What Are the Different Sizes of Air Filters?

The sizes of standard air filters may vary slightly, but they are usually half an inch smaller than advertised. Filters are labeled with three-part dimensions, which include the length, width, and depth or thickness. This is known as undercut, and all filters 1 and 2 are trimmed by a quarter of an inch to provide a little leeway and ensure it fits into the air return without being too big. This includes custom sizes, so a 27-3/8 x 23-5/8 x 2 is actually 27-1/8 x 23-3/8 x 1-¾.

This rounded measurement is known as the nominal size and is used to match the right filter to the corresponding air filter groove. From sizes to types, qualities and more, here's everything you need to know about air filters. Changing your HVAC filter is only part of the routine maintenance you have to perform on your system. But if your system needs custom-sized air filters, we offer custom sizes ranging from 8 x 24 x 1 to 25 x 30 x 1.The length, width and depth of a filter are printed in small numbers along the filter frame on the cardboard border of a Filter King brand filter. Actual size refers to the exact measurements of your air filter and the actual dimensions when you measure the length, width and depth (thickness).You can find the recommended filter size for your unit model in the table titled “Find your air conditioner and oven filter by make, model number and size”.

To understand the size of air filters, you first need to understand that there are two main types of air filters: standard and accordion. Standard filters must fit snugly into the air filter compartment with a little space around the edges. By using a properly sized filter and changing it regularly, you may experience cleaner indoor air and fewer system problems. If you measure your ventilation and find that you need a filter with a nominal size of 30x20, you will soon discover that there is no such thing as a 30x20. It's best to have some wiggle room in the slot of the HVAC unit, so you can easily slide the filter in and out without it bending or breaking.

Once you understand that nominal size is not an exact measurement, but rather a rounded number that makes searching for a filter and packing wood more efficient, you can deduce how it differs from actual size. There are also accordion air filters which are shaped like an accordion. The easiest way to identify the size of the air conditioner filter is to remove the existing filter from its slot and examine the filter frame.

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