Filter sizes are smaller than most common retailers.
I did not read the sizing detail when I ordered my filters. I had ordered two 14x20x1 earlier this year .. I thought they were a bit loose when I installed them but didn't think much about it. When it came time to replace (Summer) I just picked up 2 more (3M) 14x20x1 at either Costco or Lowes. They fit nice and snug. Two weeks ago, I decided to buy from FilterBuy again and these were so small (yeah, I measured .. they were 13.5 x 19.5 x .75) there was a very visible gap between one of the filters and the return hole it was covering. I sent FilterBuy an email to complain. I was humbled of course when the support person stated "One thing to consider about air filters is that, generally, they are listed in their nominal size. It means it is rounded up to the nearest inch. The actual size varies, thus, on our listing, we clearly state the actual size just below it." I admitted I was wrong for not reading and I will not attempt to rationalize anything here. I will make a statement however - I've lived in the same house for 25 years (several HVAC systems but same inlets and ductwork) - and I've NEVER had to care about filters not fitting IF I just purchased from Costco, Lowes, Home Depot, and once at a local grocery store. FilterBuy did nothing really wrong, and they were courteous but I will never buy from FilterBuy again.