So I've learned that "nominal size" is up for debate.So the filters I purchased are 16x25x4 just like what I needed, however, the product I received is much thinner in thickness than the name brand option. These filters are actually a full 1" thinner.The good news is that the width and height does cover my filter bay opening. I just am wondering how effective the thinner filter will be to reach the 13 rating.That was the other thing that I thought was interesting. When I received my filters, the label stated they were 8's. I immediately called to report this because I ordered 13's. I was reassured that if the check mark on the side was listed as 13"s then there were in fact what I ordered.I'm just not convinced that these are in fact 13's especially with the fact that the filter is a full 1" thinner.