What CFM fan do I need for my grow tent?

Start from the tent volume in cubic feet and aim to move that volume at least once per minute, so a 4 by 4 by 7 foot tent of about 112 cubic feet needs roughly 112 CFM before corrections. After adding a carbon filter and ducting losses, size up to the next fan above the corrected figure.

Why add a carbon filter correction?

A carbon scrubber adds significant resistance to airflow, typically cutting a fan's real output by around 25 to 35 percent. The tool divides the raw requirement by an efficiency factor when you enable the filter, so you buy a fan whose rated CFM still delivers enough through the filter.

How does ducting affect the result?

Every metre of duct, and especially every bend, adds static pressure that reduces real airflow. The tool applies an extra derating when you flag long or bendy ducting. Keep runs short and straight, and use smooth rather than concertina duct, to preserve fan performance.

Should I run the fan continuously?

During lights-on you generally want continuous extraction to remove heat and humidity and refresh CO2. The one-exchange-per-minute target assumes constant running. In flowering or hot rooms, raise the exchange rate above one per minute and consider a speed controller to tune noise and temperature.

Does tent height matter?

Yes, height is part of the volume, so a taller tent of the same footprint needs more airflow. The calculator multiplies width, depth, and height together, so always enter the true internal height rather than just the footprint.

Grow Tent Ventilation & CFM Calculator

An inline extraction fan keeps a grow tent cool, dry, and supplied with fresh CO2 by exchanging its air at least once a minute. This calculator turns your tent dimensions into a target CFM, then derates it for the real-world resistance of a carbon filter and ducting so you buy a fan that still delivers enough airflow.

How it works

The tent volume sets the base requirement, and correction factors account for the airflow lost to a filter and ducting:

volume_ft3  = width × depth × height        (feet)
base_CFM    = volume_ft3 × exchanges_per_minute
corrected   = base_CFM / (filter_eff × duct_eff × ambient_eff)

Metric inputs are converted to cubic feet first (1 m³ ≈ 35.31 ft³). Each efficiency factor is below 1 when its condition applies — a carbon filter and long ducting both raise the rated CFM you need to buy, since a fan’s real output drops under that resistance.

Example and tips

A 4 × 4 × 7 ft tent holds about 112 cubic feet, so at one exchange per minute the base requirement is 112 CFM. Adding a carbon filter (about 0.7 efficiency) and some ducting (about 0.85) gives a corrected need of roughly 112 / (0.7 × 0.85) ≈ 188 CFM, so you would choose a fan rated near or above 200 CFM and add a speed controller. Keep duct runs short and straight, and raise the exchange rate above one per minute in hot rooms or under high-power lighting.