How Do I Measure My Air Filter?

At Air Filters Delivered, a common question we hear is "How do I know what size air filter I need?" Often the nominal size is printed on the side of the filter, but when it's not, you'll need to know how to measure an air filter. Watch the video below for step-by-step instructions to find the actual dimensions and get the nominal size for your system. Keep in mind that some HVAC systems require more than one air filter, so make sure you have the right measurements for all of them.

How to measure your air filter

When you need to replace your filter, these simple steps will help you measure the right dimensions. Remember, regular maintenance of your home’s cooling and heating system is essential to ensure it operates efficiently. Proper maintenance will keep good air quality in your home, increase the life span of your system, and consume less energy lowering your bills.

Your filters should be changed every 30 to 90 days depending on the type and efficiency of your filter. However, according to Energy Star, filters should be changed every month during peak summer and winter months when your HVAC system is operating at its highest capacity.

For safety, turn off your HVAC unit before attempting to change the filter. If you can't figure out how to turn off the unit, turn off the breaker. Most air filters are located to the right or bottom of the unit and should can be easily pulled out of the slot. Although not common, some filters are found in air vents instead. If that is the case, you might need to contact a professional for assistance.

How to Measure an Air Filter Video Transcript

Hi, I'm David. Welcome to In this video, I’m going to show you how to measure your air filter. Before we start, you’re going to need a few things. You’ll need a measuring tape, a piece of blank paper, and something to write with. Now, let’s get started.

As a safety precaution, we recommend you turn off your HVAC unit before removing the air filter.

The majority of air filters are located to the bottom of the air handler and should easily pull out of the slot. Some air filters, however, can be found in the central air return register vent.

Once you’ve removed the air filter, take a look at the edges. Some air filters, like this one, have the dimensions printed on them. If the dimensions aren’t displayed, you’ll need to use your measuring tape.

First, let’s determine the length. If the air filter isn’t a square, this will be your smallest measurement. Now let’s measure the width. The final measurement we’ll need is the depth. This is the thickness of the air filter. Now, we’ll list all the measurements length by width by depth. This is the actual filter size and true dimensions.

As you can see, our actual dimensions include fractions of an inch. So, we’ll need to round up every fraction to the closest whole number. So, nineteen and a half inches will become twenty inches and three-quarters of an inch will become one inch. This new rounded up measurement is referred to as the nominal size. Standard size air filters are typically not sold by their actual dimensions, but instead sold by their nominal size. And that is how you measure your air filter.

Remember, regular and proper maintenance of your home’s cooling and heating system will maintain good air quality, increase the lifespan of your system, and consume less energy to help lower your bills.

To find out how frequently you should change your air filter, please watch our “How Often Should I Change My Air Filter” video.

Follow these 3 easy steps to measure your AC filter size:

  • Step 1 - Measuring Length and Width (L x W). (If your filter is not a square, the smallest measurement goes first).

  • Step 2 - Measure the thickness (Depth) of your filter from front to back.

  • Step 3 - Put Your measurements together (L x W x D).

  • It’s as simple as that! You are now ready to buy the right filter for your AC or furnace. Wondering what to do with your old filter now that you’ve measured it? Read more to find out how to properly dispose of your air filter or if your dirty air filter is recyclable.

Most Common Air Conditioner Filter Sizes

    At Air Filters Delivered, we offer more than just standard size filters. Our product assortment contains every hard-to-find size you can imagine, in addition to custom sizes depending on your specific needs. Below are lists of the most common air filter sizes by depth.

    • 1 inch: 16x25x1, 20x20x1, 20x25x1, 16x20x1, 14x18x1,14x20x1, 14x24x1, 14x25x1, 14x30x1, 18x20x1, 20x30x1, 24x24x1, 12x24x1, 14x14x1, 16x24x1, 20x24x1, 12x12x1, 12x20x1, 16x30x1, 18x18x1, 24x30x1, 10x30x1, 12x30x1, 16x16x1, 18x24x1, 18x30x1, 20x22x1, 25x25x1, 15x20x1, 20x23x1
    • 2 inch: 20x20x2, 20x25x2, 16x25x2
    • 3 inch: 16x25x3
    • 4 inch: 20x25x4, 16x25x4, 20x20x4, 16x20x4
    • 5 inch: 20x25x5, 16x25x5
    • 6 inch: 20x25x6