With so many air filters on the market, it’s no surprise that one of the most commonly asked questions is “what size air filter do I need?”. There are practically thousands of air filters available, which makes finding the right one for your HVAC system overwhelming. But don’t worry, we’ve got your back.

Most air filters are labeled with dimensions that you can find on the filter’s sides or frame. Typically, the standard size is bold, and the actual size is written in smaller numbers. Checking the dimensions is the easiest way to find the right air filter size you need. If there are no measurements on the filter, you can measure the air filter yourself. 

Choosing the correct size of air filter for your HVAC system is essential because it allows your HVAC system to function effectively. Plus, it will clean the air in your home. If the air filter is too small, air will flow around it rather than through it. As a result, dirty air will circulate throughout your home due to inadequate filtering. 

For most people, choosing the right air filter size can be confusing. Hence, we’ve put together this guide. This comprehensive guide will help you answer the question, “What size air filter do I need?” including how air filter sizes work. So the next time you want to buy an air filter, you’ll know exactly what size to get.

How Do I Determine The Right Size Air Filter

How Do Air Filter Sizes Work

One of the most challenging aspects of buying air filters is getting the correct size. Air filters come in various sizes, materials, and types and serve different purposes.  

Understanding how air filter sizing works is crucial, especially if you want the right size for your home or office. When you know the various sizes, you can make a smart decision to meet your specific needs. Before we start choosing an air filter size, here are a few things you should know regarding air filter sizes. 

Length, Width, and Height 

These three main measurements make up the dimensions of an air filter. You’ll notice three numbers on the air filter, such as 20x25x1, which are written in inches as height by length by width.

The width of an air filter is usually the longest side, and while the height is often the shortest side, length size comes in second. It’s also worth noting that the three numbers are usually rounded to the nearest whole number in inches.


Thickness is essential for air filter sizing, as are length, width, and height. Air filters are distinguishable by their thickness. For example, most air filters are 1 or 2 inches thick, although some might be 3 to 6 inches thick.

The thicker the filter, the more efficient it is and the longer the period between replacements. Thicker air filters also have a longer lifespan since they have a larger surface area for trapping and removing air pollutants. 

For example, you’ll need to replace a 1-inch air filter every month, while a 4-inch air filter can go six months without needing a replacement. However, the specific replacement schedules differ depending on the brand and type of air filter. 

That said, every home has unique thickness sizes of air filters. You can find 2-inch filters in some homes on rare occasions. On the other hand, 4, 5, or 6-inch air filters are found in whole house filters installed into an air handler. Air handlers are typically installed near your HVAC system, and in some homes, air handlers are installed into the ductwork. 

You may have realized that we did not include 3-inch air filters, and this is because 3-inch filters are rare to find. You have a one-of-a-kind HVAC system if you require a 3-inch air filter.

Overall, filters with 1 and 2-inch air filters are installed into standard HVAC systems, whereas 3 to 6-inch filers are installed in larger HVAC systems.  

Air filters 4 to 5 inches are often better at providing a tight fit that prevents dirty air from passing through. Likewise, they are also less restrictive and enable more filtered air, offering more outstanding filtration. 

Normal Vs. Actual Size 

The standard size is usually in bold and large text on the filter frame. Plus, it is rounded off to the nearest whole number compared to the actual size. So, for example, 16x25x4 inches is a standard size. That said, the standard size is usually the one you must focus on instead of the actual size. 

While an actual size is written in thin numbers on the frame of the filter and is typically 0.25 inches to 0.50 inches smaller than the standard size, the actual size is a more precise measurement of the filter’s dimensions. For example, 15.5×24.5×0.75 inches is the actual size of an air filter.

How Do Air Filter Sizes Work

What Happens if You Get the Wrong Air Filter Size

You should get the correct air filter sizes you need for your HVAC system for several reasons. Aside from the apparent reason for ensuring that the filter fits in your HVAC system, there are several other reasons. Read on to learn more about why it’s essential to get the right air filter size. 

