Clean Air, Filter Knowledge

Air Filter Sizes: Measure Once And Buy Faster

There are many different air filter sizes available, but how do you know what to buy?

There are many different air filter sizes available, because not all heating and cooling systems are made the same. Whether you are a commercial operation that needs to filter large volumes of air, or you need a smaller residential air filter to help keep the home clean, choosing the right size air filter is always important.

This sounds straightforward, but standard air filter dimensions are not what they seem. For example, a 20x25x1 air filter is not actually that size. If you use a tape measure, you will find that the actual air filter size is around 19.5×24.5x.75. The same applies to filters that are thicker than 1 inch, including those as thick as 6 inches.

So what’s going on? Is the manufacturer lying, or trying to save money by using less material? If you’re shopping for a replacement air filter, use the guide below to learn why actual air filter size might be different than what is listed on the packaging. Plus, you will learn how to find the right size filter so you don’t get air gaps, and you won’t find yourself in a situation where you need to return the filters for a different size.

Nominal And Actual Air Filter Sizes

The difference between nominal and actual air filter sizes is very simple.

  • Nominal – This is the number listed on the packaging. For example: 20x25x1. This is usually a whole number that is rounded up from the actual size.
  • Actual – This is the real measurement of the filter. It is typically a fraction and will be slightly smaller than the “nominal” size.

As a result of imperfections during the manufacturing process, the area where you install the air filter may not be a precise size. Meaning that a space intended to fit a 20x25x1 air filter will likely be a little smaller, though in some cases it may be larger.

If you are curious, filters are around .5 inches smaller than the nominal measurement on the packaging. The actual depth is around .25 inches thinner.

So, the reason air filter manufacturers mark their filters with the nominal measurement is to make it easy and less expensive to buy air filters. This is only possible when a single filter fits as many different heating and cooling systems as possible.

What Size Air Filter Do I Need?

You can do one of four things to figure out what size air filter you should buy.

  1. Take the air filter out of the heating and cooling system and determine the height and width by measuring it top to bottom and left to right.
  2. With the air filter in hand, look for a sticker or printing on the box that shows the nominal size.
  3. Measure the space where the air filter should install.
  4. Consult the manufacturer of the equipment where you install the air filter.

For most people, they will quickly find the air filter size they need, but in some situations, they need a filter with a custom size. For example, your filter may require a filter with the size 21.25×26.5x.75. In this situation, while rare, you would need to order a custom air filter.

Why These Dimensions Matter offers more than 250 air filter sizes for purchase! While this can seem overwhelming, the more options available to customers, the more likely you will find the exact filter you need to fit your needs.

In this blog, we covered the different sizes of filters, but not the types. If you want to learn about pleated, fiberglass, HEPA, or washable air filters, click below to find out more.

If you are ready to shop for air filters, use our Find My Filter tool, then select Auto Delivery to always have the air filters you need!

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