Free greenhouse ventilation calculator: enter greenhouse length, width, height and air changes per hour to get volume, required airflow in CFM and fans needed.
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Enter greenhouse length, width and average height in feet, the air changes per hour you want (30 is a common summer target, 60 for hot climates or heat-sensitive crops), and the rated CFM of one exhaust fan. The calculator returns greenhouse volume, the airflow needed in CFM, and how many fans of that size you need.
Ventilation is sized by air changes, not by floor area alone. A 20 × 10 × 8 foot greenhouse holds 1,600 cubic feet of air. At 30 air changes per hour, the fans must move 1,600 × 30 = 48,000 cubic feet per hour, which is 48,000 / 60 = 800 CFM. With 400 CFM fans, that is 2 fans. Dividing by 60 converts the hourly figure into the per-minute rating printed on fans, which is the step most often skipped when a greenhouse overheats despite having a fan.
Measure inside length and width, and use average height for a peaked roof (halfway between eaves and ridge is close enough for sizing). Choose air changes per hour for your climate and crop: 20 to 30 suits mild summers, 30 to 60 suits hot summers, and winter ventilation for humidity can be as low as 10 to 15. Enter one fan's rated CFM from its label, not a marketing peak figure, and the calculator rounds fans up, because half a fan cannot be bought. Air changes per minute (ACPM) is air changes per hour divided by 60, which is the figure some fan calculators ask for instead. A hoop house uses the same volume formula if you use its average height.
Air changes per hour is how many times the whole air volume is replaced each hour. Leafy greens and hardy plants cope with 20 to 30 changes, while tomatoes, peppers and seedlings in a hot climate want 40 to 60. More changes also mean more humidity removal, which matters because still humid air invites disease even when temperature looks fine. If you cannot reach the target with fans alone, add shade cloth and roof vents before buying bigger fans. To ventilate a greenhouse well, match the fan to the ventilation requirements for your crop: the air exchange rate is the same figure as air changes per hour, and some guides quote it per square foot of floor, but volume is the correct basis, so a tall house needs more CFM than a low house of the same square footage.
A small 12 × 8 × 7 foot greenhouse holds 672 cubic feet, so at 60 changes per hour it needs 672 CFM, or 2 fans of 400 CFM. A 20 × 10 × 8 foot greenhouse holds 1,600 cubic feet, so at 30 changes it needs 800 CFM, or 2 fans of 400 CFM, and at 60 changes it needs 1,600 CFM, or 4 fans. A large 40 × 20 × 10 foot greenhouse holds 8,000 cubic feet, so at 30 changes it needs 4,000 CFM, which is why big houses use louvre fans and evaporative pads rather than small domestic fans.
Put exhaust fans high on the leeward end and intake vents low on the opposite end, so air sweeps across the plants instead of short-circuiting near the roof. Intake area should roughly match fan area, or the fans will starve and rated CFM will never be reached. Shade cloth, whitewash and roof vents cut the cooling load before fans even start, and a thermostat set around 80 to 85 degrees Fahrenheit stops fans running all night when humidity, not heat, is the problem. Size intake shutters to roughly match fan area, and oversizing fans by 10 to 20 percent is common to allow for static pressure losses through screens, shutters and ducting. If you oversize greenhouse fans, use a thermostat or speed control so they do not run flat out on mild days.
Multiply volume (length × width × height) by air changes per hour and divide by 60. A 20 × 10 × 8 foot greenhouse at 30 changes per hour needs 800 CFM. Enter your size above for an exact figure.
Twenty to 30 for mild climates, 30 to 60 for hot climates or heat-sensitive crops, and 10 to 15 for winter humidity control. More changes remove both heat and humidity, which both matter for plant health.
Divide total CFM needed by one fan's rated CFM and round up. Eight hundred CFM with 400 CFM fans means 2 fans. Always round up and use the fan's rated CFM, not a peak marketing figure.
High on one end wall, with intake vents low on the opposite end, so air flows across the plants. Fans near the roof with intakes nearby short-circuit and leave the lower air still.
Roughly equal in total area to the fan openings. Undersized intakes starve the fans, so rated CFM is never reached and the house still overheats.
Yes, for humidity, even when heating is running. A low rate of 10 to 15 air changes per hour, or timed fan bursts, removes moist air that would otherwise condense and encourage disease.
Partly. Shade cloth and roof vents cut the heat load, so the same fans achieve more cooling. In a very hot climate you usually need both shade and the full fan capacity, not one instead of the other.