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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

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

Nominal VS Actual Size: Find the Right Furnace Filter Sizes

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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 3767 reviews
Enjoyed home delivery

Usually, I have to shop to find my filter since it is a common size. This seems to be a quality filter and it was so convenient to have it delivered to my door!

alternative filter

The Atomic compatible filter they now have for this size is great fits perfectly and functions same at the name brand and is a lot cheaper definitely recommend this over the name brand and I will be purchasing this from now on

Great Service and Filters

The filters arrived quicker than expected and shipping was free. the filters price is excellent. This is the second order from Atomic because of the great value and fast service. Of course will buy again.


Great price, rapid delivery, genuine OEM, perfect condition. What could be better?

As promised

Very happy with speed of delivery and price, thank you! The only thing that kept you from five stars is that the day they brought my filters it was raining hard. Luckily I was home so I could run out and get the package before it soaked though. Perhaps some plastic wrap? Otherwise, very happy, thank you.

Trane Filters

Great Price, super fast, FREE shipping. Can't ask for more!

Only buy made in america

30x30x1 filters

Before I installed my air-conditioning system I had a whole house fan that was 30 x 30 so instead of framing in the opening to put my return in are utilized the 30 x 30 opening it was a little tough finding filters until I found this site. I ordered two boxes of the 30 x 30 by one Merv eight they fit perfect very happy good price fast delivery

Carrier Furnace Filter

Bought these before on Amazon. No longer carry them. Found Atomic Filters on a search. Website easy to navigate. Ordered and shipped right away. Would recommend Atomic for for your filter needs.

Perfect fit. They do the shopping for me.


Very happy with the quick delivery and price was very reasonable

Excellent Service

Excellent, fast and courteous service! I will definitely buy from them again.

Brand name filter, quick receipt of product and free shipping!

HVAC Filter

Great product....great price....great service. no issues

Good price

Good price, free shipping & quick service

Great filter at a fraction of the price I used to pay! Thank you Atomic Filters!

Great product and service

This product is exactly as advertised and arrived at my home in a very fast and timely manner. It is the right size and I am good to go another year thanks to atomic filters. The savings are good to. So my thanks to everyone at Atomic Filters.

The "Carrier" Brand was not written on Filter when arrived!

When the filter arrived I noticed that the brand " "Carrier" was not written anywhere on the filter although according to Atomic filters website, Carrier is the first word in the name of the product (filter)!. Please be transparent to deliver exactly what you advertise on your website as filters are very important to our health.

The process of ordering the product and logistics were very good but the authenticity of the product is more important. Please send me an original Carrier filter. Thank you,

Repeat customer! Best prices, best selection. Fast shipping.

Good quality, great price and dependable company.

Good Pricing

Atomic Filters has the best price! Great product and service. Very pleased!


Filter quality and delivery were as promised. Satisfied customer.


I Am a retires Hvac Company owner { only worked on Commercial and Industrial Equipment} I rate these Filters of good Quality l order again

Fits perfect!

This product replaces our original filter brand and fits and works great and costs much less.