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All About Common (and Not So Common)
Air Filter Sizes

What air filter size do I need? It’s probably the question we hear the most often, and for good reason. The furnace or vent filter size needs to be correct, or the filter can’t really do its job. If the filter is too small, it will be loose and some of the air (and debris) won’t go through the air filter. That’s the exact opposite of what needs to happen when the filter is in place. On the other side, if the filter is too big, the frame will be crumpled, air flow may be restricted, and longevity will be compromised. Restricted air flow also hurts the longevity of your HVAC system and jacks up your utility bills.

Just like Goldie Locks, you want to find the home air filter size that’s just right. Thankfully, it’s never been easier to find the perfect fit.

What Are the Most Common Air Filter Sizes

Historically, when you’ve gone to your typical home supply store, you’d find between 10-20 size variants to choose from. These sizes reflect the most common sizes of return air vents that push or pull air into your HVAC system:

  1. 20 x 20 x 1
  2. 16 x 25 x 1
  3. 20 x 25 x 1
  4. 20 x 25 x 4
  5. 16 x 20 x 1
  6. 16 x 25 x 4
  7. 20 x 30 x 1
  8. 12 x 12 x 1
  9. 14 x 14 x 1
  10. 14 x 20 x 1

How Do I Know Which Air Filter Size I Need?

When a home air filter size fits well it doesn’t have to provide an airtight seal. It does, however, need to fit snugly in the air filter compartment with a slight bit of space around the edges. Being able to easily slide the air filter in place is another sign that the air filter is a good fit. Whatever size you need, the right one should meet these criteria

So how do you find that perfect air filter fit? It’s a combination of knowing how to measure an air filter and understanding that the dimensions you see may not be the actual size.

Measuring Your Air Filter

Finding the air filter dimensions isn’t hard to do. The two main methods for measuring furnace air filter size are:

Your Existing Air Filter
Does your existing filter fit well? Going off measurements on your existing filter is the easiest way to get a good fit.

The dimensions will include height, width and depth. In select cases, air filter measurements aren’t necessarily listed as exact physical dimensions. Sometimes, the manufacturer uses a model number instead. If you have the model number, you can figure out the dimensions. If your HVAC system uses a whole house filter (they are near your central furnace or AC unit) the model number will most likely be listed, not the dimensions.

Measure It Yourself
If the dimensions are too faded to read or there is no air filter to look at, it’s time for the DIY method.

If that doesn’t work, you may be able to find the proper air filter dimensions listed in your HVAC manual.

Read More: How to Measure Your Air Filter

Nominal vs Actual Filter Size

There’s one more thing you need to know about filter sizes. The dimensions that are listed on the filter aren’t the exact size. That is the nominal size. It’s a rough estimate that is closer to the size of the filter compartment.

The actual size of an air filter is going to be a little smaller than the nominal size. Typically, the actual size is about 1/8″ to 1/4″ smaller all the way around so it fits in the filter compartment.

Read More: Nominal vs Actual Air Filter Size

At Filterbuy we understand how important filter fit is for efficiency and good air quality. That’s why our experts have created a variety of air filter resources to help you better understand furnace filter sizes and how to determine air filter dimensions.

What Do I Do If I Don’t Have a Standard Size Air Filter?

A lot of people are surprised to find that they don’t have a standard furnace filter size. Here’s a fun fact – 70% of what Filterbuy sells falls outside of the 10 most common air filter sizes.

There’s a reason we set out to manufacture hundreds of options beyond the standard air filter sizes for homeowners and businesses in the U.S. Most people have an odd HVAC filter size. Most people won’t be able to find the air filter size they need at the local home improvement store. But you can find it on Filterbuy.com!

We can make air filters in any custom size that you need. These aren’t imperfect, pre-made cut-to-fit filters. They are built from the ground up using your exact dimensions.

Visit our custom air filter order page to start constructing the perfect filter fit for you!