Furnace Air Filter Size: How to Measure and Find the Right Fit


Furnace air filters remove dust, dirt, and other pollutants from the air that enters your furnace. They can protect your furnace from damage, improve your indoor air quality, and lower your energy bills. But to get the most out of your furnace air filters, you need to find the right size for your furnace. 

If you use a filter that is too small or too big, it can reduce the airflow and efficiency of your furnace, cause noise or leakage, or damage its components. If you are looking for 16x20x1 furnace air filters or any other size, you need to know how to measure and find the right fit for your furnace. 

How to Measure Your Furnace Air Filter Size

To choose the correct furnace air filter, measure the existing filter or consult the furnace manual. Use a tape measure to determine the length, width, and thickness in inches. Here are the steps to follow:

  1. Turn off your furnace and locate your filter slot. It is usually located near the blower fan or the return air duct.
  2. Remove the old filter from the slot and place it on a flat surface.
  3. Use the tape measure to measure the filter’s length, width, and thickness. Write down the measurements on a piece of paper.
  4. Round up the measurements to the nearest inch. For example, if your filter is 15.5 inches long, round it up to 16 inches.
  5. Write down the size of your filter in length x width x thickness. For example, if your filter is 16 inches long, 20 inches wide, and 1 inch thick, write down 16x20x1.

How to Find the Right Fit for Your Furnace Air Filter

To find the right furnace air filter size, compare your measurements to online or local store sizes. Moreover, online tools or calculators can assist you in finding the correct size for your furnace. Here are some tips to follow:

  • Find a filter that matches or closely matches your measurements. For example, if your size is 16x20x1, look for 15.75×19.75×0.75 or similar.
  • Avoid filters that are too big or small for your furnace. Oversized filters can block airflow and damage the furnace. Undersized filters let unfiltered air in, affecting performance.
  • Check the compatibility of the filter with your furnace model and design. Some furnaces may have specific requirements or recommendations for the type and size of filters they can use. 

How to Replace Your Old Filter with a New One

After finding the right fit, you should change your filter regularly depending on its type, condition, and usage. A general rule of thumb is to change your basic filter every 30 days, pleated filters every 60 days, and high-efficiency filters every 90 days. 

Additionally, you can visually inspect your filter or use other indicators to determine when to change it. Here are the steps you should follow:

  1. Turn off your furnace and locate your filter slot.
  2. Remove the old filter from the slot and dispose of it properly.
  3. Insert the new filter into the slot with the arrow pointing toward the airflow.
  4. Secure the filter in place with clips or screws if needed.
  5. Turn on your furnace and check if it is working properly.


Finding the right size of furnace air filter for your furnace is easy if you follow these three simple steps: measure existing or slot size, understand the importance of size, and locate reliable sources for purchase. This ensures effective and efficient air filtration, enhancing indoor air quality and health.