What Are the Most Common Air Filter Sizes?

air filters

Every furnace has a different suitable air filter size. You’ll find an extensive list of air filters, each catering to the unique specifications of various HVAC systems. It makes us wonder, what are the most common air filter sizes? Are there any standard filter sizes? How would I know which is right for me? Find all the answers to these questions and more in our guide. 

Two Types of Air Filters 

Air filters are classified into two broad types. Understanding these sizes is essential to get the hang of other sizes. These are:

  • 1-inch and 2-inch air filters
  • Whole house air filters

Home air filters and whole house air filters are installed in different locations. In home air filters, they are placed in the air return, into the ductwork, or the ceiling. These are usually 1″ or 2″ thick, with the latter used less frequently. 

On the other hand, whole house air filters are installed into the air handler. They are located beside the HVAC system in most cases. In this scenario, you may need more than one air filter to cool or heat the whole house efficiently. Due to their increased workload, these filters come with a depth of 4″, 5″, or 6″. 

You must be wondering why there are no 3″ filters. The reason is these are rare and not needed as often. That’s why you won’t see many air filter manufacturers or sellers offering this size.

Length and Width

The 1″ to 6″ sizes we referred to before are the depth of the air filter. Two other dimensions that are factored into the air filter size are length and width. The length and width of air filters are wide-ranging. Air filters usually have the dimensions printed on their frame as LxWxD. Now that you know everything about the dimensions of the air filter, here are some of the most common sizes used in homes. 

  • 12x12x1
  • 12x20x1
  • 12x24x1
  • 14x14x1
  • 14x20x1
  • 14x24x1
  • 14x25x1
  • 14x30x1
  • 16x20x1
  • 16x24x1
  • 16x25x1
  • 18x18x1
  • 18x24x1
  • 18x30x1
  • 20x20x1
  • 20x24x1
  • 20x25x1
  • 20x30x1
  • 24x24x1
  • 24x30x1

Believe it or not, these are only a few air filter sizes to name. There are many more variations, each suitable for a different kind of HVAC system or furnace. Likely, the ideal air filter size for your system may not be on the list. f that’s the case, you can take a look at Filter King’s filter sizes.


To understand the air filter sizes further, you’d have to familiarize yourself with undercuts. 1″ and 2″ air filters are all undercut by a quarter of an inch. This allows the air filter to slide into the compartment easily. For instance, 20x20x1 air filters are actually sized 19-¾” x 19-¾” x ¾”. 

Nearly all the air filter sizes you can think of are undercut. This is also a good place to explain the difference between the nominal and actual sizes. The nominal size is the size of the air filter printed on its frame by the manufacturer. The actual size of the air filter is the exact measurements of it, so it tells you its precise dimensions. 

It should be noted that there could be variations in how manufacturers cut their air filters. These are then rounded to the standardized numbers we listed in the previous section. Not all air filters from different manufacturers are sized the same. Consider this fact when choosing which one is perfect for your needs. 

Consult a Professional 

If you’re confused about which air filter to buy with so many to choose from, it’s best to consult a professional. You can call someone to your home and have them inspect your HVAC unit. They can measure the compartment and suggest the best-sized air filter. A professional can also help you choose an air filter with the best MERV rating for your home and the filtration level. 

If you get the size, MERV rating, and filtration level right, your air filter can perform at peak efficiency. You can enjoy better cooling and heating, improved air quality, and reduced energy bills as your system isn’t forced to overwork or accumulate as disseminate dust because of incompatibility.