 |

800 | 40 x 20 | 2361 |

### Horizontal-Oriented Return Air Grilles

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

– | – | – |

24 | 4 x 6 | 25 |

48 | 4 x 12 | 50 |

60 | 6 x 10 | 63 |

180 | 6 x 30 | 437 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

128 | 8 x 16 | 267 |

144 | 8 x 18 | 325 |

192 | 8 x 24 | 467 |

240 | 8 x 30 | 625 |

256 | 8 x 32 | 667 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

200 | 10 x 20 | 486 |

220 | 10 x 22 | 535 |

240 | 10 x 24 | 625 |

300 | 10 x 30 | 781 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

216 | 12 x 18 | 525 |

240 | 12 x 20 | 625 |

288 | 12 x 24 | 750 |

360 | 12 x 30 | 1000 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

238 | 14 x 17 | 579 |

280 | 14 x 20 | 729 |

350 | 14 x 25 | 972 |

364 | 14 x 26 | 1011 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

288 | 16 x 18 | 750 |

320 | 16 x 20 | 889 |

400 | 16 x 25 | 1111 |

416 | 16 x 26 | 1156 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

432 | 18 x 24 | 1200 |

468 | 18 x 26 | 1300 |

540 | 18 x 30 | 1500 |

576 | 18 x 32 | 1600 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

480 | 20 x 24 | 1333 |

500 | 20 x 25 | 1389 |

600 | 20 x 30 | 1771 |

– | – | – |

### Square-shaped Return Air Grilles

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

36 | 6 x 6 | 38 |

64 | 8 x 8 | 78 |

100 | 10 x 10 | 191 |

144 | 12 x 12 | 325 |

196 | 14 x 14 | 476 |

256 | 16 x 16 | 667 |

Grille Area (sq.in) | Grille Size (in) | Airflow (cfm) |
---|---|---|

324 | 18 x 18 | 900 |

400 | 20 x 20 | 1111 |

484 | 22 x 22 | 1344 |

576 | 24 x 24 | 1700 |

676 | 26 x 26 | 1995 |

900 | 30 x 30 | 2656 |

### Sizing a Return Air Grille Based AC Tonnage

HVAC units generally have about 400 CFM for every refrigeration ton. Meaning a 2.5-ton air conditioner has about 1000 CFM of airflow. So, we can also quick size the return air grille based on the tonnage.

AC Capacity | Airflow | Return Grille Size |
---|---|---|

2-Ton | 800 CFM | 20×16, 14×25, 18×18 |

2.5-Ton | 1,000 CFM | 20×18, 12×30, 20×20 |

3-Ton | 1,200 CFM | 24×18, 18×24, 22×22 |

3.5-Ton | 1,400 CFM | 36×14, 20×25, 22×22 |

4-Ton | 1,600 CFM | 38×16, 18×32, 24×24 |

4.5-Ton | 1,800 CFM | 32×20, 20×30, 26×26 |

5-Ton | 2,000 CFM | 40×20, 26×26, 30×30 |

## 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. To appropriately measure a return air grille, always measure the duct opening size and look for a grille that matches it.

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.

## How to Size a Return Air Grille?

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

### Return Air Grille Calculation Formula

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.

Fortunately, based on my research, most return air grilles sold in the United States have a free area of about 60-80%. Then, 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.

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.

With that, we’re able to calculate the grille size using the below formula:

**Grille Area (sq.in) = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144**

For example, 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 can be calculated as follows:

Grille Area (sq.in) = 1050 ÷ 500 ÷ 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, when we account for the actual 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 can be used.

Alternatively, you can also use the Grille CFM Chart I provided above to find your preferred grille size. Their airflow capacity is calculated based on variable free area. Remember I said the smaller the grille, the lesser the free area?

For instance, the 24 inches by 16 inches vertical-oriented return air grille which has a grille area of 384 sq.in is suitable for 1050 CFM. If you don’t prefer the vertical-oriented grille, you can also use a 6×30 or 14×14 grille. Both are suitable for 1050 CFM.

## How to Calculate the Free Area of a 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) ÷ Grille Area (sq.in) x 100**

Alternatively, you can refer to the AK factor of a grille to calculate its free area. AK factor is also known as the effective area of a grille. It 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%.

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

Grille size is directly proportional to the airflow. Hence, the above graph also represents the relationship between the grille free area and the grille area.

As you can see from the above graph, the free area of a grille varies depending on the airflow (grille size). Hence, the smaller the airflow (grille), the lesser the free area and vice versa.

Nonetheless, the free area of a grille varies depending on the manufacturer and the model of the grille. External weatherproof grilles is mostly 50% and below because their blades are packed much closer together to prevent the ingression of rainwater.

