The Ultimate Guide to Air Filter Sizes

The Ultimate Guide to Air Filter Sizes

Does Size Matter? 

You’ve guessed it! The answer is yes. It is essential to use the right size air filter for your HVAC unit to keep your system running smoothly and to reap all the benefits your air filter has to offer. 

If your filter is not the right size, either too small or too big, air will flow around the filter rather than through it, and as a result—not all of the air will get filtered. 

For many people, air filtration is a confusing, seldom talked about topic where questions often go unasked and unanswered.

We’ve created a simple, but comprehensive sizing guide to help you find the right filter size for your unit without a shadow of a doubt:

  1. Nominal vs. Actual Sizes
  2. 3 Ways to Find Your Air Filter Size
  3. How Tightly Should My Air Filter Fit?
  4. Best Places to Buy Air Filters
  5. Standard Size Not Fitting Well? Try A Custom Size
  6. Can you stack air filters if you need a thicker filter?
  7. Colorfil’s Nominal vs. Actual Size Chart

Nominal vs. Actual Sizes

nominal vs exact hvac air furnace filter size

When shopping for an air filter, it is important to keep in mind that the air filtration industry standard is to make filters measure slightly smaller than their slot or frame, so they can easily slide into place without getting bent or smashed. 

  • Nominal Size: The nominal size of an air filter is the big, bold, advertised size. The word “nominal” comes from the root word “name” and is the rounded-up whole-number version of the actual, measured size. 

  • Example: 16x25x1”

  • Actual Size: The actual size of an air filter is typically written in the fine print on the frame of the filter and is usually 0.25"-0.5" smaller than the advertised, nominal size. The actual size is what you’d find if you used a ruler to measure the exact dimensions of the filter. Actual sizes vary slightly depending on the brand, but in most cases, any standard size will work in a standard residential unit. 

  • Example: 15.5x24.5x0.75” or 15.75x24.75x0.75” 

  • True To Size: Oftentimes, less common sizes come true to size, meaning the nominal size is the same as the actual size. When the nominal size is the same as the actual measurements of the filter, the size is listed with “actual size” in parentheses to indicate the filter is true to size. 

  • Example: 20x21x1” (Actual Size)

    3 Ways to Find Your Air Filter Size: 

    There are three ways to find your furnace filter size: a) refer to the label on the filter’s frame, b) measure your filter, c) measure your air intake.

    A) Refer to the label on the filter’s frame. 

    The easiest way to identify your AC filter size is to remove the existing filter from its slot and examine the frame of the filter. Usually, the filter size is printed along the edge of the filter’s frame. Note that the size listed in bold on the frame of the filter is the filter’s nominal size, which likely differs from its actual size. If the actual dimensions are not listed in fine print on the frame of the filter, we recommend measuring the filter to double check.

    B) Measure your air filter.

      When in doubt, you can always measure your air filter yourself using a ruler or any other measuring device. If your filter’s size is not labeled at all, or you are unsure of the actual dimensions, follow the steps below to measure your air filter: 

      How to Measure Your Air Filter: 

      how to measure air filter size

      Step 1: Turn Your AC Off.
      As a safety precaution, first, adjust your thermostat to turn your AC handler off before removing your furnace filter. 
      Step 2: Measure Your Filter.
      Use a ruler or other measuring device to measure the exact length, width, and depth (thickness) of your air filter. 
      Example: Length = 15.5”, Width = 24.6”, Depth = 0.75”
      Step 3: Round Up.
      Round up each dimension to the nearest whole number to determine your filter’s nominal size. 
      Example: Length = 16”, Width = 25”, Depth = 1” 
      Note: Before you go ahead and buy the same size furnace filter that was installed previously, ask yourself: is this actually the right size for my furnace? If you are suspicious that the size you had installed previously may not be the right size for your system, we recommend you measure your air intake.
      C) Measure your air intake. 

        To ensure the next filter you purchase is the correct size for your system, you should measure the interior dimensions of your air filter intake. 

        How To Measure Your Air Intake: 

        Step 1: Turn Your AC Off.

        Once again, before you go messing around in your HVAC unit handler or central air return vent, first adjust your thermostat to turn your AC system off. 

        Step 2: Measure Your Intake’s Inside Edge. 

        Then, use a measuring tape or other measuring device to measure the length, width, and depth of the inside edge of the air filter’s slot or frame. 

        Step 3: Round Down. 

        In this case, you’ll want to round down to the nearest whole number to determine the nominal size of your air filter. The actual size of your air filter should be 0.25”-0.5” smaller than the slot or frame itself. 

        We recommend trying to use methods A or B, referring to the sizes on your old filter or measuring your old filter first, as measuring the air intake is more difficult, and might not produce the most accurate measurements.

        How Tightly Should My Air Filter Fit? 

