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Nominal Vs. Actual Sizes: Finding the Right Air Filter Size

Buying the Correct Air Filter Size: The Most Important Thing to Know

When you are buying your air filter online most important thing to know is Air Filters have two sizes that you need to understand the Nominal size(which is a round size often used to label the filter) and the Actual Size. Not understanding these numbers often leads to consumers purchasing a filter that does not fit their Furnace or HVAC system. See the complete list of Air Filters by Size Rating and Brand.

Nominal vs Actual air filter size by Atomic

For instance a 20x30x1 may be on the label of your filter, but if you measure the filter you will see the actual size is less than 20x30x1 Air Filter Nominal Size. It may be 19.75x29.75x.75 actual size.  

What Does Nominal Size Mean?

Nominal size refers to the rounded, standardized dimensions that air filter manufacturers use to label their filters. Nominal means "in name," indicating that the nominal size is used for naming purposes rather than to express an exact measurement. 

The nominal size is usually the actual size rounded to the nearest whole inch. For example, a filter that measures 19.5" x 19.5" x 0.75" would have a nominal size of 20" x 20" x 1". Nominal sizes help consumers find compatible replacement filters without having to measure their existing filter precisely.

What Does Actual Size Mean?

The actual size of an air filter refers to its exact dimensions, typically measured in fractions of an inch. Continuing the example above, while that filter's nominal size is 20" x 20" x 1", its actual size is 19.5" x 19.5" x 0.75”. The actual size can vary up to a half-inch smaller than the stated nominal size for length and width and up to a quarter-inch for depth.

Why Do Air Filter Manufacturers Use Nominal Sizes?

Manufacturers use nominal sizes as a standardized way to label filters, simplifying the filter selection process for consumers. If every filter was only labeled with its exact fractional dimensions, finding the right replacement filter among the hundreds of slightly varying sizes would be much harder. Nominal sizes group filters into standard standardized dimensions, making compatible replacements easier to identify and purchase.

Use Nominal Sizes to Find A/C Filters From the Same Manufacturer

Using nominal air conditioner filter sizes is usually sufficient if you're replacing your air filter with one from the same manufacturer or a replacement made for that brand. Manufacturers typically maintain consistent actual sizes across their product lines for a given nominal size. This means that if you've been using a 20" x 20" x 1" filter from Brand A, you can confidently purchase another 20" x 20" x 1" filter from Brand A and expect it to fit properly. 
Sticking with nominal sizes simplifies the replacement process and ensures compatibility when staying with the same brand.

Use Actual Sizes When Buying Air Filters From A Different Manufacturer

If you're switching to a different air filter manufacturer, it's best to use the actual size of your current filter when searching for a replacement. While nominal sizes are standardized, the actual sizes can vary slightly between manufacturers. Brand A's 20" x 20" x 1" filter might measure 19.5" x 19.5" x 0.75", while Brand B's filter with the same nominal size could be 19.75" x 19.75" x 0.625". 
Slight variations can lead to an improper fit if you only shop by nominal size. To ensure the best fit when changing brands, measure your current filter's actual dimensions and compare them to the new manufacturer's actual size specifications.

Understanding Nominal Filters Size Could Save You Time and Money

Not understanding the Nominal Filter Size is the primary culprit for incorrectly purchased filters. The Nominal Size is the usually the dimensions used to Label the filter. For instance 16x20x1. These dimensions are a rounded value on the filters actual measurements. The actual measurements on this filter may be 15.5x19.5x.75, but that could vary by brand and manufacturer. So in order to make sure you have the correct size filter, especially if you are replacing with a different brand it is important to confirm the actual size on the filter. When purchasing filters on the internet the Nominal size is usually found in the product title.

Actual Filter Size- The Key to Making the Right Replacement Filter Purchase

The actual size as you might have guessed are the actual dimensions of the filter by length, width and thickness. This size is often on the filter right below the Nominal size and is usually labeled as the actual size. When you are buying a filter online the actual size is sometime in the product bullet point or the product description. If your filter does not have actual dimensions on the outside of the filter, you can measure you filter to get the correct dimensions. Some brands have a foam gasket on the outside of the filter, in this case, you will want to make sure you have a compatible actual size and that it also has the foam to ensure a snug fit. 

