In the filter industry there are two sizes of a filter — Nominal Size & Actual Size. It is important to understand the difference between nominal and actual size when shopping for a filter.
Nominal Size is the printed, labeled or rounded size of the filter. This size will be displayed on the air filter. This is not the true size of your filter. It is usually a rounded size, for example 16x20x1. A nominal size can have several different actual sizes.
Actual Size is the true size of your filter. If you were to measure your filter, you are getting the actual size of the filter. If this size is listed on the air filter, typically it’s listed below or beside the nominal size in smaller print. An example of actual size is 15.75x19.75x0.75. If “actual size” is printed it means the actual size of the filter is the same as the nominal size.
When purchasing a filter, be sure you are getting the filter with the correct actual size. Nominal size can be deceiving as often there is more than one actual size for a nominal size.
Example:
The 1" filter below is labeled 16×25×1. This is its nominal size, but its actual size is closer to 15.75×24.75×.75, which can vary slightly by manufacturer and affect proper fit.
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