Greenhouse CFM Calculator — Exhaust Fan & HAF Sizing

It converts the house geometry into airflow targets for the three separate jobs a greenhouse fan does. Summer cooling sizes exhaust at one complete air exchange per minute — volume × 60, which for standard 8–10 ft gutter heights lands at 8–10 CFM per square foot of floor. Winter venting sizes a low stage at 2–3 air changes per hour to purge humidity without freezing the crop. HAF circulation sizes the internal fans at 1–2 CFM per square foot of floor. Enter the length, width and average height once and all three targets appear — the calculator above runs them side by side so you can see which job governs.

For summer, multiply the floor area by 8–10: a 30 × 12 ft greenhouse needs roughly 2,880–3,600 CFM of exhaust. The more precise form is volume × 60 (one exchange per minute) — a 30 × 12 × 8 ft house is 2,880 ft³, so 2,880 CFM. Winter needs only 2–3 ACH: 144–216 CFM on the same house, which is why two-speed or staged exhaust fans are standard — a single-speed fan sized for summer is ten times too much air in January. Buy the summer figure for the exhaust rating; control the winter stage with speed or staging.

Different problems. A grow tent's 20–40 ACH handles lamp heat and CO₂ drawdown in a small volume; a summer greenhouse must reject direct solar gain through glazing that transmits most of it — the sun is adding heat to every square foot of the roof, and the only removal path is airflow. The resulting standard, one exchange per minute, is 60 ACH: about double a dense grow tent's rate. This is also why the greenhouse sizing rule keys to floor area (8–10 CFM/ft²) — floor area proxies the solar load — while tent sizing keys to volume.

Horizontal air flow totals 1–2 CFM per square foot of floor, split across two or three small fans running continuously in an opposed pattern — the goal is one slow horizontal loop around the house, air speed 50–100 FPM at canopy height. A 360 ft² house wants 360–720 CFM of total circulation, delivered by two 16-inch basket fans rather than one large unit. HAF is sized separately from exhaust: it moves air that is already inside the house, and running the exhaust fan harder never replaces it.

Pad face area = airflow ÷ 300 FPM face velocity. A 3,000 CFM summer system needs about 10 ft² of pad — say a 4 ft tall × 2.5 ft wide wall — with a sump sized to keep the pad fully wetted. The 300 FPM ceiling matters: push 500 FPM through a pad and water shears off into the air instead of evaporating, cooling drops sharply, and you are pumping water straight out the fan. In dry climates a matched pad-fan system holds 15–20 °F of cooling at design conditions; undersized pads deliver a third of that.

Yes — and the inlet is where most systems quietly fail. The floor is 1 ft² of free inlet area per 700 CFM of exhaust; motorized shutters and restrictive louvers need double. A 3,000 CFM system wants at least 4.3 ft² of free inlet (8.6 ft² with louver losses) — roughly one 36×36 motorized shutter per two fans. Undersized inlets force the fan to pull through cracks, measured airflow falls 30–50% below rating, and the dust streaks at door sills show you exactly where the real inlets are.

Out — exhaust placement defines the airflow pattern. Fans on one end wall (or ridge) pulling air out, with inlets on the opposite end, sweep air the length of the house and evacuate the hottest air at the far end. Pushing air in pressurizes the house and pushes humid air through leaks unpredictably. The one exception is a pad-fan system, where the pads are the inlet and the fans still exhaust. Keep fan-to-fan spacing at most 100 ft of throw; longer houses stage fans along the length.

Same physics family, different numbers. The evaporative cooler page sizes for a 20–40 ACH living-space standard (2–3 CFM per square foot) because a house rejects only its own heat load. A summer greenhouse runs 60 ACH — 8–10 CFM per square foot — because the sun is pumping 25–30 BTU/hr through every square foot of glass, and glazing is a heat engine you cannot shut off. The two calculators share the air-change math and the pad sizing, but the design targets differ by a factor of three to five.

A grow tent is an enclosure under artificial light; a greenhouse is a solar collector with plants in it. The grow tent page sizes around lamp heat, carbon filters and negative pressure at 20–40 ACH. This page sizes around solar gain, winter humidity control and circulation at 60 ACH summer / 2–3 ACH winter, plus HAF fans and evaporative pads that tents never carry. If your structure is inside a building under LEDs, use the grow tent page; if it is under the sun, use this one.