How to Build a DIY Carbon Filter

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So you’ve been investigating the various challenges of growing an indoor marijuana crop and you’ve hit upon the important role of the carbon air filter in ensuring your grow room is free of dust, pollen, odd particles and unwanted smells. And being the handy, free-spirited type that you are, you’ve decided to make your own DIY carbon filter.

Good for you! Is there anything you won’t try?

Things to Consider When Making a Grow Room

If you want to grow marijuana indoors, then you need to consider the space needed to grow it. Building a dedicated grow room in your basement or a spare room has its challenges, which is why many casual growers opt for purchasing grow tents. Indoor-grown marijuana requires a controlled environment, and a room where doors or windows can let in light and outside particles poses a greater risk to your plants’ health.

Another grow environment option that works especially well for those with limited space is the space bucket. These self-contained units are basically just a large container with a light-tight lid outfitted with grow lights and a ventilation system.

Mind you, a ventilation system is important. Because whether you contain your plants in a grow tent, space buckets, or in a dedicated grow room, proper ventilation will be key to producing a healthy crop.

Growing Marijuana and Cannabis Plant

And as much as you may love your weed-growing operation, you will need to consider your neighbors who may not take kindly to the pungent aroma brought on by the smelly terpenoids of your plants. Including a carbon filter in your ventilation system will not only scrub the aroma from your grow room, it will contain the smell so your neighbors don’t get pissed.

If you’ve decided to go the grow tent route, there are plenty of good ventilation kits for grow tents available online. Likewise there are carbon filter options for space buckets if that’s your preferred grow method.

But you’re the handy type, right? If you’re interested in building your own DIY carbon filter for your grow room, the process is simple enough and if you keep reading we’ll show you how.

Just keep this one thing in mind: If your DIY carbon filter and exhaust fan are to be installed in an open grow room, it is very important that you make sure your windows, doors and any other openings are well-sealed before you get started. Otherwise, it will be a wasted effort. Capiche?

What Are The Essential Parts of an Air Carbon Filter?

Standard air-purifying, carbon filters have only a few basic components–a pre-filter, a filter body with activated carbon, and an air passage. Made of an air-permeable medium, the pre-filter, which surrounds the carbon filter, does just what the name suggests. It allows air to pass through while blocking dust and other particles from getting to the carbon.

The filter body is composed of an inner and outer layer of mesh with a layer of activated charcoal between the two. As air passes between these two layers, the activated charcoal traps the smelly hydrocarbons not captured in the pre-filter through a process known as adsorption. As a result, only clean, odor-free oxygen is allowed to exit the filter.

Air filter and the cabin filter

The air passage is the four to eight-inch cylinder that stands between the two flanges of the filter. In order for the process to be successful, it is imperative the flange connections connect tightly with the diameters of the ducting and inline fan.

DIY Carbon Filter for Grow Room: What Items Do You Need?

The nice thing about making a DIY carbon filter is that the components are inexpensive and often things you can find around the house:

  • Large (4.75-inch) mesh pencil cup
  • Small (4-inch) mesh pencil cup
  • Zip tie
  • A pair of women’s over-the-knee Stockings
  • Bamboo Activated Charcoal, uniform in size and finely granulated

In addition, you’ll need an ordinary drill and 5/32 drill bit for making holes, a pair of scissors, and measuring tape. Duct tape is optional.

Step-by-Step Procedure for Making Your Carbon Filter

While these written steps are fairly self-explanatory, if you do better with visuals, you can find a few helpful YouTube videos online.

  • Step 1: Prepare the Two Pencil Cups. It’s important that you pick appropriately-sized pencil cups for this DIY project. You’ll want your smaller cup to be about 3/4 of the height of the larger one. Additionally, because the smaller cup will be connecting to your ductwork, the diameter of the cup should exactly match the diameter of the ducting. Lastly, when you place the smaller cup inside the larger one, there should be a 12 to 24 millimeter space between them.
  • Step 2: Drill Holes on the Bottom Surface of the Cups. Using a 5/32 bit, drill holes all over the base of each of the cups. This will allow air to permeate from all sides. Eight to twelve holes in each should be about right.
  • Step 3: Put Both of the Cups into the Stockings. Take the pair of stockings and stretch the openings, then place one cup in each stocking with the bottom side down and push each to the bottom of the stocking so the entire surfaces of the cups are completely and uniformly covered by the mesh. This mesh will serve as the pre-filter.
  • Step 4: Fill them Up with Activated Carbon. Fill the two cups with activated carbon, but fill the larger of the two only about half way.
  • Step 5: Seal the Activated Carbon Within The Two Cups. Place the smaller stocking-covered cup inside the larger one resting on top of the carbon. The tops of the two cups will not be parallel. Pull the outer stocking (the one around the larger cup) up to the top of the inside, smaller cup so its body is fully covered. Now hold the arrangement upside down so the activated carbon gets evenly distributed in the gap between the larger and smaller cups. You’ll want to make sure there is an even layer of carbon on all sides including the space between the bottom surfaces of both cups. Once you are finished, turn the two cups upright again and adjust them as needed so that the tops of both cups are at the same level.
  • Step 6: Adjust the Socks. Now pull the stocking evenly off of the inside cup and place it evenly once more around the outside cup. The two cups should be in a secure position.
  • Step 7: Attach it to Your 4” Ducting/Fan. The last thing to do is attach your carbon filter to the 4-inch ducting or inline fan of your system. Secure it tightly with a zip tie. If you’re unsure the connection is tight, try twisting the filter around the connection. You’ll know it’s secure, if you can’t move it.

Things to Keep In Mind

If you’re going to make a DIY filter, it’s important to do it well. Make sure the pencil cups have equal diameters at both top and bottom and that the distribution of the activated carbon in the gap between the two cups are even throughout the perimeter.

Additionally, the central axes of the two cups should be on the same line and the two openings on the same level.

Cabin air filter carbon

By the way, don’t forget to change the carbon filter regularly. Nothing lasts forever and it’s best not to figure it out when things start to smell.

What if You Need A Bigger Carbon Filter?

If you’ve got a bigger growing area than a 4-inch DIY carbon filter can handle, you’ll need to improvise and come up with larger substitutes for the same basic components. For example, you can swap out the large pencil cup for a mesh trash bin and substitute a PVC drain pipe for the small pencil cup.

You can also use polyester fill media instead of hosiery and make some healthy use of duct tape. This is where having a good imagination can pay off!


As handy as you may be at making a DIY carbon filter–and we hope this primer was helpful!–it will only be as good as the ventilation it is attached to. Your exhaust fan will need to match your filter strength, and this may require a little trial and error with the amount of activated carbon you use to get the odor-free results you are aiming for.

As with everything, if you want good results, you’ll need to employ a little patience. We know you’re up to it.