Finding the right measurement for your air filter is easier than you would think. All you need to do is find you filter size, either on the filter or through measuring yourself, and know the difference between nominal and actual dimensions.
Note: the length and width are interchangeable, so a 16" x 25" is the same as a 25" x 16".
Check Your Current Filter
The easiest way to find the size of a furnace filter is to look for the size that is printed on the air filter that is currently in use. When looking for the size of furnace filters, we are really looking for the nominal dimensions.
Measure it Yourself
If your filter does not have any visible nominal dimensions, you should measure your filter for length, width, and depth. Then round each actual dimension up to the nearest inch. The rounded up measurements are your nominal dimensions. Don't worry about the filter being too large, almost all filters are undercut slightly.