Unfiltered Air Escapes 

The air filter must fit properly into the slot. If it’s too small, it won’t be able to cover the entire space in the place or vent. This means unfiltered air can escape back into your home. 

These air pollutants can lead to serious health problems like asthma or heart disease. In that case, you might as well not have an air filter in your home. So, you need to ensure the air filter fits tightly without any spaces on the sides. 

The HVAC System Will Be Unable to Close

If the air filter is too big, it will not fit into the filter vent. As a result, the HVAC system will be unable to close. This can result in unfiltered, polluted air entering your home. 

It Can Damage the HVAC System

Worse yet, if you squeeze a larger air filter into your HVAC system, it might damage it. Your HAVC system can implode in the worst-case scenario, causing permanent damage. 

That said, it’s also essential to have regular maintenance to keep your HVAC system functioning efficiently and maintain good indoor air quality. Aside from that, it also prevents any future technical issues. 

Increases your Energy Bill

When you choose the wrong air filter sizes, it strains your HVAC system to filter dirty air. So it will end up consuming more energy, and the extra energy raises your energy bill.

Poor Air Quality

Because airflow is restricted when you have the wrong air filter, it becomes hard for your HVAC system to filter out clean air, which leads to poor air quality in your home. Poor air quality can cause serious respiratory problems, allergies, and other health issues.

What Happens If You Get The Wrong Air Filter Size

What Size Air Filter Do I Need?

Let’s assume you want to replace your air filter, but you don’t know what air filter sizes you need. You might want to get a standard size. However, it might not suit your HVAC system. In addition, they come in over 30 different sizes, so it can be challenging to find the correct air filter size.

Furthermore, if you get the wrong size, it can damage your HVAC system. This leaves you with the question, “What size air filter do I need?”. Below, we’ll give you some suggestions on determining the air filter sizes you need. So no matter the situation, you’ll be able to figure out what air filter size you need for your HVAC system.

Check the Filter Label

The simplest way to answer the question, “What size air filter do I need?” is to check your current air filter label. Remove the current air filter from the vent and check the sides of the air filter. 

Typically, the air filter size is written on the edge of the filter sides. You can also check your HVAC system user manual to find the air filter you need.

It’s important to note that the size written in bold on the air filter frame is not the actual size but rather the standard size. If the actual size is not written on the sides, measure the filter to get more accurate dimensions.

Measure Air Filter

If the air filter frame doesn’t have the sizes labeled or appears incorrect, you can always measure the air filter yourself. First and foremost, turn off your HVAC system as a safety measure. Then, carefully take out the current air filter from the HVAC system. 

Use a measuring tape and measure the air filter. Make sure you measure length, width, and height correctly. Then, round off all the measurements to the nearest whole number in inches to get the standard size. If you need help measuring your air filter, you can contact a professional. 

Measure the Air Intake

Alternatively, you could measure the filter air intake to be safe. Keep in mind that this is only possible if the air intake vent is reachable.

Once again, turn off your HVAC system before taking any measurements. Then, using a measuring tape, measure the length, width, and depth of the inside of the air filter vent. 

Make sure to round off to the nearest whole number to get the standard size. It’s important to note that the actual size should be 0.25 inches to 0.5 inches smaller than the standard size.

After determining the air filter sizes, you can choose a replacement air filter type and size. Most online air filter retailers have a custom air filter page, and there you’ll find a custom sizes option. 

Alternatively, you can choose from the filters that are already in stock if your air filter size is available. If you cannot find the air filter size you need, you can order a custom-sized air filter.

What Size Air Filter Do I Need

Conclusion: What air filter size do I need?

There are several places you can purchase an air filter of varying sizes to fit your HVAC system. These include online stores as well as physical stores. 

But before you can answer the question, “What size air filter do I need?” it’s essential to have the correct dimensions. These include the actual and standard sizes. These are the common types of measurements, although the exact sizes will depend on the specs of your air filter.

