Its about 1/4 inch small. I was able to make a snug fit to ensure all the intake air goes through the filter (not the gap) by folding a long thin piece of cardboard from the shipping box and having it fit between the filter and the steel frame of the intake.
We put in a new HVAC system in our house. The air flow was twice what was generated in the old AC/gas furnace blower. It was amazing how much dust we had in the house and how great the filters have been removing those particles from the new system.
- had a hard to find size, 18x18x1 in MERV 13- easy to use website- not bombarded with emails- support excellent- arrived as expected- good quality- no complaints