What is the Difference Between a 20x25x1 Air Filter and Other Sizes?

The nominal size is the dimensions that are normally used to label air filters. These measurements are rounded values of the actual filter size, and all filters have two different sizes: an actual size and a nominal size. The actual size is the exact, unrounded dimensions, while the nominal size is the rounded dimensions of the filter. For example, on air filters 1 and 2, the nominal size does not include the notch, and the actual size does include it.

In whole-house filters, the nominal size is the rounded size and the actual size is just the unrounded size. It's important to note that all filters 1 and 2 have a quarter-inch cutout to leave some room and ensure that they fit the air return without being too big. If the filter is too large, it can cause air leaks, significantly reducing filter efficiency. On the other hand, if it's too small, it will come loose and some of the air (and debris) won't pass through the air filter. The MERV rating is a consumer standard that ensures all filters with the same rating filter in the same way. This rating provides specialized knowledge on topics related to domestic air filtration, such as HVAC filters, filtration efficiency, and indoor air quality. If you have one or two air filters that aren't on this list, chances are you'll need a custom cut.

The hardware store definitely can't help you in this case. It's best to measure it yourself if the dimensions are too faded to read or there is no air filter to look at. The 20x25x1 air filter is a very common and easy to find size. It's on a short list of sizes that are common enough for air filter companies to keep them in stock. The filter fits BlueAir Classic 480i, although not as tight as the original, but the flowing air doesn't whistle at any speed. In addition, if there is any resistance to airflow caused by using a filter that isn't the correct size, it can cause unnecessary stress on the air conditioning unit itself, which could damage the system equipment and increase your energy bills.

Why Is It Important To Use The Correct Size Air Filter?

When it comes to air filtration systems, using an incorrect size filter can have serious consequences.

Not only will it reduce your system's efficiency by not filtering out all of the particles in your home's air, but it can also cause damage to your HVAC system. Using an incorrect size filter can cause an imbalance in your system's airflow. This imbalance can lead to increased energy costs as your system has to work harder than necessary to move air through your home. Additionally, an incorrect size filter can cause damage to your HVAC system due to increased strain on its components.

How To Find The Right Size Air Filter

Finding the right size air filter for your home is essential for ensuring optimal performance from your HVAC system. The first step in finding the right size filter is measuring your existing filter or checking its label for its exact dimensions.

If you don't have an existing filter or its label has been damaged or removed, you can measure your HVAC system's return vent or ductwork for its exact dimensions. Once you have determined your existing filter's exact dimensions or measured your HVAC system's return vent or ductwork for its exact dimensions, you can then use this information to find a replacement filter with those exact dimensions. It's important to note that many manufacturers offer filters in both nominal and actual sizes so be sure to double-check before purchasing.


When it comes to selecting an air filter for your home, it's important to make sure you select one with the correct size. An incorrect size filter can lead to decreased efficiency from your HVAC system as well as increased energy costs due to increased strain on its components. To ensure optimal performance from your HVAC system, make sure you measure your existing filter or check its label for its exact dimensions before purchasing a replacement.

Ericka Boughner
Ericka Boughner

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