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

Buying the Correct Size Air Filter- 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.  

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

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 3595 reviews
Arrived with no sign it is from Carrier. but the customer service call back was fast

Returning customer. The new set of filter arrived with no sign its carrier-made filter, even all the engineering prints are the same. Different than the one I get from Homedepot. Customer service did call me back rather fast. But its an expensive filter and I need the real carrier infinity-branded one for my customers. Now I am waiting for return label.

These are OEM filters by Carrier. You can see images on the Carrier distributor website. Carrier is not written on the filter but 'Captures & Kills' is on the side of the filter which is a Carrier Trademark. Hope that helps.

Good Product

For all my expensive equipment, I am an OEM guy. As much as possible I Always use whatever was originally specified by the manufacturer. These fit the bill. I was thrown for bit of loop when they came compressed ( I didn't expect that and thought I was sent the wrong thing.) I'd never seen that before!

Self assembly was ok. unexpected though but not an issue.

These are OEM and as expected. Buy with confidence.

Thumbs up!

Arrived late, but worth the wait. Well packaged, good fit, good price.

Very happy

Atomic filters are great and the cost is reasonable, compared to some others. Can't beat Free shipping....received in 5 days. I agree with other 'posts' that shipping boxes are oversize....but, there may be a corporate reason for this....maybe it gives more protection??? Will definitely reorder when needed.

Excellent service start to finish

Received all the three cartons, intact. No visible external damage. 

Kudos to the your staff, starting from Dave who responded to my earlier emails 

asking for clarifications regarding the item I was looking for, especially assuring 

that, being a OEM product, sizing should not be an issue, and also to those who 

took care of filling the order and dispatching thru FedEx.  


Very gratifying to have come across a business, which conducts the business the 

way it should be conducted.  


Product as described. Arrived in a timely manner and packaging in good condition. Fits perfectly. This is the third time ordering from Atomic Filters, and will happily continue business with them in the future.

Exact fit

Good price, excellent quality, fast service. Is now my go to for replacement filters

Excellent Service

We've been ordering from Atomic for several years - they seem to always have the best price for our filters and once ordered, they ship quickly. This year, I was unable to put one of the parts I needed in my shopping cart as it appeared they were sold out. I called customer service and the gentleman I spoke with was very helpful. He was able to see the part had just arrived back in stock and before that was updated on the site, he was able to park one in my shopping cart for me. Excellent customer service all around!

fast and quality

fast delivery and quality product

Melanie W.

Exactly the same filters that came with our new a/c. Best price and super fast delivery. Would definitely recommend.

Very Satisfied

Very satisfied with the price & delivery. Really like the fact that these furnace filters came assembled & ready to install.

Carrier Replacement Filter

Ordering was easy. Delivery quick and well packaged. Appears to match the original equipment.

Super quick delivery- Quality product

I received my filter order within a couple of days.
It was well packaged to prevent damage.
Filter was Lennox manufactured.
Price was less than Amazon.

Fit well and ship fast

The product fit well with the correct number and it looks fine

Satisfied customer

Easy to find and order. Fast shipping came with no damage. Good price. These are not stocked locally. Will continue to purchase from Atomic

Great AC filters & at a super affordable prices.

Have purchased these before & like them very much. Fast delivery each time.

Fingers Crossed!

Perfect replacement for Carrrier filter. Price was certainly better. A few shipping hiccups got it here a few days later than promised. Hoping it works as well as Carrier filter.

Furance filter

Filter good They use FedEx for delivery and they just put the filter in front of the front gate . All other delivery service put their deliveries on our front porch

Great Service and Price

Got two filters in a very short time -- for less than half the price that my HVAC dealer was going to charge me -- and then had the HVAC technician install them during our routine Fall maintenance. Atomic Filters is the type of company that you want to do business with.

Proprietary Filter, good price

This filter does not have a generic equivalent that works properly, so it comes down to price, availability and shipping: Atomic Filters for these three items is the winner!

We have just released compatible for the PGAPXCAR1620, PGAPXCAR1625, PGAPXCAR2020, PGAPXCAR2025, PGAPXCAR2420. The new compatible have been independently tested for meet or exceed performance of the OEM.
Here is the link for the 16x20