Actual vs. Nominal Furnace Filter Measurements

For those of us who haven’t kept up with our math skills since high school, here are a couple refreshers to make your filter shopping experience a little easier!

All filters will have an actual size measurement on our site. This is exactly what it sounds like, the actual measurement of the filter. Typically measured out to a 1/8 of an inch. This measurement should match what you would get if you measure the filter at home with a tape measure.

An example of an actual measurement will read similar to this: 19.93″ x 24.68″ x 4.31″

Within all filters, there are standard sizes such as 16×20, 20×24, 20×25, (and a lot more). Nominal sizes will always be the actual measurement rounded to the closest standard size filter. Most times, this just means it is rounded up to the closet inch. However, with this technique, it can get a little tricky when it comes oddly shaped furnace filters that aren’t necessarily close to any standard size – sometimes they are categorized with a standard size that you wouldn’t automatically assume. Although for most cases, it is typically just a simple rounding of measurements.

For example, if the actual size is 19.8″ x 24.7″ x 4.3″ the nominal size will be 20″ x 25″ x 5″.