Furnace Filter Sizes

Everything You Need to Know About Furnace Filters

Sharing is caring!

Your furnace just might be the most important appliance in your home.

You rely on it to keep your home comfortable year-round, by providing heat in the winter and cool in the summer months. Sadly, it is one of the most commonly ignored appliances in any house, that is until it malfunctions or fails to perform up to par.

If you don’t perform routine maintenance on your furnace, you might end up regretting the oversight, since you’ll find that it will become susceptible to failure, especially when it’s working hardest against the weather outside in your area.

One of the most vital parts of the furnace that you must pay attention to is your furnace filter.

Of course, furnace filters are not just a ‘one size fits all’ item. There is a wide variety of filter sizes out there, and before you prepare to replace your existing filter, you need information on both the type and size of the filter you want to buy.

To make that assessment, you must first know the purpose of a furnace filter and how it works once you install it.

How a Furnace Works

Woman replacing the filter

The average furnace in a home is a forced air unit. This type of system draws air into the furnace through the return ducts, and then it pushes that air over a heat exchanger using a blower fan.

The exchanger heats the air, and then this hot air is again pushed through the ductwork in your home that branches into each room.

That hot air is dispersed through the vents in the room to warm your home. You set your thermostat for the desired temperature, and when the air reaches the desired temperature, the furnace shuts off.

Central air conditioning works similarly, using an outdoor compressor to draw in the air. then, a series of coils in the furnace cool the air before that air is pushed into the home to achieve a desired, cooler temperature.

Why Do Furnaces Need Filters?

A forced air furnace unit utilizes a filter, and that filter is necessary to maintain airflow in your home as it is heated and cooled. The filter acts as a layer of protection in several ways to the rest of the furnace and its system of parts, as well as to you and your home.

Remember, a forced air furnace brings air into the furnace, drawing it out of the ambient space in your home.

And just like the air outside can be polluted, so can the air in your home. When it comes to your home, because you seal it up and insulate it against the elements outside, many of the pollutants don’t have an outlet to leave, so they can affect the air indoors.

That means your home is full of dust, dirt, the buildup of dry skin and hair we shed, pet hair and dander, allergens, smoke if there are smokers in your home, and other things.

Close up dirty air filter

When the furnace draws in air, it’s also drawing in all of this stuff. Imagine those particles gathering on the fan that draws in the air.

The blades of the fan would eventually get caked with this stuff, which would slow it down, making it inefficient. Your furnace would have to work harder, using more energy to keep the fan running.

Eventually, the fan will not be able to move, whether because the sludge caked on it is too heavy or because it’s so thick that the blades get caught on the casing around it. This causes the fan to burn out, which can lead to more than just a burned-out fan motor, creating very expensive damage to the furnace.

In addition, particles can pass through the fan and get into the heat exchanger, or they can cake up the ducts, all of which lead to poor air flow.

Keep in mind that airflow is the most vital part of a forced air unit. Without it, the entire furnace is useless.

With a furnace filter, airflow through the rest of the system is protected. The filter works to remove tremendous amounts of particles and debris from the air sucked through the intake before it can even reach the fan, much less other areas of the furnace system.

The filter prevents a great deal of damage from occurring to the rest of your furnace. At the same time, if you don’t take proper care of your furnace filter, or if you fail to use the right filter, you’re going to be causing additional harm to your furnace, since the air won’t flow properly. How do you make sure you have the right filter to keep air flowing?

Filter Rating

One of the biggest factors in air flow to your furnace is the filter rating. Using the MERV system – minimum efficiency reporting value – furnace filters are rated according to how well they pull particles out of the air.

MERV values are rated from 1 to 16, with higher numbers indicating that the filter is going to be more efficient and effective at removing dirt and debris from the air going into the furnace system.

However, you need to keep in mind that the highest rating may do more harm than good and might not be the best solution for your airflow.

Woman replacing the filter

Considering that you are going to be breathing in the air that comes out of your furnace’s attached ductwork and that you want your system to last as long as possible, it seems intuitive to go for the most efficient rating.

