How to Measure Your Air Filter | Find Your Air Filter Size

With all the different air filter sizes available, it can be daunting to find the right one for your AC unit. There are literally thousands of air 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 guide will take you through the process of identifying your air conditioner filter size step-by-step, so you can buy a properly sized replacement air filter without any hesitation.

find custom filter sizes at Filterbuy

Check for Air Filter Dimensions on the Side of your Air Filter

Most filters come labelled 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 are listed by nominal size unless specifically labeled otherwise.

Examples of common air filter sizes and dimensions, here showing standard 1” furnace filters

This is the easiest and most obvious way to find your ac filter size, but not all filters have size measurements listed on them. In this case, finding the size of your filter will require taking a few measurements by hand.

The Difference Between Nominal and Actual Size in Air Filter Measurements

A lot of our customers can sometimes get confused by the difference between nominal vs. actual sizes listed on replacement air filters.

  • Nominal Air Filter Size – the “nominal” size lists the general dimensions often rounding up or down to the nearest whole or half number to make it easier to keep track of dimension sizes for ordering replacements. It’s a sort of shorthand that defines the size of vent the air filter itself will comfortably fit into.
  • Actual Air Filter Size – The actual size of an air filter is often .25” - .5” less and shows the air filters actual dimensional specs

The size listed in large print on the size of your filter is generally the ‘nominal’ filter size. 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.

How to Measure Your Air Filter Dimensions

If the size is not written on the side of your air filter, the next step is to take out your trusty tape measure.

Image of a tape measure used to measure air filters that do not have sizes listed or are not standard sized ac filters

You need to measure the length, width, and depth.

For air filters, the length and width dimensions are interchangeable, though conventionally the larger of the measurements is the width and the shorter of the measurements is the length. The smallest of the dimensions will almost always be the depth.

For example, if an air filter’s dimensions are 12” X 20” X 1”, that means the following:

  • Width = 12”
  • Length = 20”
  • Depth = 1”

In some cases the length and the width are interchangeable, but you’ll always need to make sure that you measure for those 3 specific air filter or furnace filter dimensions.

Below you can see an example of an air filter size chart:

air filter size chart by Filterbuy

As for the depth measurement, standard air filters sizes are nominally 1” (actual 0.75”), 2” (actual 1.75”), and 4” (actual 3.75”) deep. These standard air filter sizes are easier to find and are the most commonly used. To shop for these standard filters by size, click below.

What If Standard Filter Sizes Don’t Match Your Air Filter’s Dimensions?

If you don’t see your size, you can create it yourself using our custom filter tool. Building a custom AC or furnace filter allows you to pick specific dimensions if the standard sizes don’t work for you.

Whether you decide on custom or standard, we always provide the ability to select performance rating of filter, pick the quantity of filters, and choose whether you would like filters delivered at recurring intervals.

Image of commonly sized air filters, including a 1-inch filter, a 2-inch ac filter, and a 4-inch standard size filter

If the filter you are looking for does not match these standard measurements, then you can look for your brand on our whole house replacement filters page or request a custom size filter!

Find the Right Air Filter Size for You

Once you properly measure your air filter and get the right dimensions, you can search for it on our common filter sizes page. We recommend you start your replacement air filter search by narrowing down to your air filter’s depth.