What are nominal sizes?
Nominal size is the size you see printed on the side of your air filter. It stems from the actual size, which is rounded up to the nearest whole inch. There are hundreds of filter sizes and although manufacturers may make slight changes and offer very sight difference in their many filter sizes. the nominal size system ensures that you. the consumenr, can fall back on industry standards and not have to have cuatom air filters made every time you need some clean air.
Think of it this way: what if every brand of smart phone had a different charger? Fortunately, manufactures have agreed to standardize their cables(for the most part) so that all modern pnone charges are the same. Thankfully, the people who built your home agreed to follow a similar practice.
What are actual sizes?
The actual size is just what it says: the actual size of your air filter to very precise degree. The catual size is accurate to one eighth of one inch so for example:
Astandard filter with a nominal size of 14"x 14"x 1" might have an actual size of 133/4' x 13 3/4" x 3/4'
This actual size would be a pain to search for, which is why the nominal measurement was created.
So which one do I use?
The nominal size is the only one you really need to know when you look for air filters and it is typically printed on the card board edge of your air filter.