Choosing the correct furnace filter size is essential to ensure proper airflow, filtration performance, and compatibility with your HVAC system. Your filter's dimensions should match what your system is specifically designed to use.

Understanding Filter Dimensions

Furnace filters are labeled with three dimensions, typically in inches, such as 16x25x1. These represent:

• Width x Height x Thickness

• In this example, the filter is 16 inches wide, 25 inches tall, and 1 inch thick.

The final number—the thickness—is often used to help determine filter longevity and dust-holding capacity. Learn more about choosing the right filter thickness.

Nominal vs. Actual Size

Most furnace filters are listed by their nominal size, which is a rounded-up measurement used for marketing and labeling. However, the actual size of the filter is slightly smaller, allowing it to fit within the designated filter slot.

For example:

• A 16x25x1 filter might actually measure 15.5 x 24.5 x 0.75 inches.

Always check your HVAC system or the existing filter for the size label. If in doubt, use a tape measure to confirm the exact dimensions. You can check our actual size chart here.

Why Size Matters

• Airflow Efficiency: A poorly fitting filter can reduce airflow, strain your system, and reduce efficiency.

• Filtration Performance: Gaps around an incorrectly sized filter allow unfiltered air to bypass the filter.

• System Compatibility: Only use filter sizes and thicknesses your system was designed to support.

Pro Tip:

If your system allows for multiple thicknesses (like both 1-inch and 4-inch filters), consider upgrading to a thicker filter for longer life and improved performance—as long as your unit supports it.