How to Measure Your Air Filter | The Only Guide You'll Need

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Make the Air Quality Around You Amazing!

When it comes to working with an HVAC system, it is important to make sure that the air filter that you use has a perfect fit. A filter that does not fit well can affect both the system and in turn the air quality and people around your building. This article details questions and important information surrounding what an air filter is, how to measure one for your HVAC system, the different types and more.

What Is an Air Filter?


Air filters work to remove dust and other contaminants from the building to ensure worker health and safety. They come in different forms, work in different locations and have different applications.

Air Filter Materials

Different materials are used to make air filters, ranging from synthetic and fiberglass to aluminum and glass microfiber among others. Additionally, there are air filters that are designed with certain performance ratings, including a MERV rating. MERV is designed to indicate a filter's efficiency in capturing airborne particles and contaminants.

Here are some important points to keep in mind with MERV ratings:

  • The higher the MERV rating, the more effective the filter is at trapping small particles (between 0.3 and 10 microns (µm)).
  • Any filter rated above MERV 6 must be tested to assure its efficiency.
  • Metal (reusable) filters are generally rated MERV 4 and are not normally lab tested.

With an air filter, it will be common to associate two sizes with it that could be listed on the filter itself: nominal size and actual size. These two are slightly different but meaningful.


What Is an Actual Size?


The actual size of an air filter is the measured dimensions when using a ruler or measuring tape. It is common for the length, width and depth to be listed in 1/2 inch measurements among others. Depending on the HVAC system, it is important to compare this size to the nominal size.


What Is a Nominal Size?


With an air filter, there is also a nominal size. The dimensions, commonly known as the length, width and depth, are recognized as whole and rounded numbers. The reason why filters are made with this distinction is to make it simple for customers to find and purchase sizes that will work. One can argue the whole numbers surrounding the dimensions allow customers to avoid confusion with half or quarter-inch measurements.


How to Measure an Air Filter


Finding the right air filter for your HVAC system is a high priority, almost as important as the condition of the system. To make sure the air filter is a perfect choice, you should measure the air filter.

There is one popular technique used to properly measure and find the dimensions of a filter. It is also important to ensure that it fits into the HVAC system, finding the right dimensions for both. This can be done using measuring tools like measuring tape or a ruler. The steps to measure the filter are:

  • Find the length of the air filter
  • Find the width of the air filter
  • Find the air filter's depth

Using a filter that is the wrong size can allow particles of different contaminants (dust, pollen, mold, bacteria and viruses to name a few) to remain in the air throughout a facility. When the air filter and air quality are not up to par, it can affect people who rely on clean and healty air. This is one reason why it is important to find the right-sized air filter for your system.


How to Choose the Right Air Filter


It is important to take the material of the air filter into account just as much as the size. Filters serve different functions and are made of different materials. 

Some of the most popular filters are:

1. ASHRAE/Cartridge Air Filters

  • Pleated media made of high-strength, water resistant, glass microfiber paper
  • Designed to allow the free flow of air for exceptional filtering efficiency
  • Great choice for hospitals, laboratories, office buildings or food and beverage processing plants
  • MERV Ratings between 11 - 14

Popular ASHRAE/Cartridge Air Filter Options

ASHRAE/Cartridge Air FilterASHRAE/Cartridge Air Filter
Material Size Performance Ratings
Glass Microfiber 12" x 24" MERV 11-14
Glass Microfiber 24" x 24" MERV 11-14

2. HEPA Air Filters

  • Microglass Fiber media
  • Ideal for environments where air purity is crucial, including laboratories, pharmaceutical operations, hospitals and food processing plants
  • Options with HEPA ratings between 95 and 99.97%

Popular HEPA Air Filter Options

HEPA Air FilterHEPA Air Filter
Material Size Performance Ratings
Microglass Fiber 12" x 12" HEPA 95% and HEPA 99.97%
Microglass Fiber 12" x 24" HEPA 99.97%
Microglass Fiber 24" x 24" HEPA 95% and HEPA 99.97%

3. Mini-Pleat Air Filters

  • Microglass Media
  • MERV Ratings between 11 – 14
  • Five times the surface area of a regular pleated filter
  • Popular in hospitals, laboratories, pharmacies and computer clean rooms
  • Works great against pollen, mold, bacteria and viruses

Popular Mini-Pleat Air Filter Options

Mini-Pleat Air FilterMini-Pleat Air Filter
Material Size Performance Ratings
Microglass  12" x 24" MERV 11-14
Microglass  16" x 20" MERV 11-14
Microglass  16" x 25" MERV 11-14
Microglass  20" x 20" MERV 11-14
Microglass  20" x 24" MERV 11-14
Microglass  20" x 25" MERV 11-14
Microglass 24" x 24" MERV 11-14

