How To Measure Air Filters

Finding the right measurement for your air filter is easier than you would think. All you need to do is find your filter size, either on the filter or through measuring yourself, and know the difference between nominal and actual dimensions.

Find Your Filter Size

Check Your Filter

The easiest way to find the size of a furnace filter is to look for a part number or size on the furnace filter that is currently in use.  When looking for the size of furnace filters, we are really looking for the nominal dimensions.

Understand the difference between actual filter size and nominal filter size to find the right size filter for your air conditioner or furnace.

Measure It Yourself

If your filter does not have any visible nominal dimensions, you should measure your filter for length, width, and depth. Then round each actual dimension up to the nearest inch. The rounded up measurements are your nominal dimensions. Don’t worry about the filter being too large, almost all filters are undercut slightly.

Measuring Furnace Filter

Note: the length and width are interchangeable, so a 16″ x 25″ is the same as a 25″ x 16″.

Nominal vs. Actual Dimensions

Most filters are measured and sold using their nominal dimensions.  The nominal dimensions are the rounded actual dimensions of the filter.  The filters in the following example are nominally equivalent:

nominal vs actual

The nominal dimension is what you want to use when searching online or in store for a replacement furnace filter. Some websites, like DiscountFilters.com, will list both the nominal and the actual dimensions for the furnace filter since it is typical for the actual size to be undercut ½”.

All that is left to do is order your replacement air filters. If you still have questions feel free to reach out to our customer service online or on the phone at 888-407-5688.

Find the air filter, furnace filter, a/c filter, air conditioning filter, air conditioner filter, ac filter you need at DiscountFilters.com

19 thoughts on “How To Measure Air Filters

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  5. Jerry Schaper

    Have Carrier furnance Model 25HCB3/25HCC5 Actual size 19 1/8 x 19 3/16 x 4 5/16
    Need 2 Do you have in stock and PRICE

  6. Katie Post author

    Hello Jerry,
    It sounds like you are looking for a nominal size 20x20x5 filter.We have multiple options for that size filter here. You will need to narrow down by the style you are trying to replace. Feel free to reach out to our customer service department at info@discountfilters.com or by calling (888) 407-5688 if you need more help identifying the correct filter for you.

    Have a great day and hope this helps!

    1. Katie Post author

      Hello Dorinda,
      We do offer a selection of 20x20x1 air filters on our site. You can find them here. Let us know if you need any more help.

      -Katie

    1. Katie Post author

      Karen,
      I am looking into your question. I need a little more information. What is the width of your filter? Your filter could be 25x30x1 or 25x30x2 going up to 5″ width. We don’t want you to order the wrong size filter since ordering too small could leave gaps reducing efficiency.

      Thanks,
      Katie

  7. Matt

    I ordered the furnace 16x18x1 MERV 11 pleated filter (the yellow ones) which is supposed to be hepa-grade; I had previously been ordering the same size in a lesser quality from your site; the problem is that while the prior filters were probably a quarter inch short on the measurements, the current ones are 3/4 to almost an inch short on their dimensions, causing an excess of air to flow around them, lessening their effectiveness; is there anyway you can refer this to the manufacturer and ask them to increase the dimensions by at least a 1/4 inch?

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