That’s not exactly the case, though. Sure, furnace filters with a higher MERV rating are going to cleanse the air more than those with a lower rating. However, to do so, they are thicker, which means there are several compromises you may come across.

  • First, the thicker filter could make it so less air flows through to start with. Your furnace will have to work harder to keep up with demand, and that means you’re spending more on power bills. In addition, you could run into problems with the fan if the airflow is too low.
  • Second, because particles are removed so effectively, a furnace filter with a higher MERV rating will clog faster, with more gunk getting caught in the filter. If you are diligent enough to check the filter frequently, this may not be a problem. At the same time, you could end up with a clogged filter in no time, which then depletes air flow entirely, causing extreme damage to the rest of the furnace system.
  • Another thing to keep in mind is that most manufacturers, in the manual, will recommend a particular range of MERV ratings for filters that work best with their product. Like all other things involved with furnaces and filters, this is specific to the manufacturer and model, so each furnace is different. If you don’t have the manual, be sure to call the manufacturer or look up the information online to determine the best MERV rating for your furnace.
  • While you want to strip as many contaminants from the air as possible, most homes will be sufficiently clean with a MERV rating between 8 and 11. If your furnace specifications recommend otherwise, go with what the manufacturer says. It’s important to know where the line is drawn and stay with it so you don’t do more harm than good.

Note that the MERV rating system is the most commonly used and can be found nationwide among furnace filter manufacturers. However, it’s not the only system in place. You’ll probably notice, if you visit a major home improvement chain, they have their own rating system on their home brand of furnace filters.

You can likely relate their ratings to the way the MERV system works and adjust to their standard system with the same sort of indications. However, if you aren’t entirely sure, it’s better to buy a name brand that uses the MERV rating so that you know exactly what you’re getting and that it will be compatible with your furnace, giving you proper airflow.

Filter Types

The type of filter you use can also have a lot to do with the air flow your furnace gets. Some types of furnace filters are available readily in a larger range of sizes, while others are in limited supply, depending on the material.

The choice of type of filter depends on 3 things – personal preference, airflow capabilities, and manufacturer recommendations.

Disposable Filter

Disposable filters are the most common, and there are two types of disposable furnace filters you’ll find readily available. They are usually in stock at stores with the largest range of sizes available.

Pleated Filter

First, the pleated cardboard style is perhaps the most popular. They are inexpensive and come in the widest variety of sizes you’ll find on the shelf. In addition, you have options for ratings in this particular type of filter, which can change the effectiveness of your furnace filter and how much airflow you get. While some of the less common sizes may cost as much as $40, you’re usually looking at just a few dollars, and they need to be replaced about every 90 days (though you should check them more often).

pleated furnace filter

Fiberglass Filter

Another type of disposable filter you could choose is fiberglass. These are easy to distinguish because they are made of fibrous material stretched across a frame like multiple layers of spider webs that catch the particles as the air flows through them.

These, too, are extremely cost-efficient to replace and are readily available in most sizes, just like their paper counterparts.

All disposable filters are easy to remove and install, making them hassle-free, but you need to be sure to check them frequently.

Depending on the state of your home and the number of people or the size of your house, you may need to replace them more frequently than initially recommended. You should check them every 30 days to see if they might be clogged and should replace them at least every 90 days.
However, the cost of replacing your furnace filter 4 times a year can add up over time, and there are other options.

Permanent filters, which are sometimes called washable filters, are also available. Permanent filters can be removed, cleaned, and reinstalled over and over, meaning that you aren’t shelling out money every 3 months for a new filter for the rest of your life.

Of course, getting a washable furnace filter requires a bit more investment upfront, since these are made of sturdier materials and intended to last throughout the life of your furnace. But they can save you a lot of money in the long run, based on how much the continual cost of replacing filters will add up to.

Permanent Filters

Permanent filters typically come in a plastic casing and are made of solid aluminum, which will withstand the test of time far better than paper or fiberglass.

And while it may seem easier to simply replace a filter than it is to take the time to clean one, that’s not necessarily the case. Permanent filters are easy to wash and put back in place. You will only need water, a rag, and a vacuum with a hose attachment (the last of which you should have at the ready every time you replace a disposable filter anyway to help protect your airflow).

