If you have an old filter from your furnace that has its dimensions printed on it, size is not an issue for you. Simply order your new filters in the same size. FYI, though, those dimensions are not the actual dimensions of the opening in your furnace where the filter is installed. The dimensions are known as “nominal dimensions”—they are rounded up to the nearest inch measurement. The dimensions of the opening are known as “actual dimensions. For example, if the actual size of your opening is 15 ½” by 24 1/2”, the nominal dimensions of the filter you want is 16 by 25.
If you can't ascertain what the nominal dimensions of your filter are, measure the opening and round up to the nearest inch. For example, if the actual opening is 14 by 19 ½, the nominal dimensions you want to specify are 14 by 20. So, when it comes to furnace filters, size may be an issue, but it;s an issue that is easily resolved.