Cut to Fit Air Filters: Everything You Need to Know

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If you have a custom-shaped vent, then you don’t need to buy a custom-shaped filter just to keep your home’s air clean.

Below, I’ll discuss how cut-to-fit air filters will benefit your home, when you should use them, and how easy it is to install them.

I’ll show you how to put a filter in any vent– without breaking the bank.

When you should use a cut to fit air filter

Cut to fit air filters bring the same benefits that normal air filters do– with one addition. Not only do they keep your home’s air clean, but you can also cut them into any size or shape you need. 

Here are a few common situations where a cut to fit air filter comes in handy:

Using cut to fit air filters for a custom size vent

Unfortunately, many older homes have vents that are irregular in size or shape. Vintage homes have all sorts of weird air vents. Besides normal rectangular-shaped vents, I’ve seen circular, half-circular, and even heart-shaped vents!

It’s pretty obvious that for a circular air vent, you’ll need a custom filter shape. But there are also rectangular-shaped air vents that have an irregular size. Even though the shape is correct, you’ll be hard-pressed to find an air filter that fits securely into these vents.

Besides old homes, I’ve also seen irregular-sized air vents in apartments. If you rent or live in a place where you can’t install a normal return vent, then a cut-to-fit air filter is your only choice.

Cut-To-Fit Premium Air Vent Filters

These air filters cut and stick easily with included installation tape. They are efficient enough to filter out common household particles, but also lightweight enough to have minimal impact on airflow. 

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Using cut to fit air filters for Window ACs

Window air conditioners always come with their own built-in air filter. But what if that filter breaks or gets lost? You definitely don’t want to run your window AC without an air filter.

Some people would tell you to get rid of the AC and buy a new one. Window ACs are pretty cheap, and this is definitely a valid option. But you’re going to break a sweat swapping out what is otherwise a perfectly good air conditioner.

A better solution would be to use a cut to fit the air filter instead. Just open up your window AC’s return grille on the front, and measure the filter slot to find the filter’s dimensions.

After you get the length and width of the AC’s filter slot, cut out a piece of filter media and place it in the slot. Close the return grille and boom, that’s all there is to it!

One thing to keep in mind when using a cut to fit air filter in your window AC is the thickness of the filter. The built-in filters that come with window ACs are sized to perfectly fit in the AC’s filter slot. 

So you’ll want to make sure that your cut to fit air filter is not too thick. If your air filter is too thick, then you won’t be able to close the filter grille.

Using charcoal cut to fit air filters

What’s that smell? Is it your pets? Last night’s cooking? Or maybe it’s just one of those strange household smells that seemingly comes out of nowhere?

Whatever it is, a carbon or charcoal air filter can help get rid of it.

Charcoal cut to fit air filters on your return vents might be the one thing you need to get rid of those stagnant odors that plague your home.

Using a charcoal cut to fit air filter is the same process as any other cut to fit air filter– except it also gets rid of smells.

My favorite use case for charcoal cut to fit air filters is in a window AC that’s in a work area such as a garage or workshop.

In a work area, strange smells are a natural occurrence from the various materials and things that are stored in there. 

Things get pretty smelly in a workshop full of you and your closest buddies while wrenching on a car.

Charcoal air filters will make quick work of those smells– leaving your garage without any odors.

Breathe Naturally Universal Cut-to-Fit Activated Carbon Filters

This carbon pre-filter material can be cut to size for your air purifier. In addition to eliminating odors, this material also traps large particles, extending the lifespan of your HEPA filter.

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How to install cut to fit air filters

Installing cut-to-fit air filters is a little more complicated than just swapping out air filters as you would normally. There are two additional steps– measuring and cutting.

Measuring your air filter

In order to get the correct size air filter, you’ll need to get the correct measurement.

The first step is to get the measurement of the place that you’re going to install the air filter– whether it be a return vent, filter holder, or something else.

If you need a rectangular filter, find the length and width of the place that you want to install the air filter. 

The measurement doesn’t need to be exact. In fact, you might want to go a little bigger or smaller depending on where you need to place the filter.

For instance, if you are installing the filter on a return vent then it doesn’t hurt to overlap the sides of the vent. 

On the contrary, if you are replacing the filter in a window AC, you might want to go a little smaller so the filter fits in the unit without having to force it in.

If you have an irregularly shaped air vent, then you’ll have to think outside the box. Most times it’s helpful to trace out the template of what the filter’s shape needs to be.

To do this, place a piece of paper over the vent and draw a rough shape of what the vent’s shape is. Leave a good amount of extra space around the edges (you can cut off any excess later).

Cutting your air filter

The next step is to cut your air filter. But before you make the first slice, you need to take your measurements and mark them on the filter material.

You also need to make a filter a little bigger if you’re using air filter installation tape to secure the filter. Add about an extra inch to each side of the filter to give it more space to attach to the tape.

If you’re cutting a rectangular filter: You can use a straight edge to draw a line so you know exactly where to cut.

If you’re cutting an irregular-shaped filter: Use the template that you made earlier and trace it on the filter material.

After you put your markings on the filter material, it’s time to make the cut. For this part, you’ll want to use a sharp pair of scissors. 

Once you start cutting, don’t worry about cutting things too small– you can always go back and trim it down later if the filter is too large.

Installing your air filter

Finally, installation of the air filter will follow one of these steps:

  • If your air vent has a place to securely hold an air filter, then you’re set to install it. Just place the filter in the holder.
  • If your air vent doesn’t have a place to hold the filter, then you’ll need to secure the filter to the vent. Most cut to fit air filters come with installation tape. Installation tape is used to hold the air filter in place.

How to use air filter installation tape

Air filter installation tape goes around the perimeter of the air vent where you want to install the filter. The installation tape is made of small plastic teeth that grab the air filter material and hold it in place.

After the tape is installed, put the air filter on the vent and secure it to the tape. Now, the filter will hold in place while it’s on the air vent.

Can you cut down regular air filters to fit?

A lot of people ask: Why can’t I just cut down a regular air filter to fit the size of my grille? That seems easy, right?

The truth is that regular air filters aren’t made to be taken apart and cut down.

First of all, most pleated air filters are made of polyester or cotton sheets. These sheets have folds in them that won’t allow them to lay flat on a return vent. 

Worse yet, pleated filters have a wire mesh that holds them together. If you want to get to the filter material, you’ll need to remove the mesh. It’s like fighting with barbed wire to get it apart.

In the end, using a cut to fit air filter is a much better solution.

Hi, my name is Trey Lewis and I’m the founder and chief editor at HVAC Training Shop. My goal for this website is to help homeowners troubleshoot and maintain their home’s HVAC systems. Whether it’s changing an air filter, troubleshooting a blower motor, or just buying a new humidifier, I want to make sure that you’re covered.

6 thoughts on “Cut to Fit Air Filters: Everything You Need to Know”

  1. Hi there, my wife suffers from severe allergies, and we are living in a fairly old house. The filter in the HVAC system doesn’t seem to be able to handle enough of the filtering to prevent her from having allergy attacks from dust. I was thinking of putting an external filter sheet over the intake vent to try and cut down on some of what is being sucked into the furnace. It’s the kind of filter you can get at Amazon for eight bucks and you Velcro it to the outside. How much extra work will this cause for the HVAC system? I Had the furnace checked over and even though it’s old and got a clean bill of health, and I had the ducts cleaned, but a lot of dust is still making it through.


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