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Actual Size vs. Nominal Size
(The Size Of Your Air Filter)

We at Sheridan Supply believe that when you are looking to replace your air filter, there are essentially two different sizes you need to understand – Actual Size and Nominal Size.

The Actual Size consists of the true dimensions of your air filter when you are measuring the length, width and thickness.  This usually includes a fraction of an inch. For example: 19 1/2 x 19 1/2 x 3/4

Air Filters are NOT sold by their actual dimensions!  They are actually rounded up and that number is what is used to label filter packages. This measurement is referred to as the “Nominal Size”.  So get a pencil ready and measure and write down your numbers!

So a filter with:
• Actual Dimension of 19 1/2 x 19 1/2 x 3/4
• Will have a Nominal Size of 20 x 20 x 1
• Nominal Size is the measurement to look for when buying your replacement filters


Nominal Size
Actual Dimensions

Example A
20 x 20 x 1
19 1/2 x 19 1/2 x 3/4

Example B
16 x 25 x 1
15 1/2 x 24 1/2 x 3/4

That’s it! You now know the difference between “Actual Size and Nominal Size”.