Why Does Air Filter Size Matter?

Ever wonder why your home seems dusty even after you change the air filter? It could be a problem with the filter size! Using the right air filter size can improve your home's air quality, save you money on energy bills, and extend the lifespan of your HVAC system. Many people mistakenly think any air filter will do, but using an incorrect size can actually be detrimental. Here's why air filter size matters and how to avoid this common mistake. There's a better way to ensure you have the perfect-fitting air filter every time. Keep reading to find out how to get your perfect filter fit!

Key Takeaways 

  • Air filter dimensions are crucial for HVAC efficiency and air quality. 
  • The right size filter is essential for optimal HVAC performance. 
  • Incorrect filter sizes can have health implications such as poor air quality and respiratory issues. 
  • Choosing the correct air filter size can help save money on energy bills and prevent system strain. 

Choosing the Right Air Filter Size

It's essential to know how to choose the right air filter for your HVAC system. This process involves three key steps: 

1. Determine the Size: Check your HVAC system's manual or the old filter for the size. It's usually printed on the side and listed as length x width x height. 
2. Check the MERV Rating: Higher MERV ratings indicate better filtration but may cause your HVAC system to work harder. Balance your needs between air quality and system efficiency. 
3. Consider the Brand: Not all filters are created equal. Some brands offer better quality or specific features like allergen reduction

How To Measure Your Air Filter Size – Step by step instructions to measure your air filter size

What Air Filter Size Do I Need?

The best way to determine the correct air filter size you need is to check the dimensions of the existing filter in your HVAC system. Usually, it should be printed on the filter frame, and you can simply match the size when purchasing your new filter – but what do you do when you don’t already have an air filter in your HVAC system?

If you don't have an existing filter or can’t find the size of the current filter, you can measure the filter slot where the filter is installed. Another way to determine your air filter size is to check the owner's manual or the manufacturer's website.

How Do I Measure My Air Filter Size?

If your HVAC system didn’t come with an air filter or you’re purchasing air filters for the first time, you can find your air filter size by measuring the filter slot. Follow these steps to find the exact dimensions you need.

  1. Locate your air filter: It's usually behind a grille in your furnace or air conditioning unit.
  2. Check for printed dimensions: Look for the filter size printed on the side of the frame. There might be two sizes listed: a larger nominal size and a smaller actual size (refer to this if available).
  3. Measure yourself (if needed): If the size isn't printed, grab a measuring tape. Measure the length, width, and depth of the filter in inches. Round each measurement to the nearest whole or half inch.

Be sure to verify the dimensions by measuring multiple times and correctly rounding to your nominal air filter size. Filterbuy, as well as most brands, uses nominal size to label our filters. We recommend consulting your HVAC system manual or the manufacturer's website if you continue experiencing sizing issues. If you need additional help, refer to our article on air filter sizing.

What Is The Difference Between Nominal and Actual Air Filter Sizes?

The nominal filter size and actual filter size refer to two different measurements of an air filter. It’s typically printed on the filter packaging and frame. The nominal filter size is the actual filter size rounded up. The actual size is the precise measurement of the filter, which may be slightly smaller than the nominal size. For example, a filter with a nominal size of 20” x 20” x 1” may have an actual size of 19 ½” x 19 ½” x ¾”. This is because the nominal size is rounded up to the nearest inch.

It's important to consider both the nominal and actual size when selecting an air filter for your HVAC system. You should measure the filter slot in your system to determine the actual size of the filter you need and choose a filter with a nominal size that matches or is close to the actual size. This will ensure that the filter fits properly in your system and provides effective air filtration.

What Happens If I Use The Wrong Air Filter Size – Hands holding cash

The High Cost of the Wrong Filter Size

Using an air filter that's too small or too large can have serious consequences for your wallet, your health, and your comfort. Here's how the wrong size filter can cost you:

  • Frequent Repairs: A straining HVAC system due to improper filter size leads to wear and tear, requiring expensive repairs or even premature system replacement.
  • Increased Energy Bills: The wrong filter forces your system to work harder, consuming more energy and driving up your monthly bills.
  • Low Air Quality: Inadequate filtration allows pollutants to circulate, compromising indoor air quality and potentially triggering respiratory problems, allergies, and other health issues.
  • Restricted Airflow: A poorly fitting filter can trap debris, hindering airflow and making your system work harder to maintain temperature, leading to longer cooling or heating times.
  • Hot and Cold Spots: Inefficient filtration due to wrong size can result in uneven temperatures throughout your home, creating uncomfortable hot and cold zones.
  • Health Risks: Beyond discomfort, poor air quality from a bad filter can exacerbate respiratory problems, allergies, and other health issues.

Remember, the right filter size is key to optimal system health. Using the correct filter and replacing it regularly (at least every 3 months) maximizes efficiency, prevents costly repairs, reduces energy bills, and promotes a healthier and more comfortable home environment.

Is There A Difference Between Furnace and AC Filter Sizes?

While "furnace filter" and "AC filter" often describe the same air filter capturing dust and allergens, some homes have separate systems with unique filter sizes. Check your furnace and AC for individual filter slots to ensure you buy the correct size for each (if applicable). Using the wrong size filter reduces efficiency, air quality, and can even damage your HVAC system.

You can learn more about the difference between furnace and AC filter sizes here.

What If I Can’t Find My Air Filter Size?

No problem finding your filter size? Great! But for those with trickier situations, Filterbuy offers custom air filters built to your exact dimensions and MERV rating. Say goodbye to jamming or ill-fitting filters and hello to optimal efficiency and cost savings!

Custom Air Filters

Remember, a perfect fit isn't the only factor. Regularly replacing your air filter is crucial. Just like using the wrong size, neglecting replacements leads to poor air quality, higher energy bills, and potential system damage. Filterbuy's Autoship subscription delivers the right filter, right on time, for worry-free air quality maintenance.


Frequently Asked Questions 

Does the size of an air filter affect performance?

The size of your air filter significantly impacts your HVAC system's performance. An improperly sized filter won't capture all pollutants (too small) or restrict airflow (too large), leading to reduced efficiency, higher energy bills, potential system damage, and poorer indoor air quality - which can trigger health problems. Using the right size filter ensures optimal performance for your HVAC system, saving you money and creating a healthier and more comfortable home.

What happens if I use the wrong size air filter?

Using the wrong size air filter can lead to poorer air quality, higher energy bills, and potential damage to your HVAC system.

Is it OK to use a smaller air filter?

No, it's not okay to use a smaller air filter. A smaller filter won't cover the entire opening, allowing unfiltered air to bypass the system and circulate pollutants throughout your home.This can lead to poorer air quality, reduced efficiency, and potential strain on your HVAC system. Stick to the recommended size for optimal performance.

How often should I change my air filter for optimal performance? 

You should change your air filter every 3 months for optimal performance. However, if it's visibly dirty or dusty, change it sooner. Regular changes ensure clean air and your system's efficiency. 

Does the air filter size have any impact on the noise level of my HVAC system? 

Yes, the size of your air filter can impact the noise level of your HVAC system. If it's too small, it could strain the system, causing it to work harder and produce more noise. 

Can the wrong air filter size affect my HVAC system's warranty? 

Yes, using the wrong air filter size can affect your HVAC system's warranty. Manufacturers often void warranties for systems damaged by improper filters. It's crucial to use the right size for optimal performance and protection. 

Why Air Filter Sizes Matter – Filterbuy air filters on a desk