What is the Actual Size of a 20x20 Air Filter?

The standard size for air filters is 20x20x1, but what does this actually mean? In reality, this size is known as an “undercut” and measures 19-¾ x 19-¾ x ¾. This includes custom sizes, so a 27 x 23 x 2 inch size is actually 27 inches x 23 inches x 1 ¾ inches. It's important to understand the nominal size (which is a round size often used to label the filter) and the actual size. The actual size is the exact size of your air filter and the actual dimensions when you measure the length, width, and depth (thickness).Forcing an incorrectly sized filter can cause it to warp, damage, or reduce its ability to function properly.

Whole house filters usually go to an air controller, which is usually located next to the air conditioning system itself. Different manufacturers cut whole-house filters to different sizes and then round them up for standardized measurements, meaning that two 20x25x5 household filters made by two different manufacturers are not likely to be the same size. The fact that the filter folds extend horizontally or vertically will not affect the efficiency of the filter, so the number of unique air filter sizes on the market can be halved by always indicating the smallest measurement first instead of having a 20 x 30 and a 30 x 20 as two completely different products. Household air filters are those that are inserted into the wall in the return of air; although, in some homes, they can go to the roof or to the ducts themselves. If the filter doesn't have the actual dimensions on the outside of the filter, you can measure the filter to get the correct dimensions. To understand the size of air filters, you must first understand that there are two main types of air filters: nominal size and actual size.

Nominal size is a round number often used to label a filter while actual size is what you get when you measure length, width, and depth (thickness).It's a very common size that's easy to find, and it's on a short list of sizes that are common enough for air filter companies like us to keep them in stock. From sizes to types, qualities, and more, here's everything you need to know about air filters. When you buy your air filter online, it's important to remember that air filters come in two sizes. If you have pets, children, or allergy sufferers in your home or business, it's important to replace filters more often in order to maintain higher indoor air quality and keep energy costs down. Keep in mind that when creating a filter with this tool, you must use the “real” size of the filter - that is, the size shown by the measuring tape - and not a nominal filter size.

Jason Voisin
Jason Voisin

Unapologetic beer advocate. Passionate tv fan. Award-winning zombie enthusiast. Incurable coffeeaholic. Typical troublemaker.