What is the difference between Nominal and Actual filter sizes?
Generally speaking, filters are smaller in size than the size that is printed on them. Typically they are anywhere from ¼" to ¾" undersized. The dimensions printed on the filter is known as its Nominal size. The real physical size of the filter that you would find if you measured it with a tape measure is known as its Actual size.
The reason for this discrepancy is that when a particular system calls for a certain size, say 16 x 20 x 1, they are stating the size of the slot that the filter needs to go into. However, it would prove to be quite difficult to slide in a filter the exact size of the slot. The solution is quite simple, make the filter slightly smaller than the slot. Now the filter slides in with ease. The appropriate Actual size for this example is 15½ x 19½ x ¾.
Unfortunately, this discrepancy confuses things a bit when you are ordering filters. Most filters are marketed using their Nominal dimensions. For your convenience we also list the Actual dimensions for each of our filters. There are sometimes multiple Actual sizes for a Nominal size, making it hard to know which one you need. When this happens, we will tell you and automatically show you the Actual size that is most common for that Nominal size.
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