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.
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.
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.
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