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

When looking to replace your air filter, there are typically two different sizes you need to understand – Actual Size and Nominal Size.

Actual Size consisting of the true dimensions of your filter when measuring length, width and thickness, and typically includes a fraction of an inch. For example: 19 1/2 x 19 1/2 x 3/4

What you might not realize is that filters are not sold by their actual dimensions. Instead a rounded up number is used to label filter packages. This measurement is referred to as the "Nominal Size".

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".

Want to know more? Learn what a MERV Rating is.