What Size Air Conditioner Filter Do I Need?

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Updated February 3, 2023

If you are new to the world of indoor cooling appliances, you may wonder what size air conditioner filter you need. The best air conditioners, after all, ship in a variety of designs, each requiring differently-sized air filters. So how do you tell what kind of filter you need for your AC model? Keep reading to find out.


  • Air conditioners come with many types of filters to capture airborne particles, each with its own actual size and nominal size.
  • Check your AC unit’s instructions to find out the exact air filter needed for a replacement to keep your indoor air quality high.
  • You can also take a look at an old furnace filter, or another type, before disposing of it to find the make, model, and size.

How to Find the Right Air Filter

Before you can learn what is wrong with a central air conditioner, or any type of AC, you should become acquainted with the air filters. The filters ensure efficiency if you are wondering what to do when an AC is not cooling. Additionally, all types of air conditioner systems require different types of filters, if you just learned what a casement air conditioner is.

Insider Tip

Some air conditioners include reusable filters that don’t have to be replaced. Just wash them every couple of months.

Here are some sure-fire ways to figure out what size air filter you need for your unit, even if you just learned the best air conditioner for a 14×70 mobile home.

Check the Manual

The first thing you should do is check the instruction manual that accompanies your AC unit, no matter if it is a window unit, a portable air conditioner, or a central AC. This manual will let you know the exact type of air filter required, with the size clearly marked. The manual may even feature a web address or a phone number so that you can order official replacement filters directly from the manufacturer.

Look at the Old Filters

The vast majority of air conditioners ship with any filters necessary for operation. In other words, when it is time to replace the filters, just look at the old ones before you throw them away. There should be a model number and other pertinent information, allowing you to purchase the exact same one for a replacement. Just perform a quick search of the model number and go from there.

STAT: For safety, turn off your HVAC unit before attempting to change the filter. If you can’t figure out how to turn off the unit, turn off the breaker. (source)

Conduct Some Measurements

If you can’t find the model number or you don’t have access to the instructions, it is time to conduct some simple measurements. Take a tape measure and measure the dimensions of the air filter and the opening itself. Once completed, buy the closest approximately-sized filter as a replacement. In some cases, you may need to cut down the new filter a bit to make it fit. Many air filters feature instructions for doing just that.


Need to replace your filter?

Replace your filter every one to three months, depending on how much you use the AC. If they are not regularly cleaned out (washable filters) or replaced (disposable filters), dust and debris can accumulate in a dirty air filter and negatively affect the performance of your air conditioning unit, regardless of the size filter you have. They can also become contaminants in the air of your home. Find the correct size of filter to the nearest inch and replace it as needed. Other aspects of regular maintenance being practiced will prolong the lifespan of the unit.

How tightly should my air filter fit?

You want to closely match the actual sizes of pre-existing filters. In other words, don’t force or bend the filter to make it fit. Try to get it close to the exact size or standard size to keep energy bills down.

Where is the furnace filter located? (return air intake)

For those with dirty filters in the furnace, look at the return air intake to find the old standard filter before issuing a replacement. Generally speaking, furnace filter sizes are thicker filters than AC filters.

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