Understanding the Difference Between Actual and Nominal Sizes for Air Filters

When it comes to replacing your HVAC air filter, one of the most confusing aspects can be figuring out the right size. You might see two different sets of numbers on the packaging or in product descriptions: nominal size and actual size. While they sound similar, they’re not the same—and knowing the difference can save you time, money, and frustration.

What Is Nominal Size?

The nominal size is the rounded-up, simplified version of the filter’s dimensions. It’s the size most commonly printed on the filter’s packaging and used for marketing purposes. Think of it as the "name" of the filter size rather than its precise measurement.

For example, a filter labeled as 16x20x1 might not actually measure exactly 16 inches by 20 inches by 1 inch. Instead, it’s rounded to the nearest whole number for convenience.

What Is Actual Size?

The actual size is the filter’s true dimensions, typically slightly smaller than the nominal size. This difference allows the filter to fit properly within the filter housing or slot.

Using the same example, a filter with a nominal size of 16x20x1 might have an actual size of 15.5 x 19.5 x 0.75 inches. These fractions of an inch matter, especially if your HVAC system has a tight-fitting filter slot.

Why the Difference Matters

1. Proper Fit: A filter that’s too large won’t fit, and one that’s too small can allow unfiltered air to bypass the filter, reducing air quality and system efficiency.
2. Ordering Replacements: If you’re ordering filters online or from a store, knowing the actual size ensures you get the right fit, especially if your system requires a non-standard size.
3. Custom Filters: For systems with unique dimensions, you may need to order custom-sized filters. In this case, you’ll need to provide the actual size, not the nominal one.

How to Find the Right Size

• Check the old filter: Most filters have both the nominal and actual sizes printed on the frame.
• Measure it yourself: Use a tape measure to get the length, width, and depth of the filter. Round up to the nearest inch to find the nominal size.
• Consult your HVAC manual: It may list the recommended filter size for your system.

Quick Tip

If you're ever unsure, it's better to go by the actual size when shopping for a replacement. Many retailers list both sizes, but the actual size is what ensures a proper fit. Need Help? Our team is here to assist, contact support@airbear.com with any questions you may have.