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How to Measure Your Air Filter

With all the different air filter sizes available, it can be daunting to find the right one for your unit. There are literally thousands of filter sizes, and HVAC manufacturers are incentivized to make it as tricky as possible so that you are stuck paying for their overpriced replacements.

Don’t worry, though; we’ve got you covered. This article takes you step-by-step through the process of identifying your air filter size so that you can buy a replacement without any hesitation.


Start by checking the size written on the side of your filter.

Most filters come emblazoned with two size measurements that can be found along the filter’s side.  Often there is a ‘nominal’ size written in big print, as well as an adjacent ‘actual’ size written in smaller print. All of the filters listed on filterbuy.com are listed by nominal size unless specifically labeled otherwise.

example of air filter sizes located on the side of your air filter

image of a tape measure used to measure your air filter

No size written on your filter? No problem - you can take measurements yourself.

If the size is not written on the side of your filter, the next step is to take out your trusty tape measure. You need to measure the length x width x depth. The length and width dimensions are interchangeable, though conventionally the first number is the smaller of the two.

As for the depth measurement, standard filters are nominally 1” (actual 0.75”), 2” (actual 1.75”), and 4” (actual 3.75”) deep. If your filter does not echo these measurements, then you need to look for your brand on our whole house replacement filters page.

how to measure the dimensions of your air filter

image of commonly sized air filters

Find Common Filter Sizes

Once you find your size, you can search for it on our common filter sizes page. However, if you don’t see your size, you can create it yourself using our custom filter tool.

With these measurements in hand, you can search for your filter size on the “Common Filter Sizes” page of filterbuy.com. If you don’t see your size, though, you can create it yourself using our custom filter tool. A note on nominal vs. actual sizes: the size listed in large print on the size of your filter is generally the ‘nominal’ filter size. The actual filter size is generally .25” to .5” less. We do our best to specify actual dimensions throughout our website so that there is no confusion. However, filters within .25” or less of your existing filter are generally interchangeable.

Still confused? No problem, we’re happy to help simply call us at 855-FILTBUY or send us a note on our contact page. We answer all correspondence within 24 business hours.