We have some small room air filters in the bedroom of my home which we run pretty regularly to help cut down on some of the dust in the house that’s generated from an active family, two cats and an old force hot air heating system. The little air purifiers work pretty well, the filters were getting a bit expensive for my tastes, so I decided to see if I could just make my own air filters for a fraction of the cost.

Just buy a larger (and cheaper) air filter of the same thickness and cut it down to size.

Turns out, it’s pretty easy! You can essentially buy cheap air filters and cut them to whatever custom size you need.

I’ve had an ongoing battle with dust in my home for a while, and each thing I do makes the situation a little better. I regularly change the large air filter for my home as part of my regular furnace and air conditioner maintenance and I even wrote a review of a professional duct cleaning service. I also regularly vacuum and we try to keep our cats away from our bedrooms when we can.

I decided to also get two $30 small room air cleaners and I set one up in each bedroom. They work well, but when it came time to replace the filters I was shocked to learn that the filters it needed sold for over $12 a piece for what was essentially a piece of cardboard with filter paper in it. Even worse, I soon learned that the exact sized filters I needed were discontinued, so online sellers were basically raising their prices whenever they wanted.

Now, general air cleaners are not particularly complex machines. Most are just plastic boxes that have a fan and a filter. Yes, you can actually just place an air filter over a box fan and make a quick air filter that way. The fan pulls the dirty air in, the filter traps the dirt and dust and the clean flows out the other end of the fan. Some fans are fancier than others, but the filter paper used in the majority of air purifiers is about the same, though the exact specifications may vary. Remember: this is for air purifiers and filters that use paper filter boxes. You don’t want to go replacing HEPA filters or other spongy types of filters with simple paper filters. It may filter your air, but it’s not going to do as good of a job. Your air filter is only as good as the filter type being used.

Here’s what I did to make my own air filters:

1. Measure your old air filter: This is probably the most important step. For most air cleaners the air filter has to be an exact size to fit into the device, so you want to be sure you get as close as you can. For my air cleaner the thickness of the air filter is what mattered most. My filter was about 1 inch thick.

2. Buy a large air filter replacement: I then took my measurements and when to my local mega hardware store where I had about 700 air filter replacements to choose from. Now that I know this works I’ll probably just purchase a pack of cheap furnace filters online and save myself even more money in the future. I purposely picked a home air filter that was 1 inch thick and was larger than my small 10 x 9 inch filter. I got one with a thin wire mesh on the filter paper because I figured that would help the filter retain it’s rigidity when I cut it. If I had been smarter I would have bought a filter that was at least TWICE as large so I could maybe cut two replacement filters out of one cheap filter. I ended up buying an air filter replacement that was $3.99, which is a heck of a lot cheaper than $12!

Cardboard from a cereal box and some invisible tape are all you need to build an edge around your air filter.

3. Cut the air filter to size: Measurements are nice, but for this I ended up just placing the old air filter on top of the new one, lining up one corner. I then traced the exact size out of the old filter on the new filter. You want to line up the corner so you only have to cut two sides and can leave two sides in tact.

One important step: be sure you mark which way the air is supposed to flow. It’s usually on the edge of the air filter and it does make a difference. Air filters are designed to have air go in one way and out the other. Once everything is measured you can take a hefty pair of scissors and start cutting. You may want to wear gloves or use a mask for this, just to avoid cutting your hands on any wires or breathing in any dust particles from the filter (though they should be safe).

The finished air filter doesn’t look pretty, but it works great.

4. Reinforce the cut air filter edges: Now you have an air filter that probably has a cardboard edge on two sides and no cardboard edge on the other two sides. Those cardboard edges are important because they give the filter some form and they prevent air from flowing around the filter when it’s in place. To put new filter edges back on, just use some thin cardboard (the cardboard from a cereal box works nicely) and some simple tape. Remember, these filters only last 90 to 120 days, so they don’t have to be made out of plastic or steel. In about 10 minutes I was able to cut out and fold an edge around my air filter, keeping everything in place with a few pieces of invisible tape.

It wasn’t pretty but it met the specifications of the old air filter, so I slipped it in and gave it a shot. The whole project ended up taking me no more than about 20 minutes from start to finish and I ended up saving $8 right off the bat.

I’ve had my new DIY air filter installed in my air cleaner for about a week now and everything is working just as it was before, so I’d consider this project a success!