It's not as confusing as it sounds.
Often times, you will find two sizes on your air filters, a "nominal" size, and an "actual" size.
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 slight differences in their many filter sizes, the nominal size system ensures that you, the consumer, can fall back on industry standards and not have to have custom air filters made every time you need some clean air.
Think of it this way: what if every brand of smartphone had a different charger? Fortunately, manufacturers have agreed to standardize their cables (for the most part) so that all modern iPhone chargers are the same and most Android chargers are the same. Thankfully, the people who built your home agreed to follow a similar practice.
The actual size is just what it says: the actual size of your air filter to a very precise degree. The actual size is accurate to one eighth of one inch. So for example:
A standard filter with a nominal size of 14"x14"x1" might have an actual size of 13¾" x 13¾" x ¾".
This actual size would be a pain to search for, which is why the nominal measurement was created.
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 cardboard edge of your air filter.
From sizes to types to qualities and more, here is everything you could ever need to know about air filters.