What are the most common air filter sizes?

HVAC Logo IconBy HVAC.comSeptember 21, 2023
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Routine filter swaps are your key to a happy HVAC system. Still, understanding the most common air filter size can be a head-scratcher if you’re unsure what the numbers mean.

When it comes to changing out your air filter, size matters. Our guide helps you get an accurate measurement and find the perfect fit for your unit.

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How does filter size work?

Filters are labeled with three-part dimensions. The first number is its length, the second is its width, and the third is its depth or thickness.

For example, a filter labeled 10” X 12” X 1” is 10 inches long, 12 inches wide, and one inch thick.Here’s a look at some common filter dimensions.

  • 10 X 20 X 1’’
  • 14 X 20 X 1’’
  • 16 X 24 X 1’’
  • 18 X 30 X 1”

A filter’s advertised size is slightly bigger than its exact measurements. If you know how these two sizes relate, you’ll always get the right filter.

Signs your filter is the wrong size

The wrong size HVAC air filter can compromise your whole system. A filter that’s too small or large will not create the seal needed to filter all the air. The gaps will allow dust, dirt, and germs to bypass the filter and potentially recirculate back into your home’s air.

Here are some clues that you’re using the wrong size filter. If you’re still unsure, call an HVAC professional to confirm. 

  • If your filter makes rattling noises inside your system, it could be too small.
  • If you notice an increase in dust around your home, your filter may be too small. 
  • If you have to jam or force your filter into the slot, it’s too big. 
  • If your filter has bent corners when you take it out, it’s too big.

Filter nominal vs. actual size

Manufacturers label filters with an estimate of their actual size. This estimate is called the nominal size. If you know your filter’s nominal size, it will be a universal fit across any brand.

See how nominal and actual sizing relate.

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Actual size for filters

The actual size is an air filter’s exact size. If you measure your filter with a measuring tape, you’ll get the actual size.

An air filter’s actual size is usually ¼ to ½ inch less than its nominal size. To convert from actual to nominal size, measure your filter and round each dimension up to the nearest inch.

Nominal size

Nominal size is a rounded estimate of an air filter’s actual size. You’ll likely see the nominal size printed on the side of your filter.

Filter manufacturers use nominal sizing for industry consistency. Actual filter sizes vary by brand, which creates shopper confusion. Nominal sizing makes comparing similar products easier.

Once you find your filter’s nominal size, you can shop any brand with confidence. No matter where you are in your filter shopping journey, we’ve got you covered.

Check your filter frame for nominal size

If your filter is labeled, this is a breeze. Turn your thermostat off and remove your air filter.

Check the label, which should be on the side of your filter. The listed dimensions are your filter’s nominal size. If the filter fits properly, order a filter with the same nominal size from any manufacturer.

If your current filter isn’t labeled, take manual measurements. Measure your filter with the steps below.

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Measure your filter

Use a flat ruler or tape measurer to complete the measurements. Measure your air filter’s length, height, and thickness (depth) in inches.

Starting with the length, check each individual measurement. Round up to the nearest inch. You can shop any brand that sells your filter’s nominal size, knowing it’s a fit.

Measure the filter slot

As a last-resort way to find the correct air filter size, measure the air intake parameters. This is the most labor-intensive option, and it leaves the most room for error. Opt for this method only if you’re certain that your current filter is the wrong fit.

Find the slot that holds your air filter. It’s usually located in the blower compartment or return duct.

Measure the length, width, and depth of the space’s innermost edge. If you use this method, you’ll have to read the fine print for each filter’s actual size when you go shopping.

Your filter’s actual height and width should measure ⅛ to ⅝ inches less than the slot. If it’s the exact same size or bigger, it won’t fit properly.

If your filter sits in a vent rather than sliding into a slot, there’s less wiggle room. Your filter’s actual height and width should be about ⅛ inch shorter than its space.

We want to re-emphasize that nominal sizing won’t be helpful with this method. Every brand makes filters with slightly different actual sizes, so check each filter’s exact measurements before buying.

Best filter thickness

Most HVAC systems are built to accommodate 1”-thick filters. However, you can opt not to use the factory-installed filter racks and have an HVAC professional install an external filter rack for a thicker filter.  

Thicker filters can handle more buildup, so you don’t have to replace them as often. Plus, a thick filter traps more particles than a thin filter.But, be cautious. A filter that is too thick can hinder airflow, creating operational problems for the equipment.

Ultimately, a comfortable fit is far more important than filter thickness. Jamming in an oversized filter where it doesn’t belong can damage your HVAC system and won’t clean your air properly.

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Best air filter size

Finding the right size filter is an easier process than it may seem. Listen and look for cues your current filter may be under or oversized.

If the filter fits snuggly, use the numbers on the side to choose a replacement every 60 days, or as recommended by the manufacturer. If the filter size isn’t printed on the side, take manual measurements. Round up to the closest inch to create the nominal filter size, which is the measurement you’ll use when purchasing a filter.

Changing the HVAC filter is just one part of the routine maintenance you should perform on your system. Schedule a seasonal inspection twice yearly to extend the life of your HVAC and increase energy efficiency.

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