Actual size vs. Nominal Size

Learn the difference between the two air filter dimensions and make your purchase with confidence

Actual vs. Nominal Size

Nominal means "in name only," and is used in many industries – including HVAC and lumber – as a way of naming standardized dimensions. So, the nominal size is the one you'll look for when shopping for an AC or furnace filter. Read below for more insight.

When looking to replace your air filter, there are typically two different sizes you need to understand – Actual Size and Nominal Size. The Actual Size is the exact size of your air filter and the true dimensions when you measure the length, width, and depth (thickness). The Actual Size often includes a fraction number within the dimensions – for example, 19 1/2" x 19 1/2" x 3/4". Then the number is typically rounded up and used to label air filter packages; this measurement is referred to as the Nominal Size.

So when purchasing an AC or furnace filter:

  • If the Nominal Size is listed as 20" x 20" x 1", the Actual Size (exact dimensions) of the air filter is 19 1/2" x 19 1/2" x 3/4"
  • The Nominal Size is the larger labeled measurement you see listed on the product. This is usually what you notice first and what you look for when buying your replacement filter.

Be sure to check out our other Helpful Tips pages for more information about maintaining your business or home's HVAC system.