How to Size an Inline Fan for Your Grow Tent
Your exhaust fan should exchange the entire volume of tent air about once a minute. That single rule drives temperature, humidity, CO2 replenishment, and odor control.
The calculation
Multiply length x width x height in feet to get cubic feet. A 4x4x6.5 tent is about 104 cubic feet, so you need at least 104 CFM to turn it over once a minute.
Add back your losses
A carbon filter costs roughly 25-40% of rated CFM. Every 90-degree bend and every foot of flexible ducting costs more. Multiply your base number by about 2 to size the fan you actually buy.
- 2x2 tent: 4-inch fan, ~200 CFM
- 4x4 tent: 6-inch fan, ~400 CFM
- 5x5 tent: 6- or 8-inch fan, 400-700 CFM
Buy bigger and turn it down
An oversized fan on a speed controller runs quieter at 50% than a small fan screaming at 100%, and gives you headroom for summer.
Frequently asked questions
- Do I need an intake fan?
- Usually not. Passive intake through the tent's lower flaps is enough if the intake opening is at least twice the area of your exhaust duct.
Keep reading
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