4. V-Bank Air Filters

  • Polypropylene (PP) material
  • V-shaped banks that contaminants fall into
  • Works for both HEPA (as a pre-filter) and HVAC (as a regular filter) systems
  • Commonly used in hospitals, gas turbine power plants, clean rooms and universities
  • MERV Rating of 14 (95% filtration efficiency)

Popular V-Bank Air Filter Options

V-Bank Air FilterV-Bank Air Filter
Material Size Performance Rating
Polypropylene (PP) 12" x 24" MERV 14
Polypropylene (PP) 20" x 24" MERV 14
Polypropylene (PP) 24" x 24" MERV 14

5. Pocket/Bag Air Filters

  • Synthetic media with ultrasonically sealed stitched pockets
  • Great for hospitals, laboratories, office buildings or food and beverage processing plants
  • MERV values ranging from 9 to 14

Popular Pocket Air Filter Options

Pocket/Bag Air FilterPocket/Bag Air Filter
Material Size Performance Ratings
Synthetic 12" x 24" MERV 9-14
Synthetic 20" x 20" MERV 9-14
Synthetic 20" x 24" MERV 9-14
Synthetic 24" x 24" MERV 9-14

6. Rigid Cell Air Filters

  • High quality synthetic media
  • Popular choice in healthcare facilities, manufacturing plants, schools and office buildings
  • MERV ratings ranging from 9 to 14
  • Great against mold spores, bacteria and coal dust

Popular Rigid Cell Air Filter Sizes

Rigid Cell Air FilterRigid Cell Air Filter
Material Size Performance Ratings
Synthetic 12" x 24" MERV 9-14
Synthetic 20" x 20" MERV 9-14
Synthetic 20" x 24" MERV 9-14
Synthetic 20" x 25" MERV 9-14
Synthetic 24" x 24" MERV 9-14

7. Disposable Air Filters

  • Charcoal, Fiberglass, Polyester and Synthetic material options
  • Works against dust, pet dander, dust mites and more
  • Commonly used in hospitals, schools, government buildings, manufacturing plants or whole home and multi-unit residentials

Popular Disposable Air Filter Sizes

Disposable Air FilterDisposable Air Filter
Material Sizes Performance Rating
Charcoal

12" x 24"

16" x 20"

16" x 25"

20" x 20"

20" x 25"

24" x 24"

MERV 1 to MERV 4
 
Fiberglass

10" x 10" - 25" x 25"

Rated MERV 1 to MERV 4*

*Pleated versions may be higher

Polyester

10" x 10" - 25" x 25"

MERV 1 to MERV 4
Synthetic

10" x 10" - 25" x 25"

MERV 8 or MERV 13

8. Washable Air Filters

  • Aluminum, Galvanized Steel, 304 Stainless Steel and 430 Stainless Steel models
  • Withstand temperatures up to 900°F (depending on material)
  • Used for industrial and commercial environments, office buildings, schools, electronic equipment and air cleaners
  • Rust resistant
  • Easy wash with a spray hose or cleaning solutions

Popular Washable Air Filter Sizes

Washable Air FilterWashable Air Filter
Materials Sizes Performance Rating

Aluminum

Galvanized Steel

430 Stainless Steel

304 Stainless Steel

10" x 20"

12" x 24"

16" x 20"

16" x 25"

20" x 20"

20" x 24"

20" x 25"

24" x 24"

Rated MERV 1
to MERV 4

9. Washable Grease Filters

  • Models include: aluminum, galvanized steel, 304 stainless steel and 430 stainless steel
  • Primarily used in kitchen environments
  • Ideal to collect grease in the air to avoid fires
  • Options can withstand between 250 – 900°F

Popular Washable Grease Filter Sizes

Washable Grease FilterWashable Grease Filter
Material Sizes Performance Rating

Aluminum

Galvanized Steel

430 Stainless Steel

304 Stainless Steel

10" x 20"

16" x 16"

 16" x 20"

 16" x 25"

20" x 16"

 20" x 20"

 20" x 25"

 24" x 24"

25" x 16"

25" x 20"

25" x 25"

N/A

How Do I Know When I Need to Replace My Air Filter?


It is important to keep an eye on your filtration system over time to make sure it is still in good condition. A popular question when going over air filters and HVAC systems is: “When exactly do the filters need to be replaced?”

It is recommended that people who use air filters check out and change out their air filter every month. Some factors that may affect the timing include material of the filter, how often the HVAC system is used, the surrounding air quality, and more.


Shop Our Wide Collection of Air Filters!


The Cary Company offers a wide variety of filters for your HVAC system, from disposable and washable to mini pleat and v-bank among others. In addition to different types and frames for the filters, we pride ourselves in having a vast selection of filters in different sizes.

For more questions on HVAC systems, filters and more, email our specialists at sales@thecarycompany.com for more information or give us a call at 630-629-6600 today!

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