Furnace filter - Washable

You can find permanent filters in the most common sizes and some expanded sizes. However, size is not the only factor to consider.

For example, you might not be able to find a compatible filter rating – based on the MERV scale – in the type of filter you’re looking for. You may have to choose a different type of filter until you can locate the one you’re looking for in the correct size and rating.

In recent years, some people have started to opt for the placement of electrostatic furnace filters due to their efficiency in removing particles from the airflow.

These filters hold an electrical charge, which is created specifically by the flow of air through them. This charge works like a magnet, which pulls in the debris and dirt in the air more effectively so that the air going into the furnace system is even cleaner.

This type of filter is often preferable to homes that have special problems, including people with severe allergies or asthma, as well as other respiratory issues. You can also benefit greatly if there is an indoor smoker in your home or if you have pets that leave hair and dander everywhere.

The problem is that not all furnaces are compatible with electrostatic filters. Before you decide to buy an electrostatic filter, you should consult the manual or call the manufacturer to determine if you might be harming the furnace and its other parts by installing one of these, especially since they can be costly compared to standard furnace filters.

If you don’t have your manual, and you can’t get in touch with your manufacturer, you can usually find information online.

There are plenty of resources available, including the manual itself, usually offered to read or download, so you have the information you need ready at your fingertips.

Furnace Filter Sizes

There are several details to consider when dealing with your furnace filter so that you can make sure you’re not inhibiting the flow of air through your system.

Perhaps the first thing you’ll want to guarantee is that you’re using the right size filter for your particular furnace.

Like cars, furnaces are made by multiple manufacturers, and parts are not always one size fits all and interchangeable. That holds true for furnace filters. Because all furnaces are created with different specifications, they have different size filters.

That means you’ll find a broad range of filter sizes available when you go shopping. The good news is, more recently, furnace manufacturers have worked to maintain a certain standard so that there are more common sizes used for furnace filters. For these, you’ll find a broad range of brands, types, and ratings available.

furnace filter

Unfortunately, there are still some furnaces that use custom sizes. Also, older furnaces may use one of a broader variety or something customized. While some of the non-standard-sized filters can be found on a typical shopping trip, others are going to require a call to the manufacturer or a specialized provider so that you make sure you have the perfect fit.

Keep in mind you can’t substitute another size since the slot into which the furnace filter goes is made to have a tight fit with an exact standard. So, how can you figure out the proper size?

  1. Start with the manual. Just like all appliances, your furnace comes with a manual, and all the specifications, as well as required maintenance activities, are detailed in the manual. You can find the required filter size, as well as any specifics regarding rating requirements and filter type compatibility inside the manual. You’ll also find instructions on removing, cleaning, and replacing furnace filters.
  2. Look inside the furnace itself. Locate the furnace filter, which is between the furnace and the air intake. Next to the slot or on the door that covers it, you’ll typically find a tag, sticker, or stamp that tells you everything you need to know about what size furnace filter is required for the unit.
  3. Take a look at the filter. If the information has worn off the inside of the furnace and you’ve lost the manual, you should still be able to identify the size filter you need by looking at the existing filter. The casing of the filter typically has the size written on the exterior for you to see, though you may have to remove it from the slot to read it.
  4. If it turns out that this, too, is a bust (which should only be the case if the filter has already been left for too long), take the filter out of the slot and measure it. You can use a yardstick or a measuring tape, so long as it’s long enough to get an exact measurement of all three dimensions. Typically, for home units, your filter will only be an inch thick, but some larger systems and older furnaces do have a 4-inch-thick filter. Once you’re armed with the numbers, put the filter back into the slot. A used filter is better than no filter.

If you’re concerned you haven’t measured right, or you don’t have a measuring tool, you should call the manufacturer for the details on the right furnace filter before making a purchase.

Inserting the wrong size filter can be detrimental to your furnace, so you need to be clear on what you’re getting. Typically, a furnace filter will be between 10 and 30 inches on each side. To make it easier for you to discern if you have a standard size or something less common, note that the most popular sizes are (in inches) 16 x 20, 16 x 25, 14 x 25, 20 x 25, and 25 x 25.

