Calculate the exhaust fan CFM your greenhouse needs from its dimensions, plus winter airflow, intake vent area, and HAF circulation fan sizing.
Greenhouse Fan Sizing Formula
CFM = L * W * H * ACM * SF * EF
- CFM is the planning airflow target (cubic feet per minute); check actual fan output at operating static pressure
- L is the interior length of the greenhouse (ft)
- W is the interior width of the greenhouse (ft)
- H is the average interior height (ft); for a gable roof use (sidewall + peak) / 2
- ACM is the selected air changes per minute (1 is a summer design rule of thumb, not a universal requirement)
- SF is an editable heat allowance multiplier (the1.2 unshaded and1.0 shaded presets are illustrative assumptions)
- EF is the illustrative elevation allowance, EF = 1 + 0.04 * (elevation in ft / 1000), not a manufacturer fan-performance correction
The exhaust fan mode multiplies the interior volume of the greenhouse by the air exchange rate, then applies a 1.2 heat factor when there is no shade cloth and an elevation correction for thinner air at altitude. It also reports a winter design-capacity reference of 2 CFM per square foot from UConn guidance, not a universal minimum humidity-control rate. Intake free area uses an editable150ft/min assumption and the product free-area percentage.
HAF CFM = 2 * L * W
The circulation fan mode sizes horizontal air flow (HAF) fans at 2 CFM per square foot of floor area. These fans do not exhaust air; they keep it moving in a loop inside the greenhouse to even out temperature and humidity.
Intake Area = CFM / 150
The intake vent mode divides design airflow by an entered face velocity to give free intake area, then divides by the entered free-area fraction for gross louver area. The150ft/min and65% defaults are planning assumptions; confirm product free area and pressure drop.
Typical Fan Sizes and Elevation Adjustments
The table below shows arithmetic planning examples at one air change per minute and sea level, using illustrative heat multipliers1.0 and1.2; it does not establish minimum fan sizes for every climate or crop.
| Greenhouse (L x W x H ft) | Volume (ft³) | CFM with shade | CFM no shade (x1.2) |
|---|---|---|---|
| 8 x 6 x 6 | 288 | 288 | 346 |
| 10 x 8 x 7 | 560 | 560 | 672 |
| 12 x 10 x 8 | 960 | 960 | 1,152 |
| 20 x 12 x 8 | 1,920 | 1,920 | 2,304 |
| 30 x 16 x 10 | 4,800 | 4,800 | 5,760 |
| 96 x 30 x 12 | 34,560 | 34,560 | 41,472 |
The table illustrates the retained linear4% per1,000ft altitude allowance. It is not a fan-curve or heat-balance correction. Use manufacturer data for density and operating static pressure; the factor alone does not prove equipment suitability.
| Site elevation | Elevation factor | Example: 2,000 CFM base becomes |
|---|---|---|
| Sea level to 1,000 ft | 1.00 - 1.04 | 2,000 - 2,080 CFM |
| 2,000 ft | 1.08 | 2,160 CFM |
| 4,000 ft | 1.16 | 2,320 CFM |
| 6,000 ft | 1.24 | 2,480 CFM |
| 8,000 ft | 1.32 | 2,640 CFM |
Example Problems
Example 1. A 12 ft by 8 ft hobby greenhouse has an average interior height of 7 ft, has no shade cloth, and sits near sea level. Volume = 12 * 8 * 7 = 672 ft³. At one air change per minute the base requirement is 672 CFM. With no shade cloth, apply the 1.2 heat factor: 672 * 1.2 = 806.4 CFM as a planning target. Select a fan that provides the design airflow at actual screen, louver and duct resistance; a universal percentage size-up is not reliable.
Example 2. A 30 ft by 20 ft greenhouse has a 6 ft sidewall and a 10 ft peak, uses shade cloth, and sits at 5,000 ft elevation. Average height = (6 + 10) / 2 = 8 ft, so volume = 30 * 20 * 8 = 4,800 ft³. The shade factor is 1.0 and the elevation factor is 1 + 0.04 * 5 = 1.20, giving 4,800 * 1.20 = 5,760 CFM. The winter design reference is 2 * 600 ft² = 1,200 CFM, and the fan needs about 5,760 / 150 = 38 ft² of free intake area on the opposite end wall.
Frequently Asked Questions
Why is one air change per minute the standard?
Solar gain through glazing heats greenhouse air fast enough that interior temperature can climb several degrees in under a minute. One volume per minute is a common summer mechanical-ventilation design rule of thumb. The actual inside/outside temperature difference depends on solar load, glazing, shading, crop and system performance; this arithmetic does not predict it. Winter humidity-control rates and cooling capacities are different design questions.
Do I still need an exhaust fan if I have roof vents?
Passive roof and side vents work well in mild weather when there is a breeze and the vent area is at least 15 to 20 percent of the floor area. On hot, still days natural ventilation stalls, and that is exactly when the greenhouse overheats. Most growers in warm climates use both: passive vents for spring and fall, and a thermostat-controlled exhaust fan sized with this calculator for summer peaks.
What is the difference between exhaust fans and HAF circulation fans?
An exhaust fan pushes inside air out and pulls fresh air through intake vents, controlling temperature and humidity. HAF (horizontal air flow) fans hang inside the greenhouse and move air in a horizontal loop without exchanging it. They eliminate hot and cold pockets, keep humidity off leaf surfaces, and strengthen stems. Size exhaust capacity at one volume per minute and HAF capacity at about 2 CFM per square foot of floor, and run the HAF fans whenever the exhaust fan is off.