Lennox Air Filter X0851 with Foam strips

What if My Air Filter Size Is Too Big?

If an air filter is too big for the slot in your HVAC system, it won't fit properly and may allow unfiltered air to bypass the filter. This can let in dust, allergens, and other particles, reducing indoor air quality. You may also damage an oversized filter when installing it. Aim for a snug fit, and go down a size if needed.

What if My Air Filter Is Too Small?

An undersized air filter can also cause problems. Gaps around the edges of a too-small filter will allow unfiltered air into your HVAC system, negating the filter's purpose. 

Additionally, a filter that's too small may not have enough surface area to effectively clean your air, and it will get dirty and clogged faster. This reduced airflow makes your HVAC work harder, decreasing efficiency and increasing wear on the system. Measure carefully and buy a slightly larger filter if you're unsure.

How Do I Know What Size Air Filter I Need Without a Filter?

There are so many possible configurations of HVAC systems for furnaces and Air conditioners that finding the correct size filter may be relatively easy or it may be more complicated depending on the HVAC manufacturer and the setup of your system. It is always best to consult a professional because you want to make sure you get the correct filter size and efficiency for your system.

Common locations where the air filter is located are in returns in the wall, ceiling or closet and sometimes they are in a compartment, rack or slot in the HVAC equipment by the blower. If you have a return in the wall that has a metal grille that does not have a filter in the wall, make sure there is not already a filter in the blower compartment or filter slot in the air handler in the HVAC unit.  

If you don't have your old filter to measure, and are trying to replace a filter in a wall or ceiling return make sure the filter is small enough to fit in the opening but large enough to cover the inset. Once the unit is turned on the air will pull the filter securely against the inset of the return. Filters are usually labeled by a nominal size which is a rounded size. This size may be rounded up or down. A typical 1 inch filter may have a nominal size of 16x25x1 with an actual size of 15.5x24.5x.75. If your wall has a 16x25 opening with a 1 inch inset and an opening of the inset of 15.25x24.25, you would choose a 16x25x1 filter with an actual size of 15.5x24.5x.75 as long as the filter would completely cover the opening of the inset. If you need a filter in a rack or slot or specialty whole house filter for your system, the best thing to do is contact an HVAC company that specializes in the Brand of your home's HVAC equipment. For instance if you have a Carrier unit, call the local Carrier contractor or dealer in your area and schedule a maintenance on your system.

If you did not have a filter in the system you may need the system services and the coils cleaned. You can also consult your user manual for your HVAC system. If you do not have the user manual, you can usually find one online.  If you’re unsure which nominal size is correct, have an HVAC professional who specializes in your Brand of HVAC equipment help you select the correct filter. 

Examples of Nominal and Actual Sizes

 Size  Example 1  Example 2
Nominal Size 20x20x1 16x20x1
Actual Size 19.5x19.5x.75 15.75x19.75x.75


 Furnace filters should fit securely but should not have to be forced into position. If you have to force the filter into its slot, then it is probably too big. Forcing an improper filter size in a filter can cause it to buckle, damaging the filter or reducing its ability to function properly. Filters are a smaller than their slot to allow for easy replacement. Some HVAC units may need a filter with dimensions that are unique or uncommon. In these cases, a custom filter needs to be ordered.

AC Filter Sizes: How to Find the Right Replacement Filter

Find Your Filter Sizes Easy in 1" Deep Filters , 2" Deep Filters, 4" Deep Filters

Related Links

Furnace Filters

Best Place to Buy Air Filters Online

16x25x1 Air Filter

16x20x1 Air Filter

20x20x1 Air Filter

20x25x1 Air Filters

Lennox Air Filters by Size and Model Number

Lennox X6672 HCF16-16 Healthy Climate 16x25x5 Merv 16 Filter

Nominal VS Actual Size: Find the Right Furnace Filter Sizes

How to Choose The Right Home Air Filter Everytime

Buy a Replacement Air Purifier

Atomic Has Been Privileged to Support

Atomic Has Been Privileged to Support

Founded by the the son of a US Navy veteran, Atomic Filters has been proud to support the following Charities that support our veterans and their families.

