nominal vs actual dimensions

Nominal vs. Actual Sizes

When it’s time to change your HVAC system’s air filter, it is important to order the correct size. Typically, looking on the old filter or referring to the owner’s manual are the two best ways to determine the size filter that a heating and cooling system requires. If neither of those work, you can measure the filter yourself.

When dealing with filter sizes there are two terms you need to know: Nominal Dimensions and Actual Dimensions. It is important to know the difference between the two because filters are sold by their nominal dimensions.

Nominal Dimensions

Most air filters have numbers listed on side. These numbers are the filter size by height, width, and thickness. A filter that has the number 16 x 25 x 1 would be read as sixteen-by-twenty-five-by-one. These are the nominal measurements of the filter, which are measured in inches. The first two numbers are the length of the sides of the filter. The last number is the thickness of the filter. A 25 x 16 x 1 is the same size filter as a 16 x 25 x 1. The first two numbers, the height and width, are interchangeable.

Filter Sizes

A nominal filter measurement is the actual dimensions rounded up to the nearest 1-inch.

Actual Dimensions

The actual dimension of an air filter is its exact size. Generally, actual dimensions are smaller numbers than nominal measurements. A 16 x 25 x 1 nominal measurement may have the actual dimensions of 15 1/2 x 24 1/2 x 3/4. To determine the actual measurements of an air filter, simply measure each side of the filter with a tape measure.

This chart visualizes how nominal and actual dimensions work together:

nominal vs actual sizes

Fitting a Filter into the Slot

Furnace filters need to fit snugly but should never have to be forced into position. If you have to force the filter into its slot, then it is too big. Forcing in a filter that does not fit properly can cause it to buckle, destroying the filter or reducing its ability to function properly. Remember, filters need to be a little bit smaller than their slot to allow for easy removal of the old and installation of the new. Some HVAC units may need a filter with dimensions that are unique or uncommon. In these cases, a custom filter needs to be ordered.

If you are unsure what size air filter your HVAC system needs, contact one of our Filter Specialists. We can help you determine the size and best filter for your system to satisfy your filtration needs.

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6 thoughts on “Nominal vs. Actual Sizes”

  1. I just knew the basics, and I have already learned some important things. I have also learned how much more I need to know and about how many more areas I have to know about. I just started reading and I have a whole lot more to learn, then do, to really know the basics!
    I am so happy I found you! Thank you. Jeannie

    • Hi Gary,

      We have custom filter options. Contact our customer service team at 888-407-5688 and they’ll be able to get you what you need.


  2. I do not have a slot, my filter leans up against the opening and I was told to have a 20×20. Since my filter leans against the side wall, I think the depth of the filter (1″, 2″, 3″ etc.) is flexible. What criteria do I use to choose the depth of the filter?


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