Nominal vs Actual Filter Size

When you are looking to find a new filter, there are two different sizes that you will need to be aware of. The Nominal Size and the Actual Size. You will want to make sure you are purchasing the correct actual size as well as the nominal size. 

 

The Nominal Size is the simplified size that is used to label the filter, which makes searching for a filter much easier. You’ll want to note that not all filters with the same nominal size have the same actual size. Most filters measure slightly smaller than the size they are labeled as. For example, our 20x25x4 Glasfloss ZL Series MERV 10 filter (20x25x4 being the Nominal size) measures as 19½ x 24½ x 3¾ inches (which is the Actual size). There are a few exceptions to this rule, such as the 20x25x4 MERV 10 Glasfloss Replacement for Honeywell, which measures 20 x 24-7/8 x 4-3/8 inches as the actual size. Both filters are considered 20x25x4 filters, but they are not interchangeable. The nominal size may make it a lot easier to find your filter, but it can also make it easy to pick the wrong size if you aren’t careful. 

 

The Actual Size of the filter is the exact measurements if you were to get out a tape measure and measure it yourself. Some brands print this number on the filter in smaller print underneath the nominal size. It is important to be aware of the actual size of your filter when ordering for the first time, instead of only going by the nominal size.