Shop by
Here's something that surprises most homeowners: buying the wrong air filter size is one of the most common mistakes in home maintenance—and it can cost you hundreds in wasted energy and HVAC repairs. At Filterbuy, we're obsessed with getting the details right because after manufacturing millions of air filters since 2013, we've seen exactly what happens when sizing goes wrong.
Most people walk into a big box store, grab a filter that looks "close enough," and never realize that air is bypassing their filter entirely, circulating dust, allergens, and pollutants through every room. Or they force an oversized filter into place, restricting airflow and making their HVAC system work 40% harder than it should.
Here's what we've learned from producing over 300 different filter sizes in our American facilities and working directly with homeowners nationwide: understanding air filter sizing isn't complicated—you just need someone obsessed enough to explain it right.
Getting the right air filter size can feel confusing. You might wonder why a 20x25x1 filter doesn't actually measure exactly 20 by 25 inches. Or maybe you're unsure how to measure your filter slot properly. Don't worry, understanding air filter sizes is simpler than it seems.
This guide covers everything you need to know about air filter dimensions, from standard sizes to custom options. You'll learn how to measure correctly, understand the difference between nominal and actual sizes, and find the perfect fit for your HVAC system. Whether you need a common size or something custom-made, Filterbuy has you covered with both standard and made-to-measure filters.
Air filter sizes follow a simple format: Length × Width × Thickness. These measurements tell you the dimensions needed to fit your return air vent or filter compartment properly.
The size directly impacts how well your HVAC system works. A correctly sized filter creates the right airflow while capturing dust, pollen, and other particles. Size also affects filtration efficiency and your system's lifespan. Too small, and air bypasses the filter. Too large, and it won't fit at all.
Most residential filters range from 1 to 4 inches thick, with 1-inch being the most common for basic systems.
This confuses more people than any other aspect of air filter shopping, so let's make it crystal clear with insights from our manufacturing floor.
This is where things get interesting. The size printed on your filter is called the nominal size.
The nominal size is the rounded measurement you use for ordering and shopping. It's the size printed in large text on your filter frame and the size you'll search for online. Examples: 20x25x1, 16x20x1, 14x20x1.
Think of nominal size as the "filing system" for air filters—it's how the industry organizes and sells filters for easy identification.
The actual size is the precise physical measurement of the filter. It's typically 0.25 to 0.5 inches smaller than the nominal size on each dimension. Examples:
- Nominal 20x25x1 → Actual 19.5x24.5x0.75
- Nominal 16x20x1 → Actual 15.5x19.5x0.75
- Nominal 14x20x1 → Actual 13.5x19.5x0.75
This smaller actual size is intentional, not a manufacturing defect. The difference allows the filter to slide into your filter slot without binding or scraping.
Why does this matter? When shopping online or from different brands, knowing both measurements helps ensure you get the right fit. Some manufacturers list actual dimensions, while others use nominal sizing.
Filterbuy clearly labels both nominal and actual measurements on our product pages, so you never have to guess.
Here are the most common standard air filter sizes you'll encounter:
1-Inch Filters:
4-Inch Filters:
Filterbuy carries all standard sizes and offers custom sizing for those hard-to-fit slots.
Choosing the correct air filter size is essential for maintaining optimal air quality and ensuring your HVAC system operates efficiently. Knowing how to measure your air filter properly will help you find the perfect fit and avoid potential issues caused by an incorrect size.
The easiest method is checking your current filter. Look for size markings printed along the edge—they're usually there in black ink.
If the label is faded or missing, check your HVAC manual or look up the model number online. Many systems use standard sizes based on the unit specifications.
When in doubt, grab a tape measure and follow these steps:
Remember to measure the actual opening, not an old filter that might be warped or compressed.
Measure once, order with confidence from Filterbuy—we make it easy to find your exact size.
Not all HVAC systems use standard dimensions. Builders sometimes create custom-sized filter compartments, especially in older homes or unique installations.
If your measurements don't match any standard sizes, you'll need a custom filter. Common custom scenarios include:
Filterbuy offers made-to-measure filters shipped directly to your door. Just provide your exact measurements, and we'll create the perfect fit.
