How to find the correct air filter size.
Choosing the correctly sized air filter is important in order to keep air clean in your home. An improperly sized filter may have limited performance. Determining the proper air filter size can be confusing, since filter packages generally display two sets of dimensions. It is important to understand that these dimension sets are the nominal and actual dimensions of the filter, and differences between the two.
Nominal Size:
The dimensions listed on the side of air filter frames and printed largest on air filter packaging are normally the nominal size dimensions. These measurements indicate the length, height, and depth (thickness) of the air filter, rounded up to the nearest inch. An example of nominal dimensions would be 16” x 24” x 1”.
Actual Size:
These are the exact dimensions of your air filter when measuring length, width, and depth (thickness). On air filter packaging this number is typically listed under or to the side of the nominal size, and is normally in a smaller font size. Most actual size measurements include fractions. An example of actual size dimensions would be: 15 ½” x 19 ¾” x ¾”. 
Air filters are grouped and sold by their nominal size, but it is important to know the actual dimensions in order to get the best fit. Different brands may have the same nominal sized air filters, but the actual size could be slightly different.
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