Atomic Filters has supported Special Operation Warrior Foundation, UDT Seal Foundation and Fold of Honor.

Founded by the the son of a US Navy veteran, Atomic Filters has been proud to support the following Charities that support our veterans and their families.

Atomic Filters has supported Special Operation Warrior Foundation, UDT Seal Foundation and Fold of Honor.

Based on 3824 reviews
Perfect Fit, Exactly as Described

My Carrier HVAC needed a new filter. I couldn't find OEM replacement filter on Amazon, so a Google search led me here. I am very pleased because it is the same exact filter that I replaced. I'll be back for more once my filters are used up. Thank you!

Second Order

This is my second order soon to be a third. Easy ordering, fast shipping and a good product. Will order from again.

Atomic Filters 2 Pack

Very easy to order and quick delivery. This is my second order and both have been handled great.

Great fit and price

Tried replacement filters from others but system rejected those as unrecognized. Very frustrating but the replacement from atomic Filters fit perfectly and worked instantly. These devices are hard to find so I will be buying from Atomic so long as I have my Carrier!. Best price I could find as well!

Filter Purchase

Good product at a good price. Have been buying products for my system here for years.

Ultravation 91-005 16x25x5 MERV 11 Replacement Furnace Filter - 2 Pack

Great price - excellent delivery time via FedX - well packed for shipment - no damage.. Will definitely buy the 2 pack again!!

Great product!

Great product and a very competitive price and delivered as promised. Will definitely order again!!!

Right fit

It was easy to navigate your website and find the item I need. Great price. Check out process
was easy. My item
arrived in a timely

Good price good service

Purchased a pool filter at a price that was substantially lower than other companies, yet is identical to what others offer. The site was easy to navigate and the item was shipped promptly. Thank you. I will be ordering again in the future.

Good but issues.

Good filters but are not compatible with the air purifier. It seems there is some sort of connection issue with the tabs on the ends of the filter. So even though the filters fit, I can't use my air purifier. So I'll use them for now but unfortunately I won't buy again.

Great communication and fast delivery! Great filter!

Great Price and Service

I have purchased these filters in the past. They are the OEM and fit perfectly. The price is usually less than most other vendors. The shipping is always very fast and arrive in good condition. Not a single complaint or problem!


Perfect fit, quick shipping and fairly priced. Glad I came here for our furnace filters.

Bryant / Carrier Air Purifier Cartridge - GAPBBCAR2025

Genuine OEM product at a fair price. No issues and will be ordering more.

Great Service!

Great service as always. Have bought multiple furnace filters and always fast delivery. Highly recommend.

Ultravation Filters

Great filters and fast service

Great product

Well built to last the duration of the life of the filter. Great price

Pleased customer

I ordered Trane quick box filters. I was pleasantly surprised when they came and was already assembled. I’ve ordered from different companies prior and had to assemble them and get all cut up in the process. Definitely will order again. Prices are fantastic.

A/c Replacement Air Filters

While I have not yet used the A/C filters received, I am confident I ordered correctly. Reaching out to share the original order was not received as stated. I made contact with Kevin King and Dave in support and they immediately took charge and replaced my order, which I received within a few days. To say the least I am very impressed with Atomic's customer service and will use them in the future.


Excellent filter …. The price was fantastic and the shipping was very fast… I will buy more when I need them.. excellent experience and this was my first time buying from atomic but definitely not my last
Thank you

Good price on original equipment filters. Fast shipping.

OEM at a great price

Original replacement, at a fraction of what my service people wanted to charge me. Easy to change yourself. My fourth year buying from Atomic, never any issues, fast delivery by Fed Ex.

Good filters at a fair price.

Purchased a twin pack of Merv 13 filters for Bryant furnace. Filters were identical to original filters provided by HVAC installation company but at a much cheaper price. Delivery was fast and accurate and would use Atomic Filters again when time comes for new filters.