All About Air Filter Sizes

What air filter size do I need? It’s probably the question we hear the most often, and for good reason. 

You're probably wondering why air filter sizes matter. They're crucial for maintaining your home's air quality, ensuring your HVAC system runs efficiently, and even reducing energy costs. 

But it's not always easy to know which size you need. That's where we come in. This guide will explain everything you need to know about air filter sizes, how to measure them, and tips for choosing the right one. 

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

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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.

To determine the size of the air filter you need, locate the dimensions printed along the edge of your current filter, typically listed as length x width x height (L x W x H). If the filter is missing or unmarked, measure the filter slot in your HVAC system for these dimensions. Always ensure the size matches exactly, as even slight differences can affect filtration efficiency and system performance.

How to Measure Your Air Filter

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

1. Use 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.

  • Turn the AC system off.
  • Remove the existing air filter from the compartment.
  • Look for the dimensions printed on the filter frame.

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.

You may also be able to find the proper air filter dimensions listed in your HVAC manual.

2. Measure It

To ensure you're getting the correct filter for your HVAC system, it's essential to know how to accurately measure your air filter. This simple task requires a tape measure and a little bit of your time. Here's a step-by-step guide: 

Step 1: Measure the Length

This is usually the longest side of your filter. 

Step 2: Measure the Width 

This is the shorter side of your filter. 

Step 3: Measure the Thickness 

This refers to how 'deep' the filter is. 

Always remember, the measurements are typically listed in this order: Length x Width x Thickness. So, if you've got a 20x25x1 filter, it's 20 inches long, 25 inches wide, and 1 inch thick. Get your measurements right and you'll never buy the wrong filter again. 

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

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!

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!