Standard Furnace Filter Sizes (With Drawings)

Stacy Randall
by Stacy Randall

Choosing the correct furnace filter is critical for the overall air quality in your home and your HVAC system’s functioning. Air filters make sure contaminants don’t make their way into your HVAC system and into the air you breathe. It’s important to choose correctly from the standard furnace filter sizes so that your filter works properly.

Standard furnace filters come in various sizes ranging from 10 x 20 x 1” to 20 x 25 x 4”. These sizes coincide with air returns in most houses, and the most common size furnace filter is 10 x 20 x 1”. If you use the wrong size filter, it can lead to poor air quality and overwork your HVAC system.

These standard-size furnace filters are what you would likely find in most home improvement stores. Regularly replacing your furnace filter is an essential part of HVAC maintenance. Knowing the size you need makes the process seamless.

What Are the Most Common Furnace Filter Sizes?

Furnace filters come in an assortment of sizes and thicknesses, making it easy to find the right fit. When thinking about furnace filters, you have three primary measurements to consider. You need to know the width, length, and depth or thickness.

Most standard filters come in thicknesses ranging between 1 and 4 inches. But some filters are 5 or 6 inches thick. There are many standard-size filters you can purchase, but here are some of the most common.

  • 12x12x1
  • 14x14x1
  • 10x20x1
  • 14x20x1
  • 16x20x1
  • 20x20x1
  • 16x24x1
  • 16x25x1
  • 14x25x1
  • 15x25x1
  • 20x25x1
  • 18x30x1
  • 20x30x1
  • 16x25x4
  • 20x25x4

A Quick Note About Furnace Filters vs. A/C Filters

Furnace filters and A/C filters come in similar sizes. In many cases, the central HVAC system shares one filtration point. In these cases, your filter will filter both heated and cool air. Sometimes, people call this a “whole-home air filter.”

If you have a separate heating and cooling system, your furnace filter typically goes in the blower compartment. Either way, both filters serve the same purpose and come in similar sizes.

Source: Amazon

What Is the Difference Between Nominal and Actual Furnace Filter Sizes?

When you purchase a filter, the numbers on the side represent the filter size. However, these aren’t the exact dimensions of the filter. Instead, these are the nominal measurements compared to the actual dimensions.

Basically, the nominal measurements are rounded-up figures that make it easier for consumers to identify their filter size. It keeps things consistent from manufacturer to manufacturer. The actual measurements of a filter will typically be about ½” to ¼” smaller than the nominal measurements.

For example, a 10 x 20 x 1” filter could have an actual size of 9.5 x 19.75 x 0.75”. A 16 x 25 x 1” filter could have an actual size of 15.75 x 24.75 x 0.75”.

Source: Amazon

Does An AC Filter Have to Be the Exact Size?

Using a filter when your run your furnace is important. But it’s also essential that your AC or furnace filter be the exact size you need; otherwise, it won’t work optimally. Using a too-small filter will allow dirt and other contaminants to pass through the return into your HVAC system.

The result of these extra contaminants is poor air quality which can lead to a host of issues. Plus, they also can make your HVAC system work harder than it needs to since they clog up the system. Eventually, you’ll end up with a furnace or A/C that burns out earlier than it should, leading to costly HVAC repairs.

What Standard Furnace Filter Sizes Can You Purchase at Home Depot?

Home Depot and other big-name home improvement stores are popular places to shop for air and furnace filters. You can find pretty much any size furnace filter you need at these stores or via their online storefronts. Home Depot offers filters ranging in thicknesses from ¾” to 6 inches, but between 1 and 4” are still the most common.

You can use their easy online search tools to find the size you need. They offer the most common standard sizes plus even more standard options. Some of these include filters ranging from 8 x 20 x 1” to 30 x 30 x 1” and 24 x 36 x 1”.

How Do I Know What Size Filter to Buy For My Furnace?

Since using the proper size filter is so crucial, it’s worth knowing how to determine the size you need. You can always look at the nominal measurements on your existing filter.

Also, in terms of thickness, the thicker the filter, usually the more contaminants it will filter out. However, sometimes thicker filters can restrict airflow. So make sure you keep up with changing out your filter on schedule.

It’s worth checking the filter occasionally, as sometimes it might be dirty and packed before the usual replacement period. In these cases, it’s worth replacing the filter early to get maximum results.

Explainer Video: Choosing the Right Furnace Filter Size

The other thing to consider when choosing your furnace filter is your budget. Don’t get more than you need when it comes to a filter, but don’t skimp either. Remember, the most important thing is to change your filter regularly so it can work effectively.

Trying to save money by buying cheap filters or not replacing them often enough could ultimately lead to a struggling HVAC system. If your system has to work harder, you’ll end up costing yourself more money in the long run.

Source: Amazon

How To Determine Your Furnace Filter Dimensions

You can try measuring the filter compartment. Turn off the system, access the compartment, and use a tape measure to measure the length, width, and depth. Measuring your return will tell you exactly what you need. (Just in case your existing filter is the wrong size.)

However, if the existing filter fits correctly, it’s easier to measure it. If the filter isn’t labeled with nominal measurements, simply measure the length, width, and depth. Then, round up to the nearest whole number to get the nominal size you need for your new filter.

Explainer Video: Measuring Your Furnace Filter


Can I Stack Two Filters Together?

If you want to have the extra filtration provided by a thicker filter, then you need to buy a thicker filter. Trying to stack two thin filters will not give you the same filtration.

It could also restrict your airflow more than the thicker filter. Plus, it’s actually going to end up costing you more.

Two one-inch filters will cost you more than purchasing one two-inch filter. For example, a 12 x 24 x 1” filter might cost you $5. The same size, but 2 inches thick, might cost $8. It’s cheaper to get one for $8 than stack two $5 filters.

Wrapping Up Standard Furnace Filter Sizes

Using the right size furnace filter is essential to the optimal performance of your HVAC system and ideal air quality. Typically, filters feature nominal measurements on the side, making it easy to identify the size you need. As long as it fits properly in your unit, you can simply replace it with one of the same size.

The most common standard furnace filter sizes range from 10 x 20 x 1” (the most common) to 20 x 25 x 4”. However, you can find standard sizes smaller, larger, and thicker than these. Any standard size you need, you can find online or at stores like Home Depot or Lowe’s.

Remember, the nominal size on the filter is rounded up from the actual size, which is between ½-inch and ¼-inch smaller. So, if your filter doesn’t have numbers on it, measure the length, width, and depth. Then round up to the nearest whole number to get the nominal size.

Stacy Randall
Stacy Randall

Stacy Randall is a wife, mother, and freelance writer from NOLA that has always had a love for DIY projects, home organization, and making spaces beautiful. Together with her husband, she has been spending the last several years lovingly renovating her grandparent's former home, making it their own and learning a lot about life along the way.

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