CFM (cubic feet per minute) is a volumetric airflow rate. Fan manufacturers quote it at zero static pressure, meaning free air with no ducting or filter, so real-world airflow in a grow setup is always lower. The metric equivalent is cubic metres per hour (m³/h).
In practice
Basic sizing: calculate the grow space volume, choose an exchange rate, then add for restrictions.
Example: a 1.5 × 1.5 × 2 m (5 × 5 × 6.5 ft) tent is 4.5 m³, about 160 ft³. One exchange per minute needs 160 CFM. Add about 25% for a carbon filter and 10–20% for each long run or tight bend, giving a fan rated around 220–250 CFM. Run it on a controller at partial speed most of the time; headroom matters on hot days.
Match the carbon filter's rated CFM to the fan. A filter too small for the fan restricts flow, lets air pass too fast to scrub odour, and saturates quickly. A slightly oversized filter is fine; it just costs more.
Units and formula
- 1 CFM = 1.699 m³/h
- 1 m³/h = 0.589 CFM
- Required CFM ≈ volume (ft³) × exchanges per minute × restriction factor (about 1.25–1.5)
Don't confuse with
- Air speed (m/s or ft/min): how fast air moves at the leaf, not total volume.
- Static pressure: the resistance the fan works against; it reduces actual CFM.
