# CFM

> CFM (cubic feet per minute) is the volume of air a fan moves per minute; growers use it to size exhaust fans to their grow space volume.

- Canonical: https://genetixbay.com/knowledge/glossary/cfm.html
- Also written: cubic feet per minute, ft³/min, cfm rating, m³/h, airflow rating
- Glossary category: Environment and light · Evidence: Grower practice
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/carbon-filter.md) 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.

## Related terms

[Exhaust fan](https://genetixbay.com/knowledge/glossary/exhaust-fan.md), [Carbon filter](https://genetixbay.com/knowledge/glossary/carbon-filter.md), [Negative pressure](https://genetixbay.com/knowledge/glossary/negative-pressure.md), [Airflow](https://genetixbay.com/knowledge/glossary/airflow.md)

## Read more

- [Airflow: air exchange, canopy air speed and fan placement](https://genetixbay.com/knowledge/environment/airflow.md)