Measuring and Replacing Your Air Filters
All air filters have a nominal size and an actual size. The nominal size is a shorthand number rounded to the nearest whole or half inch, normally in a larger font, that helps simplify the purchasing process (ex: 20" x 20" x 1"). Whereas the actual size is an exact measurement, normally in a smaller font, that depicts the precise dimensions of the filter (ex: 19.5" x 19.5" x .75").
If your current air filter does not have a dimension listed on the frame, grab a tape measure, and record the length x width x depth to find the actual size. You can then round these measurements to the nearest inch to find a corresponding nominal size, which will be easier to locate when searching online.
As a rule of thumb, we recommend replacing your filter every 90 days to keep your system running efficiently and your air clean. However, this is subject to change depending on the filter size as well as other variable factors for pet owners, allergy-sufferers, households with children, etc.