Nominal Size vs Actual Size for Air Filters

Actual Size vs. Nominal Size

When measuring air filters, it is important to note that the nominal size is not the same as the actual size. Learn the difference between the two dimensions to find the correct size filter for your HVAC unit and properly capture airborne contaminants to keep your system running smoothly.

What is Actual Size?

The actual size for air filters is the exact dimensions of your filter, often shown in precise fractions, and is usually 1/2” – 3/4” less than the nominal size. This is also described as the filter’s “undercut”.

What is Nominal Size?

Nominal dimensions are the actual size rounded up or down to the nearest whole or half number. The nominal size is what you typically use when shopping for a replacement filter.

If your filter doesn’t have a size printed on it or if you don’t have a filter to reference you can follow the simple steps on our "How to Measure An Air Filter" blog to measure your existing air filter or air intake. 

View our standard size list, which includes our nominal and actual sizes.

Don’t see your size? No problem, you can build your custom filter here!