A Guide To Furnace Filter Sizes (Nominal Size vs. Actual Size)

Choosing the correct filter size seems straight forward but there actually some common traps that can be a bit misleading.

Some of our customers either accidentally purchase the incorrect filter size or worse, do not even realize they have the incorrect filter size.

Why is it so important to choose the correct filter size?

Too small - air can leak out the sides of your filter, meaning dust/particles will not be trapped.

Too big - your filter may not even fit where it is supposed to go.

Either mistake - can be quite costly because you would have to ship the incorrect filters back to us (and believe us, it's not cheap to ship filters).

So in order to avoid selecting the incorrect filter, follow the simple steps below.

Nominal Size vs. Actual Size

The most common mistake we see is customer confusing nominal size and actual size.

Nominal size is the nice "rounded" number that makes it easy to market and categorize filters.

Actual size is the exact dimensions of the filter .

For example, 20x20x1 is considered the nominal size. But taking a closer look at the exact dimensions, the actual size is 19¹¹/¹⁶ x 19¹¹/¹⁶ x ¾.

Your filter or HVAC unit should have both the nominal and the actual size written on it. If you can't find the actual size, it's a good idea to measure it just in case.

Our website categorizes filters by the nominal size, making it easier to navigate and find your filter.

Finding Your Filter Size on PureFilters

After you have both your nominal size and actual size handy, head over to our website.

On the home page, first select the product type (furnace filters), then select the nominal furnace filter size (thickness) of your filter. It's the last number written on your filter, usually between 1 to 6.

In the example picture above the size is "1 inch".

Next up you need to select the nominal dimensions (length and width) of your filter. It's the first 2 numbers on your filter, there are several dimensions we carry, so scroll down until you find yours.

In the example picture above the dimensions is "20x20".

If you cannot find your nominal size/dimensions by following the instructions above, don't worry, we make custom size filters as well.

To request a custom size filter, please inquire here. One of our indoor air quality experts will followup with a quote.

An alternate method in finding your filter is searching by model number.

Searching by Model Number

Many branded filters have a model number, usually written on the filter or on the HVAC unit.

An example of a model number would be "Lennox X6670".

You can search this model number on our website in the top right corner.

Product Page

Once you have found the product that you believe is the correct size for your HVAC unit, you can scroll down and see the actual measurements in the description section.

Usually if the exact measurements are within an eighth of an inch, you should be okay.


If all else fails, and you are still unsure about the size of your furnace filter, don't hesitate to call us at 1-888-979-4284 or email us at support@purefilters.ca and one of our air quality experts will be glad to help you!