Finding the right measurement for your air filter is easier than you would think. All you need to do is find your filter size, either on the filter or through measuring yourself, and know the difference between nominal and actual dimensions.
Find Your Filter Size
Check Your Filter
The easiest way to find the size of a furnace filter is to look for a part number or size on the furnace 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.
Note: the length and width are interchangeable, so a 16″ x 25″ is the same as a 25″ x 16″.
Nominal vs. Actual Dimensions
Most filters are measured and sold using their nominal dimensions. The nominal dimensions are the rounded actual dimensions of the filter. The filters in the following example are nominally equivalent:
The nominal dimension is what you want to use when searching online or in store for a replacement furnace filter. Some websites, like DiscountFilters.com, will list both the nominal and the actual dimensions for the furnace filter since it is typical for the actual size to be undercut ½”.