Last Updated on October 9, 2026

Greenhouse Ventilation Calculator: Fan CFM and Vent Area

Size a greenhouse exhaust fan at 8 to 12 cubic feet per minute (cfm) for every square foot of floor, then multiply by an elevation factor if you are above 1,000 feet. Fan-cooled commercial greenhouses use 8 to 10, which UMass Extension says moves about one greenhouse full of air per minute. The National Greenhouse Manufacturers Association (NGMA) asks for at least 12 in a hobby greenhouse. At that rate a 12×20 ft backyard hoop house needs about 2,880 cfm of fan and a 4.3 sq ft intake. Without a fan, it needs at least 36 to 48 sq ft of side vents plus the same again up high.

Greenhouse ventilation calculator

Enter the floor size, wall and peak height, and roof shape. Pick the rate that fits your greenhouse and your elevation. The results show the summer fan size, a low speed for cool sunny days, the intake opening, and the vent area if you ventilate without a fan.

Greenhouse fan size chart

For a greenhouse that averages 8 feet tall or less, the floor area sets the fan size. Find your size and read across. Add the elevation factor below if you are 1,000 feet up or more.

The 30×96 row matches the worked example in Rutgers University’s greenhouse environment guide: 23,040 cfm at 8 and 28,800 cfm at 10.

How the calculator works

1. Floor area times a rate. Fans are sized by floor area because the heat comes from the sun, and solar heat load rises with floor area (NGMA ventilating and cooling standard). Rutgers designs for 8 to 10 cfm per square foot, dropping to 8 when summer shade screens are used, and UVM Extension uses 8 for high tunnels in the hot season. Hobby greenhouses need at least 12, the NGMA minimum.
2. A volume check. University of Florida IFAS Extension calls one air change per minute the generally accepted summer minimum. Volume is length times end wall area. A round roof counts as a half oval on the sidewalls (exact for a half-circle hoop), a peaked or lean-to roof as a triangle. The calculator uses the bigger number. In a low hoop house the floor rule wins, so the default moves about 2.5 air changes a minute.
3. Elevation. Thin air carries less heat, so NGMA multiplies the fan size by an elevation factor (table below).
4. Lean-to adjustment. A lean-to on an east, south or west wall gains heat from the building, so NGMA adds half the building wall area to the floor area. The calculator takes that wall as length times back wall height.
5. Intake. University of Alaska Fairbanks Extension gives 1.5 sq ft of vent opening per 1,000 cfm of fan.
6. No-fan vents. The ASABE greenhouse standard EP406 says total side vent area should equal total ridge vent area, each at least 15 to 20 percent of the floor. Roll-up height is that side area divided by the wall length.
7. Circulation. UVM sizes horizontal air flow fans at 25 percent of the volume per minute. That mixes the air; it does not replace it.

Elevation factors

Source: NGMA standard, Table 1. At 5,000 feet, the 12×20 hoop house needs 2,880 x 1.20 = 3,456 cfm.

Do the commercial numbers apply to a backyard greenhouse?

Mostly, yes. NGMA says small hobby greenhouses face the same cooling conditions as commercial houses, so the commercial rules normally apply; its hobby section raises the minimum to 12 cfm and adds the lean-to adjustment. The Rutgers, UMass, Florida and ASABE figures were written for commercial growers, and UVM’s for vegetable high tunnels. Treat every number as a design starting point, not a promised temperature. Sun, wind and how full the beds are all change the result.

Setting up the fan and intake

• Fan at one end, intake at the other. UMass Extension puts the fans on one end wall and the louvers in the opposite end. Fans work best when the air travels less than 150 feet.
• My hoop house works the same way. In my auto vent build, a power vent at the door end pushes hot air out, an intake at the far end lets cool air in, and a box fan over the intake helps. I chose that over a single roof vent, which lets warm air out but does not move fresh air through.
• Exhaust with the wind. If the fans must blow into the prevailing wind, ASABE says to add at least 10 percent.
• Compare fans at static pressure, not free air. Rutgers says fans should be rated at 0.1 inch of static pressure. The UAF fan table shows the gap: one 16 inch fan moves 1,095 cfm in free air but 720 cfm at 1/8 inch.
• Check the intake with the fan running. UMass says that if the plastic pulls tight against the frame or the door is hard to open, the intake is too small.
• Get two speeds. NGMA says hobby greenhouse fans should always have two-speed motors, because winter ventilation tops out at about half the summer rate.

