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 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 labeled 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.

How to Measure Your Air Filter - Image of custom size air 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 the 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 filter's 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.

How to Measure Your Air Filter - Image of a tape measure to measure air filter sizes.

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:

How to Measure Your Air Filter - Image of how to identify which part is the depth, length and width which will help you how to measure your air filter.

As for the depth measurement, standard air filter 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 the performance rating of the filter, pick the number of filters, and choose whether you would like filters delivered at recurring intervals.

How to Measure Your Air Filter - Image of different types of air filter from the most common air filter size like a 1-inch filter, a 2-inch ac filter, and a 4-inch standard size filter up to the custom size air conditioner filters.

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 it down to your air filter’s depth.

How Do I Know My Air Filter Size?

To determine your air filter size, you need to measure its length, width, and depth. These measurements are usually printed on the side of the filter. If not, you can use a tape measure to get the dimensions.

How Do You Calculate Filter Size?

The size of the filter is calculated by measuring its length, width, and depth. Always measure from edge to edge. Remember to write down the measurements in the same order: length, width, and depth.

How Is Air Filtration Measured?

Air filtration is measured in MERV (Minimum Efficiency Reporting Values), a standard that rates the overall effectiveness of air filters. The higher the MERV rating, the finer the filtration and the more particles it can filter out.

How Do I Know What Size Air Filter My Car Needs?

To know what size air filter your car needs, consult the vehicle's owner's manual. If it's not stated there, you can remove the existing air filter and measure it. It's important to get the right size to ensure optimal performance and protection for your engine.

What Is the Normal Air Filter Size?

Normal air filter sizes vary depending on the system they are intended for. In general, standard sizes for home HVAC systems range from 10" x 20" to 25" x 25" and depth can range from 1" to 6".

What Is a Standard Air Filter Size?

A standard air filter size for home HVAC systems is often 20" x 20" x 1". However, the size can vary greatly depending on the specific system. Always measure your existing filter or check your system manual to ensure you get the right size.

How to Measure Air Filter on AC Unit?

To measure the air filter on an AC unit, remove the filter and measure its length, width, and depth. Always measure from edge to edge and round up to the nearest inch if necessary.

How Often Should I Change My Air Filter?

It's recommended to change your air filter every 60 to 90 days. However, if you have pets or allergies, you might need to change it more frequently, possibly every 30 days.

Do You Need an Exact Size Air Filter?

Yes, you need the exact size air filter. Using the wrong size can lead to inefficiency and potential damage to the system as it may allow unfiltered air to pass through.

Can I Use the Wrong Size Air Filter?

Using the wrong size air filter is not advised. It can lead to decreased efficiency, increased energy costs, and potential damage to the system as unfiltered air is allowed to pass through.

How Do I Choose an Engine Air Filter?

When choosing an engine air filter, consider the vehicle's make, model, and year to ensure compatibility. Also, consider the MERV rating for the filter's effectiveness in removing particles and pollutants.

Are Car Air Filters a Standard Size?

No, car air filters are not a standard size. The size varies depending on the make, model, and year of the vehicle. Always refer to your vehicle's owner's manual or measure the existing filter to determine the correct size.