How to Determine Air Filter Size

Let’s face it: figuring out the right air filter size for your HVAC system isn’t exactly a thrilling task, but it is an important one. The wrong size can reduce your system’s efficiency, waste energy, and even impact your indoor air quality. In this guide, we’ll walk you through how to find the correct size air filter for your unit—no guesswork, no measuring tape anxiety, just clear steps to keep your air (and your system) clean.

Key Takeaways

• Always check both the nominal and actual filter size. Never assume the size printed on the frame is the exact fit.

• Measure length, width, and depth accurately using a tape measure and round up when necessary.

• Consider MERV ratings based on your indoor air quality needs, but be mindful of airflow restrictions.

• Use manufacturer recommendations or consult an HVAC professional for non-standard sizes. 

• An incorrectly sized filter can reduce efficiency, increase energy costs, and degrade air quality. Choosing the right size is essential for optimal system performance.

Understanding Air Filter Measurements

Air filter measurements may seem confusing, but they’re pretty straightforward once you understand them.

Nominal vs. Actual Size

Air filters are typically labeled by nominal size, which refers to the rounded dimensions printed on the packaging (e.g., 20x25x1). However, the actual size is slightly smaller and reflects the filter’s precise measurements. This is usually printed in smaller text on the filter’s frame. This discrepancy of about 1/8" to 1/2" exists so filters fit easily into standard housings. While nominal size is what you’ll usually shop for, it’s the actual size that ensures a proper fit.

Tip: Before purchasing a replacement, check the actual dimensions of your existing filter. If those aren’t visible, measure the filter yourself.

Importance of Exact Sizing

Using the wrong size filter can cause a few issues. A filter that’s too small may leave gaps that let dust and allergens pass through unfiltered. One that’s too large might not fit at all, or could be forced in, which restricts airflow.

Incorrect sizing can reduce your system’s efficiency, increase energy use, and even shorten the life of your HVAC unit. To get the best performance—and the cleanest air—it’s important to make sure your filter fits exactly as it should.

Tools and Tips for Measuring Air Filter Size

If you’re unsure what size filter your system needs and the old one doesn’t have the size clearly labeled, measuring it yourself is a simple and reliable way to find the right fit. All you need is a basic tape measure and a few minutes.

Using a Tape Measure

To measure your air filter:

1. Remove the old filter from your HVAC system.

2. Lay it flat on a level surface.

3. Measure the length, width, and depth (thickness) using a tape measure. Take your time to get accurate readings.

4. Round each measurement up to the nearest whole inch to find the nominal size (what you’ll search for when shopping).

5. Write the measurements down and keep them somewhere handy. It’ll save you time next time you need a replacement.

Filter Size Chart Reference

A filter size chart can be a useful tool if you’re looking for standard sizing or trying to find a close match. Most charts, like the one below, list the most common nominal sizes and help you quickly identify compatible options. 

 

Choosing the Right Air Filter for Your Needs

Not all filters are created equal—some are designed for basic dust control, while others offer advanced protection against allergens, odors, and pollutants. Here's how to choose the best option for your system and lifestyle.

Evaluating MERV Ratings

MERV (Minimum Efficiency Reporting Value) ratings measure how effectively a filter captures particles of different sizes. A higher MERV rating means better filtration, but it can also mean more airflow resistance, so it’s important to strike the right balance for your system. The available MERV ratings are:

• MERV 1–4: Basic protection. These filters capture large dust particles and lint, but won’t do much for allergens or smaller contaminants.

• MERV 8, 11, and 12: A solid choice for most homes. These filters trap pollen, pet dander, and mold spores, making them great for allergy sufferers and pet owners.

• MERV 13–16: High-efficiency filters. They remove even finer particles, including bacteria and smoke. Best for those with asthma, respiratory conditions, or living in areas with poor outdoor air quality.

• Carbon Filters: These use activated carbon to reduce odors, gases, and VOCs (volatile organic compounds). They’re ideal if you’re sensitive to smells from cooking, smoke, or cleaning products.

It’s important to note that not all systems are built to handle high-MERV filters. Before upgrading, check your HVAC unit’s manual or consult a professional to make sure your system can maintain proper airflow.

Custom Filter Solutions

Sometimes, standard filter sizes just won’t cut it. If your HVAC system is older or was built with a non-standard slot size, you may need a custom filter. When ordering custom filters:

• Provide exact actual measurements (not nominal)

• Choose a material and MERV rating suited to your needs

• Ask about lead time, as custom filters may take longer to ship
  

If you're unsure, it's always a good idea to get input from an HVAC technician to ensure you're making the right choice.

Consulting Manufacturer Resources

When in doubt, go straight to the source. Your HVAC system’s manual should list the recommended filter size, type, and MERV rating. You can also visit your manufacturer’s website for digital manuals, support articles, or even built-in filter finder tools.

Consequences of Incorrect Air Filter Size

Installing the wrong air filter size might seem like a small mistake, but it can lead to bigger problems for both your HVAC system and your indoor air quality. 

System Efficiency and Damage Risks

A filter that’s too small or too large can disrupt airflow and place unnecessary strain on your HVAC system. When airflow is restricted or inconsistent, the blower motor has to work harder, which can lead to increased wear and tear and a shorter system lifespan.

Poor airflow can also result in frozen evaporator coils, overheating, or even full system shutdowns if the issue isn’t caught in time. On top of that, your energy bills can start creeping up as your system burns more power trying to maintain comfort levels. In short, a poorly sized filter can cost you more in monthly utilities and repair or replacement expenses.

Impacts on Air Quality

Filters are designed to trap airborne contaminants, but they can’t do their job if they don’t fit properly. A loose or undersized filter allows dust, pet dander, pollen, and other particles to bypass the filter and circulate through your home. Over time, this can lead to poor indoor air quality, which may worsen allergy symptoms, aggravate asthma, or contribute to other respiratory issues, especially in sensitive individuals.

Measuring Your Air Filter with Wonderfil 

Finding the right air filter size might not be the most exciting part of home maintenance, but it’s one of the most important. A properly sized filter keeps your HVAC system running smoothly, your energy costs in check, and your indoor air fresh and healthy. Whether you’re replacing a standard filter or exploring custom options, taking a few extra minutes to double-check your measurements can save you time, money, and headaches down the road.

Need help choosing the right air filter? Wonderfil makes it simple. Browse our selection of high-quality filters, contact us for expert guidance, and enjoy fast, reliable delivery right to your door.