Quick answer: Work out your tent's volume (width x depth x height in metres), then choose a fan that can change all the air in the tent roughly once a minute, with extra capacity to make up for the carbon filter and ducting. Match the carbon filter's diameter and airflow rating to the fan. For most 1.2m tents a 5-inch fan and filter is a solid choice; a 4-inch set suits smaller tents. Use acoustic ducting and a speed controller if noise matters.
Why extraction matters
A grow tent is a sealed box with a light in it. Without extraction, heat and humidity build up quickly, plants struggle, and mould becomes a real risk. An extractor fan pulls warm, stale, moist air out and draws fresh air in. Add a carbon filter and the air leaving the tent is cleaned of smells on the way out, which matters if you are growing strong-smelling herbs or using organic feeds in the house.
Step 1: Work out your tent volume
Multiply the width, depth and height in metres. For example:
- 0.6m x 0.6m x 1.6m = about 0.6 cubic metres
- 0.8m x 0.8m x 1.8m = about 1.2 cubic metres
- 1.0m x 1.0m x 2.0m = 2.0 cubic metres
- 1.2m x 1.2m x 2.0m = about 2.9 cubic metres
- 1.5m x 1.5m x 2.0m = 4.5 cubic metres
Step 2: Turn that into a fan size
Fan airflow is usually given in cubic metres per hour (m³/h). A common rule of thumb in grow shops is to aim to change the air in the tent about once a minute. That means multiplying the tent volume by 60.
So a 1.2m x 1.2m x 2.0m tent at about 2.9m³ works out at roughly 175m³/h.
That is the minimum. A carbon filter, long ducting runs and bends all slow the air down, and a hot light adds to the load, so it is sensible to choose a fan with plenty of headroom and turn it down with a controller. A fan running at part speed is also quieter than one flat out.
Typical fan and filter sizes by tent
Here is how we put our own tent kits together, which gives you a tried-and-tested starting point:
- Bay6 1.2m budget kit: RVK 100A1 4-inch fan with a Mammoth 4-inch carbon filter (0.345m) and 4-inch combi ducting.
- GorillaBox 1.2m kit: RVK 125A1 5-inch fan with a Mammoth 5-inch carbon filter (0.345m) and 5-inch acoustic ducting.
As a general guide, 4-inch sets suit small and medium tents, 5-inch and 6-inch sets suit 1.2m and 1.5m tents, and 8-inch and above are for larger tents, multiple tents or whole rooms. If you are running a powerful light, growing in a warm loft, or planning to add a second tent later, go up a size.
Choosing a carbon filter
Match the diameter
Your filter flange should be the same size as your fan. If you need to join different sizes, use a metal duct reducer, but keep in mind that squeezing air down to a smaller duct reduces airflow.
Match the airflow
Every filter has a maximum airflow it is designed for. Pushing more air through than it is rated for means the air passes too quickly for the carbon to work properly. Choose a filter rated at or above your fan's output. Our Dutch Touch filters, for example, are listed with their ratings, from the 10-inch 75cm at 2250m³ to the 12-inch 1m at 5000m³.
Keep humidity in check
Carbon does not like very damp air. Dutch Touch state their filters can last up to 18 months when run with humidity at or below 70%. Mammoth quote 12 to 18 months depending on use. Keeping your tent humidity under control makes your filter last longer.
Fit it the right way round
Most growers hang the filter inside the tent at the top, where the warmest air collects, with the fan pulling air through the filter and pushing it out of the tent. Make sure air flows into the filter, not out of it.
Ducting
Combi ducting
Combi flexible ducting has a PVC outer and aluminium inner layer with a steel wire helix. It is flexible, affordable and does the job well.
Acoustic ducting
Acoustic ducting has a perforated inner layer and a fibreglass insulation layer that reduces noise breaking out of the duct and acts as a silencer. For best results, use it on both sides of the fan. If your tent is in a bedroom or a shared house, this is money well spent.
Keep runs short and straight
Every bend and every extra metre of ducting slows the air. Pull the ducting tight so it is not sagging, avoid sharp kinks, and seal joints with jubilee clips and metallised tape so you are not leaking air.
Fan speed and controllers
Running a fan at full speed all the time is noisy and, in winter, can pull too much heat out of the tent. A speed controller lets you set the fan to what the tent actually needs. Standard AC inline fans such as the RVK range work with compatible speed controllers. EC fans, such as the Rhino Ultra Silenced EC fan and K-Line Silent EC fan, offer smooth speed control and lower power use, and work with their matching controllers. Always check the controller is compatible with your fan type.
Quiet options
If noise is your main worry, look at silenced fans. The Isomax has a silencer built into the housing, and the Rhino Ultra has an acoustic foam-lined casing. Hanging the fan from a rope ratchet or strap rather than bolting it to a hard surface also cuts vibration.
Do not forget air movement inside the tent
An extractor changes the air; a clip fan moves it around the plants. Gentle air movement strengthens stems, evens out temperature and helps prevent damp spots. The Vortex 8-inch EC clip fan fits tent poles from 15 to 30mm and has 10 speed settings.
FAQs
What size extractor fan do I need for a 1.2m grow tent?
For a standard 1.2m x 1.2m x 2.0m tent, a 4-inch or 5-inch fan with a matching filter works for most growers. Go for 5-inch or 6-inch if your space is warm, you use a powerful light or you want to run the fan quieter at lower speed.
Can I use a bigger fan than I need?
Yes, and many growers do, as long as you control it with a suitable speed controller and your filter is rated for the airflow.
Do I need a carbon filter?
If you want to stop smells leaving the tent, yes. It also stops dust and debris being blown around the room.
How do I know when to replace my carbon filter?
When you start to notice smells getting past it. Depending on use and humidity, Mammoth quote 12 to 18 months and Dutch Touch up to 18 months.
Need help sizing your extraction?
Tell us your tent size and where it lives and we will match a fan, filter and ducting for you. Visit us at Unit 2 BlackLake, West Bromwich, B70 0PJ, call 0121 553 7872 or WhatsApp us. Browse the full fans and filters range. Free UK delivery over £99, and we price match.