Air filters come in many shapes and sizes and are referred to in different ways; Nominal size and Actual size. Let's talk about the difference in these sizes.
Actual Size: This is the exact demensions of the air filter. For example, when you measure your filter you may find that it measures 15-1/2" x 24-1/2" x 7/8" (15.5" x 24.5" x .875"). This size is important to know, but unfortunately, most air filters are not labeled this way. Most filters (but not all) are a slight bit smaller than the size they are labeled as. That's where the Nominal Size comes in.
Nominal Size: This is the size that the air filter is labeled as, so the 15-1/2" x 24-1/2" x 7/8" (15.5" x 24.5" x .875") in our previous example would be labeled as a 16x25x1, this is the filter's nominal size and how you would order it.
One exception would be some air cleaner filters. occasionally these can be larger than the nominal size. For example, a Honeywell FC100A1003 is labeled as a 16x25x4 air filter, but the actual size of this filter is 16" x 19-7/8" x 4-3/8" (16" x 19.875" x 4.375") which is thicker than the 4 inch nominal listing. So you should take extra care in ordering replacement filters if you have a name brand air cleaner.