The easiest way to determine what size you need is to remove the current air filter from your HVAC system. After, you can check the dimensions on the sides of the air filter.

Once you determine the dimensions suitable for your HVAC system, you can purchase a filter. Make sure you double-check the air filter sizes to avoid getting the wrong size.  

Inserting the wrong air filter sizes can damage your HVAC system. It can lower the efficiency of the HVAC system and raise your energy bill. The biggest concern is unfiltered dirty air being recirculated into your home, affecting your health. That’s why it’s essential to get the air filter sizes you need.

You can reach out to our experts if you need help getting the correct dimensions for your air filter to get the correct size. Having amassed a wealth of expertise through years of experience, our HVAC industry professionals will work according to your needs. Thanks for checking out what size air filter do I need.

FAQ’s about What Size Air Filter Do I Need

Is It Possible to Put Many Air Filters on Top of Each Other?

Even if you are trying to fit numerous air filters into a small space, it is not advisable to stack them on top of each other. Installing multiple filters on top of one another can damage your HVAC system because they are not designed to handle the level of airflow blockage.

How Often Should I Change My Air Filter?

You should change your air filter every month. However, this varies depending on the type, manufacturer, and how dirty the air filter is. But, we recommend changing reusable filters every 30 days for optimum HVAC system efficiency. You can read your air filter manual for the exact instructions and specifications. 

Where Is the Air Filter Located?

Most air filters are located in the vent or slot located in the return air intake within the HVAC system. You can also find home air filters on your wall in the air return. While sometimes, some can be found in the roof or ductwork. A plate should cover the vent where the air filter is installed. If you don’t have an HVAC system, the air filter will be in a separate vent inside the home.

Is a Thicker Air Filter Better?

Most HVAC and furnace air filters include a 1-inch thickness. While this is a standard requirement for most homes, some need thicker filters, ranging from 4 to 6 inches. These can handle more built-up, meaning you’ll need to change them less often. A thicker filter can also trap more particles than a standard filter. However, these filters can occasionally limit airflow, causing system and air quality problems.

How Do I Know When to Change an Air Filter?

Most air filters have a replacement schedule specified in the user manual or on the company’s website. It provides you with a good idea of when to change your filter. However, if you cannot determine how frequently you should change the filter, there are several warning signs to look out for, such as if your air filter is dusty or no longer working effectively, it’s time to change it.

How Much Should I Budget for New Filters?

The price is determined by the type and size of an air filter. Fiberglass 1-inch filters, for example, are often the least expensive air filters on the market, costing at least $1. A pleated air filter with a thickness of 1 inch can cost as little as $5 if purchased in bulk. The price of HVAC filters can range between $25 and $100.

Can I Use a Smaller Air Filter?

No, using a smaller filter than the required size is not wise. If the filter is too small, dirt particles such as dust, mold, and other air pollutants might slip through the gaps. These dirt air particles can recirculate back into your home, leading to poor air quality. Poor air quality can cause serious health issues. 

Does the Thickness of an Air Filter Matter?

Yes, the thickness of your air filter is essential when determining the air filter size you need. Different HVAC systems accommodate different thicknesses, ranging from 1 to 6 inches. A thicker one may be preferable since it has a longer lifespan and is more efficient. However, it’s crucial to determine the thickness size you need for your HVAC system. 

How Does an Air Filter Function?

They function by capturing and removing small air pollutants that pass through them. This prevents and reduces the build-up of dirt, dust, pet dander, and other air pollutants. As a result, they improve the air quality in your home or office and assist your HVAC system in operating efficiently.

How Do I Replace an Air Filter?

Replacing an air filter is a simple process. For starters, turn off your HVAC system as a precautionary measure. Next, find the location of your HVAC system’s air filter. Most filters are located in the slots or vents of the HVAC system. Remove the air filter by opening the vent lock. If you have difficulty replacing the air filter, you can get it replaced by a professional.

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