Small grilles are usually less in the free area as their blades occupy a larger percentage of their total mass. So, the free area graph is a good reference. Manufacturer’s datasheet always supersedes my recommendations.

If you’re sizing return air grilles for new construction/project, get the grille size excel calculator in my Design Engineer Starter Pack to speed up the process.

## 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.

Supply air diffusers are sized differently than return air grilles. If you want to learn how to size supply air diffuser, check out my blog post: .

This article was originally published on aircondlounge.com. Actions will be taken for unauthorised republication of this article.

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## FAQs

### What is the formula for calculating CFM? ›

Cubic feet per minute

The number represents the volume of air moved through a given space every minute. To calculate CFM, **multiply cubic feet by 60 seconds**. So, for example, a fan moving 1 cubic foot of air per second would be equal to 120 CFM.

**What is the flow rate of a return air grill? ›**

Return Grille Sizing

The speed of the air moving through of a return grille should typically be kept in the **300 FPM (Feet per Minute) to 500 FPM** range to reduce noise through the grille. It's easy to hear a grille that exceeds this velocity range as it is usually accompanied by an irritating level of noise.

**How do I know if I have enough return air? ›**

More Air Is Blowing In Rooms Near the AC Unit

A telltale symptom of an undersized return air duct is **if the room closest to the air conditioner has more airflow than rooms farther away from it**. That's because air pressure is high near the unit. But the farther you go, the lower it is.

**How many return air vents per square foot? ›**

Generally, the size of your home determines how many vents you will need per room. **If your room is larger than 100 square feet, you will need more than one vent (at least two) to really get adequate airflow to the room.** **If the room is smaller, you only need one**.

**What is the thumb rule for calculating CFM? ›**

As per rule **1 cfm is required per 1 sqft of floor area**. Hence 2000 sqft needs 2000 cfm. 400 cfm required per ton.

**What is the rule of thumb for CFM per square foot? ›**

**1.3 cfm per sq ft**

If the cooling load is relatively small, the system may require closer to 1 cfm/sq ft. In spaces that are more density populated, like conference rooms, gymnasiums or auditoriums, the space may require closer to 2 cfm per sq ft (or sometimes more!)

**What is a CFM calculator? ›**

This fan calculator is **typically used to calculate the CFM or cubic feet per minute of air exchange that may be desired in a building**. Whether exhausting air or bringing fresh air into a structure, the calculation produced should help to figure out the size of fan(s) required to accomplish the air exchanges needed.

**How large does a return vent need to be? ›**

The Size Of Your Return Ducts

In most cases, if you are installing ducts in each room where there is a supply, you will want to go with the same size as your supply vents. If you are installing just one or two return vents, you will want to go with a **12-inch return for 2-ton units and lower**. 14- inches for 3 and up.

**What is code for return air vent? ›**

Return air openings for heating, ventilation and air-conditioning systems shall comply with all of the following: 1. **Openings shall not be located less than 10 feet (3048 mm) measured in any direction from an open combustion chamber or draft hood of another appliance located in the same room or space**.

**Should return air be equal to supply? ›**

**Your return and supply ducts are supposed to maintain a balanced air supply**. In other words, there should be an equal amount of air entering and leaving your HVAC system. If there is a difference in pressure, expect comfort and efficiency problems.

### How many CFM do I need for a 12x12 room? ›

To adjust the CFM, you can change the speed of the blower or alter the ductwork size. Increasing the blower speed won't have much effect if a room's duct is too narrow. A 12' x 12' foot room is 144 square feet. You generally need at least **1 CFM per square foot**, meaning that room's duct should have a 144 CFM capacity.

**How much CFM do I need for 2000 square feet? ›**

Total area of home (square feet) | Continuous ventilation rate |
---|---|

1,000 square feet | 50 CFM |

2,000 square feet | 100 CFM |

3,000 square feet | 150 CFM |

**How many CFM do I need for cold air return? ›**

ACs need roughly **400 CFM per ton**. Undersized: There will be too much pressure in the ductwork, which will be noisy and might eventually cause it to come apart. Your system will work harder.

**How big does my air return need to be? ›**

The Size Of Your Return Ducts

If you are installing just one or two return vents, you will want to go with a **12-inch return for 2-ton units and lower**. 14- inches for 3 and up. 16-inch should be suitable for 4 ton systems, whereas 5 might require 18-inch returns.

**How do you calculate CFM for outdoor air? ›**

The outdoor air intake = **summation of Vbz in each zone divided by the calculated Ev value**. For our example the summation of Vbz= 600 CFM, Ev = 0.6, then the outdoor air intake = 6000.6 = 1000 CFM.