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If you’ve ever stood in a store aisle staring at a wall of furnace filters, wondering which size to grab, you’re not alone. Many homeowners guess the size, which can lead to poor airflow, higher energy bills, and extra strain on your HVAC system. Using a high-quality pleated filter is one of the best things you can do for your home’s air quality and system health, but only if it’s the right fit.
The good news is that figuring out how to find the right furnace filter size is easier than you think. You don’t need to be an HVAC expert; you just need to know where to look. This guide will walk you through three simple methods to find your correct filter size and make your next purchase with confidence.
We'll cover how to check your current filter, what to do if the size isn't listed, and how to use an HVAC filter size chart to be absolutely sure. Once you have the right size, you can get high-quality MERV 8–13 pleated filters from Filterbuy delivered right to your door, making the process seamless.
Your furnace filter has two important jobs: protecting your HVAC system from damaging dust and debris, and improving your home’s indoor air quality. For it to do either job well, it has to fit perfectly.
A filter that’s too small leaves gaps, allowing unfiltered air to bypass the filter. This dust and debris can build up on your system's sensitive components, like the evaporator coils and blower motor, leading to costly repairs and breakdowns. A filter that's too large, or crammed into the slot, restricts airflow, forcing your system to work harder. This can lead to a noisy furnace, higher energy bills, and even system damage. Getting the size right ensures a quieter, more efficient system and fewer repair calls.
It's tempting to think "close enough is good enough," but when it comes to furnace filters, precision is key. Even small gaps or a slightly bent frame can compromise performance, leading to reduced air quality and increased strain on your HVAC system over time.
The fastest way to find your furnace filter size is to check the one that’s already installed. You can usually find the filter in a slot in the return air duct, near the blower compartment, or behind a return air grille in your wall or ceiling.
Carefully slide out the old filter and look at its cardboard frame. The dimensions are typically printed along one of the edges. You'll see three numbers, like "20x25x1." This is the filter's nominal size.
It's important to understand the difference between nominal size and actual size. The nominal size is a rounded, convenient number used for labeling and ordering. The actual size is the exact measurement of the filter. For example, a filter with a nominal size of 20x25x1 might have an actual measurement of 19.5" x 24.5" x 0.75". This slight difference ensures the filter slides easily into the slot.
The three numbers on a filter label represent its dimensions: Length x Width x Depth. Some brands might swap the length and width, but as long as both numbers match your system’s slot, it will fit correctly. When you're ready to buy a new filter, you'll order it using the nominal size printed on the frame.
Sometimes the label on your old filter is worn off, missing, or the filter itself looks bent and misshapen. In these cases, your best bet is to measure the filter slot directly.
Here’s a simple, step-by-step process for measuring your filter:
Pro Tip: If your measurement falls between standard sizes, use an HVAC filter size chart to confirm the correct nominal size instead of guessing.
An HVAC filter size chart is a great tool to eliminate any lingering doubt. These charts list standard furnace filter sizes, showing both the nominal size and the corresponding actual dimensions. By comparing your measurements to a chart, you can confidently identify the correct nominal size and avoid ordering a filter that’s just a little too big or small.
Let's say your furnace filter slot measures approximately 19.6" x 24.6" x 0.75". You can look at an HVAC filter size chart from a trusted source like Filterbuy and find the entry for the nominal size 20x25x1. The chart will show that its actual dimensions are very close to your measurements. Now you know for sure that a 20x25x1 pleated filter is the right choice for your system.
Once you know your exact furnace filter size, you’re one step away from cleaner air. Choose a pleated MERV 8–13 filter from Filterbuy that matches your size and have it shipped straight to your door.
Most homes use 1-inch thick filters, but some systems are equipped with media cabinets designed for thicker 4-inch or 5-inch filters. Always match the filter thickness to what your system was designed for; you cannot force a thicker filter into a 1-inch slot.
If your system can accommodate them, thicker pleated filters offer several advantages. They have more surface area for capturing particles, which means they can last longer and often maintain better airflow, even at higher MERV ratings. Filterbuy offers pleated filters across all common thicknesses, so you can find the right fit for your home.
Some homeowners worry that high-efficiency pleated filters are "too much" for their system, but that's a common misconception. When sized correctly and replaced regularly, a pleated filter is a significant upgrade for both your home's air and your HVAC system.
MERV, or Minimum Efficiency Reporting Value, is a simple 1-16 scale that rates how well a filter captures airborne particles.
A properly sized MERV 8–13 pleated filter is not too restrictive for most modern residential systems. The benefits are clear: better indoor air quality, cleaner ducts, less dusting, and superior protection for your HVAC system's motor and coils.
Once you know your furnace filter size, the rest is easy. Filterbuy specializes in high-quality, American-made MERV 8, 11, and 13 pleated filters for residential systems. We offer a huge range of standard furnace filter sizes, plus hundreds of "odd" and custom sizes you won't find on a store shelf.
Ordering from Filterbuy means you get consistent quality every time, and home delivery saves you a trip to the hardware store. You can even order a multi-pack to be set for the whole year.
The easiest way is to check the size printed on the frame of your current filter. If it’s not visible, measure the length, width, and depth of the filter slot and round up to the nearest whole inch to find the nominal size.
The nominal size is the rounded dimension (e.g., 20x25x1) used for labeling and ordering. The actual size is the filter's exact measurement (e.g., 19.5" x 24.5" x 0.75"), which is slightly smaller to ensure it fits properly in the slot.
If the size isn't printed, you'll need a tape measure. Measure the length, width, and depth of the filter slot, then round each number up to the nearest whole inch to determine the nominal size you need to order.
No, you should always use the exact size recommended for your system. A filter that's too small allows dirty air to bypass it, while one that's too big can restrict airflow and strain your furnace.
An HVAC filter size chart lists both the nominal size and the exact "actual" dimensions. This helps you confirm you're ordering the correct filter by matching your slot measurements to the corresponding standard size, eliminating guesswork.
Yes, switching to a pleated filter is a great upgrade for your home's air quality. As long as you use the correct size, a pleated MERV 8–13 filter will capture more airborne particles without harming your system.
While very high MERV filters (14+) can restrict airflow in some systems, a MERV 8–13 rated filter is designed for residential use and provides an excellent balance of filtration and airflow, especially when changed regularly.
Yes, thickness is crucial. You must use the thickness your system was designed for. You cannot fit a 4-inch filter into a 1-inch slot. If your system has a media cabinet for a thicker filter, using one can provide longer life and better airflow.
For most homes, it's recommended to replace 1-inch pleated filters every 1–3 months. Thicker 4- or 5-inch filters can last from 6 to 12 months. Check your filter monthly and change it when it appears dirty.
Once you know your correct nominal size, you can easily order high-quality MERV 8, 11, or 13 pleated filters online from Filterbuy. We offer a wide range of standard and custom sizes delivered directly to your home.
