How do I know if my air filter is too small?

Author: Dorian Considine Sr.  |  Last update: Saturday, February 28, 2026

So, if you buy an air filter online without testing the length or width in person, it will most likely be too big or too small. When you place the air filter in the slot, and it doesn't glide in, it's too bid.

Is it OK if my air filter is a little small?

The size of your air filter really doesn't matter much, but it has to seal correctly to whatever piece it fits into so that all the air gets drawn in through the filter like you can't put a bigger filter in your smaller air cleaner. It won't work correctly.

Do air filters have to be exact size?

actual size of an air filter can affect filter compatibility if it doesn't fit properly into the HVAC system filter slot. Ensuring the actual size aligns with your HVAC system's requirements, even when the nominal size matches, is important if you want system efficiency, proper filtration, and the best performance.

How snug should an air filter be?

When a home air filter size fits well it doesn't have to provide an airtight seal. It does, however, need to fit snugly in the air filter compartment with a slight bit of space around the edges. Being able to easily slide the air filter in place is another sign that the air filter is a good fit.

Can I use a 20x25x4 instead of a 20x25x5 filter?

You cannot use a 20x25x4 filter in place of a 20x25x5 filter because the thickness is different. The 20x25x5 filter is designed for specific systems that require extra thickness for proper airflow and filtration. Always use the filter size recommended by your HVAC system's manufacturer to ensure optimal performance.

How to Find the Right Replacement Air Filter by Size or Model Number avoiding common mistakes

Can I use the wrong size air filter?

Check your furnace and AC for individual filter slots to ensure you buy the correct size for each (if applicable). Using the wrong size filter reduces efficiency, air quality, and can even damage your HVAC system.

What is the actual size of Filtrete 20x25x5?

Actual Size 19 5/8" x 24 3/16" x 4 15/16." Case of 2.

What happens if a filter is too small?

Unpleasant Odors: Filters that are too small can become saturated quickly, leading to musty or stale odors circulating throughout your home. If pollutants are not adequately captured, they can contribute to unpleasant odors that linger in the indoor environment.

What is the most popular air filter size?

The most common air filter sizes for residential HVAC systems typically include 16x25x1, 20x25x1, and 16x20x1. However, it's essential to measure your existing filter or consult your HVAC system manual to ensure you choose the right size.

What happens if your air filter is too big?

It's normal for home bills to rise during winter or summer when temperatures fluctuate, but if it's a consistent problem throughout the year, you need to check the air filter size. If the air filter is too large, it will cause the system to run slowly and reduce the efficiency of the filtration process.

How do you know what size filter to use?

To determine your air filter size, you need to measure its length, width, and depth. These measurements are usually printed on the side of the filter. If not, you can use a tape measure to get the dimensions.

What is a good MERV rating for a house?

A good MERV (Minimum Efficiency Reporting Value) rating for residential HVAC systems is typically between 8 and 13. MERV 8 filters capture basic particles like dust, pollen, and pet dander and are adequate for most households.

Does my air filter need to be the exact size?

Use Actual Sizes When Buying Air Filters From A Different Manufacturer. If you're switching to a different air filter manufacturer, it's best to use the actual size of your current filter when searching for a replacement. While nominal sizes are standardized, the actual sizes can vary slightly between manufacturers.

Is it OK to use cheap air filters?

Cheap air filters are often less efficient at trapping particles, which means that more harmful airborne contaminants can enter your HVAC system and circulate throughout your home or office. This can lead to poor indoor air quality and increased health risks.

Can I use 16x25x4 instead of 16x25x5?

UNDERSTANDING FILTER SIZES

Therefore, for example, a 16x25x4 filter is 16 inches long, 25 inches wide, and 4 inches thick. It's important to note that filters with different dimensions are not interchangeable due to variations in frame size and media area.

What happens if air filter is backwards?

The most common problem you will face with a backwards facing filter is simple inefficiency. If your furnace is forced to blow air through the non-porous end of a filter, it will take more energy to do so. The blower will be overworked and you will pay more money for your heating.

How long after changing the air filter will AC work?

After replacing the filter, you should wait at least 12 hours before running the system. If the AC is still not blowing cold air, it could signify that you have a refrigerant leak, a damaged condenser, or your entire mechanical system needs maintenance.

What happens if air filter is not fitted properly?

While you might find an air filter made for a different vehicle that has the same dimensions as the one made for your vehicle, it may not seat perfectly onto the filter mount. This can allow unfiltered air to slip past and damage your oil and engine.

How tight should an air filter fit?

How Tightly Should My Air Filter Fit? If you have to bend, force, or jam your air filter into place: your filter is too big. On the other hand, if your filter does not come within 1-2 fingers' width of all four corners of your air system handler's slot or frame: your filter is too small.

What happens if you use the wrong size filter?

If your filter doesn't fit properly, it can interfere with the flow of air from the system to your home. Your air filter may also get sucked into the system, causing damage. Improper installation of the filter or the absence of a filter allows dust and other debris to build up inside the system.

Do thicker air filters restrict airflow?

This extra depth gives the filter more room for airflow. That means that as the filter collects dirt and other debris from your air, it won't restrict the airflow in your furnace as much. This prevents damage to the furnace and gives the filter itself a longer lifespan.

What is the most common air filter size?

Currently, the most common size furnace filters are:
  • 20 x 20 x 1.
  • 16 x 25 x 1.
  • 20 x 25 x 1.
  • 20 x 25 x 4.
  • 16 x 20 x 1.
  • 30 x 30 x 1.
  • 16 x 25 x 5.
  • 14 x 20 x 1.

Are all 20x25x5 air filters the same?

Not all 20x25x5 air filters are alike. They differ in material, MERV rating, and effectiveness, affecting performance and HVAC compatibility.

Why does my furnace have a 5 inch filter?

Stiffer builds for steadfast seal: 1-inch filters have less material to control, which means the frames are thinner. Thin frames have flimsy seals which let through unfiltered air, defeating the purpose of your filter. 5-inch frames are thicker and fit into the furnace more seamlessly.

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