"Actual vs Nominal Size"

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

First:The Actual Size consists of the true dimensions of your filter when measuring length, width and Depth(thickness). These dimensions typically includes a fraction of an inch within them.
For example: 19 1/2 x 19 1/2 x 3/4

Second: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".

Third:So a filter with actual dimensions of "19 1/2 x 19 1/2 x 3/4" will have a nominal size of "20 x 20 x 1", and that will be the measurement to look for when buying your replacement filters.

Method
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.

Back to top