Finding Your Home HVAC Filter Sizing: A Step-by-Step Guide

Finding the right HVAC air filter size might not sound exciting, but it plays a major role in how well your heating and cooling system works. 

A properly sized filter helps maintain strong airflow, keeps dust and allergens out of the air you breathe, and protects your HVAC unit from unnecessary strain.

Using the wrong size can lead to poor air quality, higher energy bills, and even long-term damage to your furnace or AC system.

Whether you're replacing an old filter, shopping for a new one, or just not sure what those numbers on the side of the filter mean, this step-by-step guide will help you figure out exactly what size you need, why it matters, and how to get it right the first time.

Step 1: Locate Your HVAC Filter

Before you can find your filter size, you need to locate where your air filter is installed. Most filters are found in one of these locations:

• Furnace: If you have a central heating system, your filter is likely located inside or next to the furnace unit. Furnaces are commonly found in basements, attics, garages, or dedicated utility closets.
• Return Air Vent: Some homes have air filters placed in large return vents, often located on walls, ceilings, or hallways. These vents pull air into the HVAC system to be heated or cooled.
• Air Handler Unit: If you have a split HVAC system, your filter may be inside the air handler, which is usually in a closet or attic.

If you’re unsure where to look, ask a family member who’s familiar with your home’s heating and cooling system - or check your HVAC user manual.

Step 2: Check the Existing Filter for Size

Most standard air filters have their dimensions clearly printed on the cardboard frame. Most filters are labeled using the standard width x length x depth format (example: 16x25x1). If you’re searching online, use this number to shop for replacement HVAC filters.

This nominal size (rounded for convenience) is what you’ll use when ordering a replacement filter. However, if exact measurements are needed, you can check the actual size—which is usually slightly smaller than the nominal size.

Homeowner’s [Pro HVAC Tip]: If your filter has a size printed on it, take a photo with your phone so you don’t forget it when shopping for a replacement. You can also store it in a folder on your phone so you aren’t endlessly scrolling to find it.

Step 3: How to Measure Your Filter Slot (If No Size is Listed)

If your old filter doesn’t have any size markings, you’ll need to measure it manually. Here’s how:

1. Find a tape measure. (Trust us, it makes this process a lot easier.)
2. Measure the width, length, and depth of the existing filter or filter slot.
3. Round your measurements to the nearest inch to determine the nominal size.

Tip: If your measurements come out as 15 ¾ x 24 ¾ x ¾ inches, the correct filter size would be 16x25x1.

Step 4: Still Not Sure? Contact Yoor Air for Help

Still confused? No worries! Send a photo of your HVAC unit and filter slot to our support team. We’ll help you identify the correct air filter size for your system. Give us a call or fill out ourcontact formand we’ll guide you through the process of finding the perfect furnace filter size or AC filter size for your home.

Tip: If you’re having trouble, take a picture of your HVAC unit and filter slot before calling customer support. This can help them identify the right filter for your system.

Step 5: Choose the Right Filter for Your Home

Now that you know your size, choose the best air filter for your home’s needs. Filters come in different types based on efficiency (MERV rating) and air quality needs.

• Basic Filters (MERV 1-4): Good for dust and lint, but not great for allergens.
• Standard Filters (MERV 5-8): Captures common household pollutants like pollen, mold spores, and pet dander.
• High-Efficiency Filters (MERV 9-12): Ideal for allergy sufferers and homes with pets.
• HEPA & Ultrafine Filters (MERV 13+): Best for trapping tiny airborne particles, but may restrict airflow in some HVAC systems.

Tip: If you have allergies or pets, consider a higher-efficiency filter to trap more airborne particles and improve air quality.

When to Change Your HVAC Filter (And Why It Matters)

Even the perfect filter won’t work forever. Over time, filters become clogged with dust, pollen, pet hair, and other airborne particles, making it harder for air to pass through.

When that happens, your HVAC system has to work harder to push air through, which can lead to wear and tear, higher utility bills, and poor indoor air quality.

So how do you know it’s time to swap it out? Here are a few signs your filter might be overdue:

• You notice more dust collecting around vents or on furniture
• Your system is running longer than usual to reach the same temperature
• Allergy or asthma symptoms are flaring up indoors
• It’s been over 60–90 days since your last filter change

Still unsure? A quick visual inspection can help. If the filter looks gray or dusty, it’s time for a fresh one.

How Often Should You Replace Your Filter?

If you have pets, allergies, or live in a particularly dusty area, changing your filter every one to two months is recommended. For most households with standard HVAC usage, replacing the filter every two to three months is sufficient.

If you rarely use your heating or cooling system, you may only need to replace the filter every six months. To stay on track, write the replacement date on the filter or set a calendar reminder every 30–60 days.

Why the Right Filter Size Is Critical

You might be thinking, “How do I know what size air filter I need?” and “What happens if I use the wrong size?” Here’s why it’s crucial to get the correct fit.

A filter that is too small leaves gaps, allowing unfiltered air to bypass the system and circulate contaminants. A filter that is too large won’t fit properly and may get crushed when installed, reducing its effectiveness.

Risks of the wrong filter size:

• Gaps that let dirty air bypass the filter
• Filter gets crushed or bent during installation
• Reduced airflow = higher energy bills
• Extra strain on your system = shorter lifespan

An improperly fitted filter can also restrict airflow, forcing your HVAC system to work harder, increasing energy costs, and shortening the lifespan of the unit. Selecting the right furnace or AC filter size helps maintain proper airflow, improves indoor air quality, and ensures your system operates efficiently.

Final Air Filter Size Checklist

Before purchasing a new filter, make sure you have identified the location of your HVAC unit and filter slot, checked your current filter for size markings, or manually measured if no size is listed.

Be sure you know how often your filter needs to be replaced and are ready to choose a filter that best suits your home’s needs. With the right filter, you can enjoy cleaner air, a more efficient HVAC system, and a healthier home.

Found your size? Shop Yoor Air’s Antimicrobial AC & Furnace filters for better protection against dust, allergens, and airflow issues.