I bought a house a year ago with the 20-year-old furnace. The previous owners and the HVAC company were using 20/20x1. In the winter time I would collect a lot of dust and dust bunnies in the house. So I measured the opening and it measured 21x20x1. So I ordered a non-standard size from filter by and it has solved my problem No more heavy dust or dust bunnies.
I ordered several different filters that said they were the size for my Furnace but they didn't fit. Filterbuy was a perfect fit. This will be the place I'll purchase my future air filters.
Finally! I finally have filters to fit my builders a/c unit! I was about to think that I was always going to have to use a customizable filter. These filters fit and work perfectly!