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.
Worked example
A 4 × 4 × 7 ft tent holds about 112 cubic feet. At one air exchange per minute the base is 112 CFM. Now add real-world losses:
| Condition | Efficiency factor | Effect |
|---|---|---|
| No extras | 1.0 | 112 CFM required |
| Carbon filter | ~0.70 | raises required to 160 CFM |
| Long/bendy ducting | ~0.85 | raises required to ~132 CFM |
| Both filter and ducting | 0.70 × 0.85 ≈ 0.595 | raises required to ~188 CFM |
You would choose a fan rated at 200 CFM or above and add a speed controller to tune it down when temperatures are moderate.
Understanding the correction factors
Carbon filter: The activated carbon bed adds static pressure resistance. Typical correction factors range from 0.65 to 0.75. Buy a filter rated to at least your uncorrected CFM requirement, and match it to a fan manufacturer that publishes performance curves — a mismatched filter and fan is the single most common reason ventilation underperforms.
Ducting losses: Every metre of smooth duct adds a small resistance. Flexible aluminium duct (the concertina type) adds much more per metre because of its corrugated interior. Each 90-degree bend is roughly equivalent to adding one or two additional metres of straight duct. Keep runs under 3 m, use smooth duct where possible, and take bends gently.
Ambient temperature: Hot ambient air outside the tent means the fan must work harder to create a meaningful temperature differential. In summer or in hot climates, raise the exchange rate to 1.5 or 2 exchanges per minute to compensate.
Fan and controller sizing tips
- Always size the fan to the corrected CFM, not the raw volume. Under-sizing is the most common mistake.
- A variable-speed controller lets you match extraction to actual conditions — run at 70 percent during mild weather, full speed during hot periods or peak plant transpiration.
- Passive intake should be roughly 15–20 percent larger in area than the exhaust opening. If intake airflow is restricted, even a correctly sized fan cannot reach its rated output.
- For tents over 4 × 4 ft, consider a separate intake fan at one-third to one-half the exhaust CFM rather than relying on passive intake holes.
Keep duct runs short and straight, and raise the exchange rate above one per minute in hot rooms or under high-power lighting.