How to Size a Filter in an HVAC System

Properly sizing an air filter for a HVAC (heating, ventilating and air conditioning) system will not only make the unit efficient but keep the air inside the home cleaner and healthier.
Air filters are usually placed just inside the HVAC air intake grill. This allows for easy access to the filter and keeps the entire duct system clean.

Step 1

Understand that the size of the air filter is based solely on the air movement of the blower fan contained inside the HVAC unit. This air movement is measured in cubic feet per minute (CFM). The CFM of HVAC units can be found on the side of the blower fan housing. Typically it will be placed on a metal tag. For example, it may read 2,000 CFM.

Step 2

Find the required size for the air filter. A safe recommendation for air filters is to use a base formula of 2 CFM of airflow for every square inch of disposable air filter. This number ensures that the air filter will not clog too fast or collapse in the air stream. For example, divide 2,000 CFM by 2. The result is equal to 1,000 square inches of air filter material.

Step 3

Find a commercially equivalent filter to match this surface area. In described example, two air filters that are 20 inches by 25 inches (500 square inches each) should be used.

Things You Will Need

  • Cubic feet per minute of HVAC unit


  • It is always better to oversize the filter intake area than under size the opening.