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Nominal Size vs. Actual Size: What Does It Mean

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 because often times there are more than one actual size to nominal size.**


Both Carrier replacement filters below have a nominal size of 20x25x5 but two completely different actual sizes.