Furnace Filter Air Flow

If you stop to think about it, what appliances in your home do you rely on most heavily? You might say your washer and dryer, or you might say your microwave. But you’d be wrong. Although no one thinks about it, the most important appliance in most homes is the furnace. We count on our furnaces to keep us warm through the harsh cold of winter or to run our central air conditioning unit in the summer. And when it fails, it’s almost certainly in the worst heat wave or snowstorm of the year.

When the furnace fails, it’s usually due to our oversight and the fact that we tend to ignore its existence as long as it’s functioning properly. But the minute our pleasant, comfortable environment disappears, we start to panic. The funny thing is when it comes to the way your furnace functions and keeping it going, it’s simple enough.

All you have to do is maintain the furnace filter airflow with some regular cleaning and maintenance, and your furnace is far less likely to become a problem. Why is airflow so important? To understand that, you have to first understand how a furnace works and the purpose of the furnace filter.

Where is My Furnace Filter?

Typically, the furnace filter is quite easy to find. All furnace filters are located between the intake (look for a grate, and see if you can feel air being drawn into it when the furnace is running) and the furnace itself (wherever the grate is located, the actual furnace unit should be nearby).

When you open the slot, you should see the filter, and if you haven’t found it before, it is likely covered in dirt and debris. To learn more about the size of your filter, check the side of the frame, which should give you the dimensions.

You’ll probably want to replace the furnace filter, now that you’ve seen its condition. If you’d like to clean it temporarily, and it’s not the washable kind, put on a mask and take it outdoors to shake off what you can.

Front view of a dirty furnace filter taken out

If you’re going to insert a furnace filter, be sure that the arrow on the side points toward the furnace itself and not toward you as you install it.

This assures it’s facing the right direction. If it’s inserted backward, your filter will clog very quickly, and your fan could cease to function. Also, make sure the filter isn’t sliding around in the slot. This could mean you have the wrong size and should check again the dimensions of your furnace filter.

Last Word on Location

Learning where your furnace filter is could be essential not only to the comfort of your home but to the lifespan of your entire HVAC system and your health and wellness. Be sure to check the filter once a month to make sure you don’t have a clog. Watch for extremely high electric bills, in case something is wrong with the filter.

A man's arm and hand seen replacing disposable air filter in home central air furnace.

Never leave it for more than three months without changing or cleaning, depending on the type of furnace filter you have installed. Early discovery of a problem and proper maintenance is the key to a long-lasting system. Please comment below if there are any questions or recommendations!


Your home is full of electronics and appliances that are important creature comforts and make life easier and more enjoyable. But when it comes to the furnace filter, this is essential to living, and you shouldn’t ignore its existence or its need for regular care.

It’s as simple as checking and changing it on a regular schedule to keep the furnace filter airflow direction going so that you don’t end up with bigger problems with your overall furnace system. The last thing you want is for the fan to burn out in the dead of winter or the sweltering heat of summer.

Make sure you clean or replace your furnace filter at least every 90 days, or on a more regular basis as defined by the manufacturer’s specifications to prevent any restriction.

Also, ensure that you use the right size filter and adjust your maintenance cadence based on the type of filter you’re using.

Choose the proper MERV rating, using the manufacturer’s recommendations or a diagram to make the right decision when you go shopping so that you get efficiency without causing detrimental clogging that could end up damaging your furnace.

When you get ready to check the filter or change it, take proper safety measures. Turn off the furnace so you aren’t burned or have contaminants flushed into your face. Consider wearing a mask, and make sure you throw away disposable filters instantly to keep you safe and clean.

Don’t buy an electrostatic filter unless you have clearance from the manufacturer. This could actually hurt your furnace if it’s not compatible with the way your system functions, and it would be a waste of money.

And while you’re considering the financial impact, decide whether you want disposable filters, which are less costly immediately, or reusable one, which is less costly over time.

What Is A Furnace Filter?

Sharing is caring!