        If you have to bend, force, or jam your air filter into place: your filter is too big. On the other hand, if your filter does not come within 1-2 fingers’ width of all four corners of your air system handler’s slot or frame: your filter is too small. 

        HVAC unit slots are usually found near the bottom of your air handler. It is better to have a little wiggle room in your HVAC unit slot, so you can easily slide the filter in and out without it getting bent or smashed. If you have to jam your air filter in to install it, you should try a different furnace filter brand or custom size air filter with a slightly smaller actual size. 

        For common HVAC system brands like Goodman, Lennox, Trane, Rheem York, Coleman, and Carrier, most standard air filter sizes will work. You may be able to find the filter size that is recommended for your unit model here in the table titled “Find Your AC and Furnace Filter by Brand, Model Number and Size”. 

        Central air return vents are usually found in the hallway ceiling or hallway wall of your home. If your return vent is in the ceiling, you may want to go with a filter that has a slightly larger actual size, so the filter doesn’t fall down on you when you open the ventilation grate. 

        If you live in an older home, with a non-standard sized air return frame, you may want to use adhesive foam weather-stripping tape to slightly increase the dimensions of your preferred filter or order a custom size air filter. 

        Best Places To Buy Air Filters 

        Most white, pleated air filters are inherently the same within each efficiency level—varying slightly, if at all, between brands. There are lots of options out there to purchase air filters with convenience, whether that is a) online, b) via your HVAC service company, or c) in-store. There are some pros and cons to each: 

        A) Online Sellers

          Due to the current state of the world we are all living in, online shopping has become the new normal—now more than ever. For your own safety and the safety of others, it is wise to limit the number of trips you make to the store and to only invite service technicians into your home when necessary for your annual HVAC maintenance check-up, as recommended by Hannabery HVAC. There are a number of excellent choices out there to buy air filters online and have them conveniently delivered to your door. In order to compete with Amazon, Colorfil offers free 2-day shipping as well! 

          Top 5 Places to Buy Air Filters Online 2020: 

          1. Amazon - largest range of brands, quality, and price
          2. Colorfil - most advanced technology and quality customer service with 100% satisfaction guarantee
          3. Second Nature - scheduled subscription service
          4. DiscountFilters - offers both water and air filters
          5. AirFilters Delivered - large filter selection and easy to use 

          B) HVAC Service Companies

            HVAC service companies typically provide air filter replacement services in addition to system repair and unit installation services. Since the HVAC technicians come into your home and evaluate your specific unit, they know what type of filter your furnace can handle and what is best for your system. However, typically they add an additional surcharge to the filter to compensate for the services they provided. Another caveat is, given vendor-supplier relationships, they might be incentivized to upsell on certain types or brands of air filters. 

            C) In-Store Retailers 

              Most big-box retailers, such as Walmart, Target, and Home Depot, carry air filters, though with a different variety of options and brands to choose from. For example, Home Depot primarily carries Honeywell brand air filters, while ACE Hardware primarily carries Filtrete. Given the limited shelf space, retailers typically only carry the most common 10-15 sizes in-store. If you can’t find your size in-store or need a custom-made air filter, online sellers are likely the best option for you. 

              Top 5 Places to Buy Air Filters In-Store 2020: 

              1. Home Depot 
              2. Walmart 
              3. ACE Hardware 
              4. Lowe’s 
              5. Menard’s

              Standard Size Not Fitting Well? Try A Custom Size. 

              If you are looking for a retailer near you that sells custom size air filters, you may be hard-pressed for luck. However, there are a number of online retailers that make custom size air filters delivered to your door—some at no extra cost!

              When purchasing a custom size air filter, it’s important to remember the difference between nominal vs. actual sizes. Once you determine the exact dimensions that work best for you, place an order for your filter size to the exact fraction or decimal of your preferred measurements. Otherwise, you may get a different actual size than you were expecting. 

              At, our customer service team reaches out to you directly if we need to clarify the exact dimensions for your custom order to get you the perfect fit on the first try. 

              Can You Stack Air Filters?

              stacking filters

              Do not stack multiple smaller air filters on top of one another to fit them into a larger air handler. Installing multiple 1” air filters in a 2” slot, for example, can cause costly damage to your AC unit, as residential furnaces are not equipped to handle this level of airflow restriction. 

              You wouldn’t want to put one 1" air filter in a 2" slot either because air takes the path of least resistance, and will flow around the filter rather than through it if it is not the right size. 

              Colorfil filters, originally built for NASA, help eliminate chemicals and odors at the molecular level that regular air filters can’t. Learn more about the next generation of air filtration technology at



              Colorfil’s Nominal vs. Actual Size Chart 

              Nominal Size (inch)
              Actual Size (inch)


              Go back to the product page by clicking here.


              Atomic Filters

              Hannabery HVAC

              NASA Spinoff


              Learn More  | Shop Colorfil |  Contact Us 

              Back to blog

              Leave a comment

              Please note, comments need to be approved before they are published.