What Size Air Filter Do I Need For My Furnace

Last updated on May 5th, 2024 at 03:13 pm

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The simplest way to determine what size filter to purchase for your furnace is to remove the existing filter and measure its dimensions. Typically, these are printed on one of the sides of your filter. You should be aware that the actual size of your filter will be slightly less than what is printed on it, sometimes by as much as a half-inch in each dimension.

For instance, your filter may read 16x25x1, but if you measure it with a ruler, you may have somewhat different results. This is because air filters must be somewhat smaller than the holes they fit into so you can easily replace them. If your filter and its slot were the same sizes, it would be difficult to install and remove it. 

If you cannot locate the dimensions listed on your current air filter, do not worry. Simply measure the height, length, and width of your filter and round the measurements to the nearest whole number.

Finding the Right Filter Size

A home’s air filter is a crucial component. These items have several benefits, including filtration of dangerous pollutants and maintaining HVAC system efficiency. While filters provide numerous benefits for your system, house, and health, you must replace them frequently. Additionally, it is essential to know the filter size you need.

The Most Common Furnace and AC Filter Sizes

These dimensions are represented by (Length x Width x Thickness). These are the sizes you’d find in a home improvement store. These figures refer to the standard size of your home’s return air vents, which control air flow to and from your HVAC unit.

  • 10 X 20 X 1’’, 12 x 12 x 1”, 14 X 20 X 1’’, 14 x 14 x 1”
  • 16 X 24 X 1’’, 16 x 20 x 1”, 16 x 25 x 1”, 16 x 25 x 4” 
  • 18 X 30 X 1”, 20 x 20 x 1”, 20 x 25 x 1”, 20 x 25 x 4”, 20 x 30 x 1”

How to Know You Have the Wrong Size

Unfortunately, a lot of people install filters that are the wrong size, which might cause some issues. For starters, this may have a significant impact on the number of air pollutants your system filters out, resulting in subpar indoor air quality. Second, this problem can make your HVAC system work harder than it should, which might result in various issues.

Here are a few indicators if you’re choosing the incorrect size:

  • If you hear rattling noises, the filter may be too small.
  • The filter is too big if it has to be pressed into place to fit.
  • If your house is dustier than usual, the filter could not be large enough.
  • When the filter is removed, if you observe rounded corners, this may indicate that it is too large.

Checking Your Filter Size

Not sure which filter size is best for your house? Although determining the size can seem simple, it may be trickier than you think. There could be severe issues if you purchase the incorrect size. You might experience greater energy costs, a malfunctioning system, or health issues.

It’s crucial to remember that the air filter and frame don’t have to be completely airtight. But there should be some room around the edges, and it should fit rather tightly. You probably need a different size if you have to shove the filter into position. The filter should be able to slide into the frame with some ease and just enough room to move around.

It can be necessary to measure if you’re unclear about the required filter size. However, it’s critical to comprehend the distinction between nominal and actual filter sizes before moving on to this phase.

What Is the Perfect Fit?

The manufacturers estimate the sizes of AC and furnace filters. The nominal size is what is meant here. The nominal size of the filter will remain the same regardless of the brand you select. However, it’s crucial to comprehend how this nominal size differs from the actual size.

Nominal Size

The manufacturer provides only an estimate for the nominal size. Typically, this number is placed on the side of the filter. The nominal size is a rounded approximation utilized for uniformity. 

During the manufacturing process, the dimensions of each filter will vary. If the manufacturer measured the true size, shoppers would be confused by the varying numbers. This standard size ensures consistency across the board, providing consumers with greater peace of mind and fewer misunderstandings. This also facilitates the comparison of filter types.

Actual Size

Simply put, an air filter’s precise size corresponds to its actual size. You’d have to measure the filter to get these precise values. Nominal measurements for AC and furnace filters are typically 1/4″ to 1/2″ larger than actual sizes. Fortunately, determining the nominal size of a filter is simple if you know its actual size. Simply round each dimension to the nearest inch.

DIY Measurements

To determine the size, power down the system, take the filter from the compartment, and measure it. If your filter fits without any concerns, this is the correct size. To determine the precise size, you need to take measurements. Use a tape measure or flat ruler to measure each side of the filter to get started. This covers the length, width, and depth or thickness. These should be measured in inches to determine their exact measurements.

Filter Compartment Measurements

Alternatively, size can be determined by measuring the frame the filter fits in. However, this might be a challenging procedure. To begin, power off the system and open the container. Then measure the slot’s length, width, and depth. Keep in mind that the dimensions of the filter compartment should be smaller than the AC and furnace filter diameters. We recommend measuring the filter instead, as this method can be difficult and lead to erroneous measurements.


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