When looking to replace your air filter, there are typically two different sizes you need to understand – Actual Size and Nominal Size. The Actual Size is the exact size of your air
filter, and the true dimensions when you measure the length, width, and Depth (thickness). The Actual Size typically includes a fraction number within the dimensions - for example: 19 1/2" x 19 1/2" x 3/4". What you might not realize
is that filters are not sold by their actual dimensions. Instead, a rounded up number is used to label filter packages, this measurement is referred to as the Nominal Size.
So when purchasing an filter:
If the Nominal Size is listed as 20" x 20" x 1"
The Actual Size (exact dimensions) of the air filter is 19 1/2" x 19 1/2" x 3/4"
The Nominal Size is the larger labelled measurement you see listed on the product. This is usually what you notice first, and what you look for when buying your replacement filter.
Want to know more? - Learn how to properly measure your air filter