Roll-up sides and ridge vents

Passive vents need no fan to buy or run and nothing fails in a power outage, but they depend on wind (Florida IFAS). Roll-up sides work well in hoop houses given good summer breezes and space between structures, though on hot days the warm air rising out of the roof does almost nothing on its own, so wind does the real work (UMass, Rutgers). Keep a passively vented tunnel away from large buildings or dense tree lines, and pair roll-up sides with gable vents high in the end walls for a steadier draw (UVM).

Treat the vent figures as a minimum. Sidewall vents alone work only when the wind is strong enough; add a ridge vent and warm air exits out the top while fresh air comes in the sides (Florida IFAS). Small vents can open on their own: openers driven by expanding wax or liquid need no motor or thermostat and work with vents under 10 to 15 pounds (UAF). The cold frame version is simpler: open the lid on warm days, as in 5 tips for using a cold frame.

When to turn the fan on

The ASABE standard says greenhouse temperatures should generally stay under 86 to 90F for most crops, and that cool season plants grow best at 50 to 68F. I aim my hoop house much lower. I set the thermostat controller to 60F, so the vent fan comes on at 60 and shuts off as the air cools. The goal is a steady 40 to 64F for spinach, arugula and other cool weather crops. Big temperature swings stress those crops and make them bolt, so stable beats warm.

Shade, light and humidity

Florida IFAS Extension calls shade curtains or whitewash very effective at lowering inside temperature. It adds that shading only makes sense for plants that grow well in lower light. If you add shade cloth, check your light with the DLI calculator. The same publication says winter ventilation should be at least two air changes per hour, even with the heater running, to keep humidity down. Humidity over 90 percent favors fungal disease. It also notes that more winter ventilation raises the heating load, so size the heater with the greenhouse heater size calculator.

Quick questions

How many CFM do I need for my greenhouse?

Multiply the floor area by 8 to 10 cfm per square foot for larger greenhouses and high tunnels, or at least 12 for a hobby greenhouse per the NGMA standard. Above 1,000 feet, multiply by the elevation factor. A 12×20 ft hobby hoop house needs about 2,880 cfm.

What does one air change per minute mean?

The fan moves the greenhouse’s whole volume of air out once a minute. University of Florida IFAS Extension calls it the generally accepted summer minimum. A 12×20 ft hoop house 6 ft tall at the peak holds about 1,131 cubic feet.

How big should the intake be for a greenhouse exhaust fan?

University of Alaska Fairbanks Extension gives 1.5 square feet of opening per 1,000 cfm of fan, so a 2,880 cfm fan needs about 4.3 square feet. UMass Extension says if the plastic pulls tight or the door is hard to open while the fan runs, the intake is too small.

How much vent area does a greenhouse need without fans?

The ASABE greenhouse standard says total side vent area should equal total ridge vent area, each at least 15 to 20 percent of the floor. For a 12×20 ft greenhouse that is 36 to 48 square feet of each.

Does elevation change greenhouse fan size?

Yes. Thin air carries less heat, so the NGMA standard multiplies fan cfm by 1.04 at 1,000 feet, 1.20 at 5,000 feet and 1.36 at 8,000 feet. A 2,880 cfm fan becomes about 3,456 cfm at 5,000 feet.

Grow under cover, then plan the beds

Once the air is moving, plan what goes in the beds. Plan My Garden is free to start: enter your zip code and it finds your frost dates, draw your beds, pick your crops, and it places each plant at square-foot spacing and builds a planting schedule from those dates.

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