Filter Knowledge, HVAC, Uncategorized

Find the Air Filter Size Right for my HVAC System: Nominal vs Actual

We answer the question what air filter size I need.

Numbers dictate our budgets and aid us in crafting formulas to study the universe, but they also tell us what air filter size I need. Finding a filter that simply fits into my air filtration system is not enough. Installing the right size will help prevent air gaps and damage that results from stuffing the filter into the HVAC cabinet.

As a curious shopper you may find yourself inside a retail store looking over the rows of air filters with large printed dimensions. If you break out your tape measurer you will notice something almost immediately. The filter is slightly smaller than advertised. Take for example a 20x30x1 air filter – a popular size. The numbers listed on the packaging denotes the actual dimensions, whereas the nominal size (what you measured) might be anywhere from 1/8” to 1/2” smaller.

That is not an insignificant difference from what is listed, yet there is a reason for this discrepancy and it is not to intentionally mislead consumers. The air conditioning system and ductwork running throughout every building in the world is manufactured be one of perhaps thousands of different companies using their own materials and methods of production. As a result it is difficult to make a one size fits all filter. So as filter material moves down the line workers are advised to cut each HVAC filter smaller.

Here are a few examples of this in action (sizes shown are in inches):

(Nominal – Actual)

1. 20x30x1 – 19.5 x 29.5
2. 16x25x1 – 15.5 x 24.5
3. 12x24x1- 11.5 x 23.5
4. 20x25x1 – 19.5 x 24.5
5. 14x14x1 – 13.75 x 13.75

The above nominal sizes are what you would expect from the filters we ship. The actual size might be a little smaller or larger. Note that the depth of the filter might be up to 1/2” smaller than the 1” advertised.

Measuring the air filter size you need is relatively easy. Most home users will not need an exact size. Meaning that if filter does not entirely fill in the space inside the cabinet or wherever you are installing the filter, you won’t be losing anything. When your air conditioning system switches on, pressure from the fan will push the filter forward and create a fairly reliable seal. Just so long as you are not forcing the filter into the space, and there are no gaps where air could flow around the filter.

To figure out what size air filter you need you have 3 great options:

  1. Remove your old filter – Be careful not to shake lose any dirt on the filter. There should be printing or a sticker on the filter with the size. Reorder that size and enjoy.
  2. Measure the cabinet or area where the filter installs – You should come close to a round number like one listed above. Choose a filter closest in size to what you measured so long as the actual size is smaller than the space allotted.
  3. Refer to the manufacturer supplied information or contact them directly – There is a good chance that somewhere on your equipment is the name of the manufacturer or contact information.

Replacing HVAC filters should not be a challenge. Except there are no guarantees that the nominal size on the label will match the actual dimensions. Just remember that when you shop for your air filters, buy the same size that has worked for you in the past or measure the area where you need to install the filter and check that the actual size is a little smaller.

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