My Rheem apparently requires an oddball size filter. Filterbuy makes them precisely the custom size it needs, and they're made in the USA. This is my 3rd order over 3 years (I change them every 3 months), and they're perfect every time, right down to the millimeter.
I bought the Merv 11 its amazing. No dust at all and its been 2 weeks. Unfortunately, I measured without slotting room for the metal door to close . Its a little too big. It did force it inside but for the money I should have been more careful. It needed to be 1/2 inch smaller
Actual size is less than normal filters with this dimension (by around 1/4") leaving a gap around the edges and resulting in an air leak; not ideal for air filters. 20 x 20s are not actually 20 x 20 but it was my assumption that all 20 x 20s would be the same. My mistake, I guess, but irritating. I'll tape around the sides but I shouldn't have to do that.