This is a common question. Air filters are sold in nominal dimensions. That is to say the printed or advertised dimensions are approximate. The actual dimensions of the filter will be determined by the manufacturer of your filter cabinet.
A good example is Honeywell filters. Honeywell was one of the originators of the "Media" filter, which is referenced also as "Deep Pleats". These are filters that are advertised and printed as being either 4" thick or 5" thick. But the actual dimensions of a Honeywell filter are neither 4" nor 5". The actual measured thickness of a Honeywell filter is 4 -3/8" (actually, none of the dimensions of the filter are what they are printed or advertised to be, but that is a topic for another day).
Since Honeywell was an innovator of media filter cabinets, many other manufacturer's cabinets were designed by or modeled after the Honeywell cabinet. Some of the manufacturers cabinets who use the "Honeywell" filter design are Lennox, Bryant, and Carrier, as well as a few others.
Confusing? It can be. But once you know who manufactures your filter cabinet you can easily identify the correct filter for your system. If you need help finding the correct filter for your system, please click here and we will be happy to assist.