How big should my passive intake be?
Most growers should use a passive intake: the exhaust fan pulls, and fresh air follows in through holes low on the other side. Make the holes about two to three times the size of your exhaust duct, so the tent pulls in slightly without starving the fan or whistling.
Your result
How it works, in plain words
Area, not fan size
Exhaust duct area = π × (diameter ÷ 2)². A 6-inch duct is 28 in² (182 cm²); 2.5 × that is about 71 in² (455 cm²) of open intake.
Mesh counts
Bug mesh or a filter sock blocks about 40% of the hole, so we divide by 0.6 to keep the open area the same.
The tent check
With the fan running, the tent walls should pull in slightly. Flat walls: too much intake or too little fan. Walls sucked in hard, a whistle or a straining fan: open more intake.
When to go active
Big airflow (over about 800 CFM), intake ducted from far away, or a small source room. Keep any intake fan weaker than the exhaust so the space stays under negative pressure. A sealed AC room needs no intake while the lights are on.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
INTAKE_RATIO | 2.5 × exhaust duct area | Passive intake opening area (KB rule of thumb 2–3×; we default to the middle) | Grower practice |
MESH_FREE | 0.6 open fraction | Bug mesh or a pre-filter sock leaves roughly 60% of the hole open | Estimate |
INTAKE_FAN | 0.75 × exhaust airflow | If you do fit an intake fan, keep it at about 70–80% of the exhaust so the space stays under negative pressure | Grower practice |
ACTIVE_CFM | 800 CFM | Above about 800 CFM (an 8-inch fan running hard) passive ports get large: consider an active intake | Grower practice |
INTAKE_RUN_M | 3 m | Intake ducted from further than about 3 m (10 ft) away needs a fan to pull through it | Grower practice |
Sources
- GenetixBay Knowledge Base. Airflow: air exchange, canopy air speed and fan placement (one tent volume every 1–3 min; +25–50% for a carbon filter and ducting; passive intake ≈ 2–3× exhaust duct area). https://genetixbay.com/knowledge/environment/airflow.html
- GenetixBay house position (founder's field experience, decades of indoor rooms): passive intake for most growers; oversize lights and dim down; replace HPS watt for watt unless the LED is top tier. https://genetixbay.com/editorial-policy
