Understanding Furnace Filter Dimensions

Usually, people are confused about the size and dimensions of their air filters. However, there is a list of standard or very commonly used commercial air filter sizes that you can use as a quick reference and hopefully you will get what you are looking for.

Air Filter Standard Dimensions

As a usual practice, commercial air filters while resizing are cut nominally. Below are the industry standards:

10 X 20 14 X 20 16 X 24 18 X 30
12 X 12 14 X 24 16 X 25 20 X 20
12 X 20 14 X 25 18 X 18 20 X 24
12 X 24 14 X 30 18 X 20 20 X 25
12 X 30 15 X 20 18 X 24 20 X 30
12 X 36 16 X 20 18 X 25 24 X 24
25 X 25

Replacing a Commercial Air Filter

There are a few things that you should consider before replacing your commercial air filter. These are:

  • Make a check that your furnace already has a filter
  • Make a check if the existing air filter’s size is fitting perfectly
  • Have a through look if the corners of the air filter have been squashed to fit it in properly

How to Measure your Commercial Air Filter

Before you measure your commercial air filter, remember there is a difference between the actual size and the nominal size.

An actual size comprises of the true dimensions of the air filter. This is usually:
19 ½ x 19 ½ x ¾

On the other hand, nominal size comes out after the manufacturer round off the above numbers. This is:
20 x 20 x 1

It is crucial for you to change your air filters between 30 and 90 days of its usage. This depends on the type, usability and thus efficiency of your air filters. This change will allow you to keep a proper cooling and heating system at your home and ensure good air quality. Not merely this, but it will also help you to lower your electricity bills as efficient air filters will help consume less energy while enhancing the life of your system. The best time to examine and change your air filters is during the months when the summers are at peak and same when the winters are on the extreme. This is because it is the time when your system is functioning frequently as well as at its highest capacity.

Below are some easy steps that will guide you to measure your commercial air filter’s size properly:

  1. Measure the length and width of your air filter. Make sure to put the smallest measurement/size first in case your air filter is not in a square shape. In this step, you take L x W dimensions.
  2. Next, you will need to measure the depth or thickness of the air filter. This measures
  3. Lastly, merge all the above dimensions together in this sequence – L x W x D


If you are a furnace technician, you need to know what filter sizes to use per system. If you are a homeowner, it is just as important that you know the right size for your unit. Whenever you are unsure of what size to actually use, you can now refer back to this article.

Tip: Download the page for quick reference.