A properly fitted filter sits snug in the compartment without bending or forcing. You should see minimal gaps around the edges—small spaces are normal and won't hurt performance.
Signs of a good fit include:
Wrong-sized filters cause problems like:
Filterbuy makes finding your perfect air filter size simple:
Browse our complete size selection or request a custom filter—we've got the expertise to keep your air clean.
Understanding air filter sizes ensures better air quality and HVAC efficiency. The key is knowing your exact dimensions, whether that's a standard size or something custom-made.
Remember that the printed size is nominal (rounded), while the actual size is slightly smaller. When measuring, be precise and round up to find your nominal size.
Whether you need a common 20x25x1 or a custom size like 17.5x23.75x1, Filterbuy makes ordering simple. We clearly label dimensions and offer both standard and custom options to fit any system.
Browse filters by size or request a custom filter at Filterbuy today.
A lot of people are surprised to find that they don’t have a standard furnace filter size. Here’s a fun fact – 70% of what Filterbuy sells falls outside of the 10 most common air filter sizes. In this case, don’t worry because Filterbuy stocks over 300 sizes and upwards of 1300 unique products. And we even have custom sizes if you can’t find the one you’re looking for!
Air filters come in a variety of sizes, with some of the common ones being 10 x 20, 14 x 20, 16 x 24, 18 x 30, 20 x 30, and 24 x 24. These measurements are in inches and represent the length, width, and sometimes the thickness of the filter.
The numbers on air filters usually indicate the dimensions of the filter. This includes the length, width, and thickness. For instance, a filter labeled 20 x 20 x 1 has dimensions of 20 inches by 20 inches by 1 inch. There are typically 2 sets of sizes listed on filters, one is the nominal size, and other is the actual size. You can use either to find your filter size.
Absolutely, the size of your air filter matters! Your air filter must fit correctly into your HVAC system for it to function efficiently. An ill-fitting filter can allow unfiltered air to pass, like leaving the front door wide open. This reduces your system's efficiency and potentially damaging it.
Don't take your indoor air for granted! The most common air filter size in American homes is 20x20x1 inches, closely followed by 16x25x1, 16x20x1, and 20x25x1. The "1" represents the standard 1-inch thickness that fits most basic residential HVAC systems.
However, here's what many homeowners don't realize: while these four sizes represent roughly 60-70% of residential installations, your home might be different. After manufacturing filters for over a decade and working with millions of customers, we've learned that nearly 40% of homes actually require non-standard dimensions due to custom HVAC installations, older construction, or regional building practices.
The most common size air filter for a house is usually 20 x 20 x 1, but this can vary depending on the specific HVAC system installed.
A bigger air filter does not necessarily mean more air. However, it can mean better filtration as it provides a larger surface area for trapping particles.
A larger air filter can be better as it can last longer before needing to be replaced, due to its larger surface area. However, it must fit correctly in your system to function efficiently.
Air filters are different sizes to fit different systems. The size of the filter required depends on the size and design of the HVAC or engine system it's meant for.
To measure filter size, simply measure the length, width, and thickness of the filter using a tape measure. These measurements are typically given in inches.
The MERV (Minimum Efficiency Reporting Value) filter you need depends on your specific needs. For residential homes, a MERV rating of 8 to 11 is usually sufficient. Learn how MERV ratings affect filter performance and sizing.
While you can technically use a different size air filter, it's not recommended. An ill-fitting filter can allow unfiltered air to pass, reducing your system's efficiency and potentially causing damage. Always use the manufacturer's recommended filter size.
There’s a reason we set out to manufacture hundreds of options beyond the standard air filter sizes for homeowners and businesses in the U.S. Most people have an odd HVAC filter size. Most people won’t be able to find the air filter size they need at the local home improvement store. But you can find it on Filterbuy.com!
Order custom-sized filters manufactured to your exact dimensions. These aren’t imperfect, pre-made cut-to-fit filters. They are built from the ground up using your exact dimensions.
