Greenhouse Exhaust Fan Sizing Calculator
Enter the greenhouse dimensions, summer conditions, and a candidate fan. The calculator estimates delivered exhaust airflow, fan count, intake opening area, and winter and spring/fall staging.
Required delivered airflow24,000
- Base summer airflow
- 24,000
- Design correction factor
- 1
- Required nameplate airflow
- 26,666.67
- Number of candidate fans
- 3
- Installed airflow capacity
- 32,400
- Capacity margin
- 35%
- Minimum clear intake area
- 26.51
- Minimum gross louver opening
- 37.87
- Winter ventilation stage
- 6,000
- Spring/fall ventilation stage
- 18,000
- Air-path warning
- No long air-path warning for this end-to-end layout.
Correction factors
| Factor | Value | Use |
|---|---|---|
| Elevation | 1 | Part of the climate correction |
| Light intensity | 1 | Part of the climate correction |
| Temperature rise | 1 | Part of the climate correction |
| Combined climate correction | 1 | Compared with velocity |
| Short-house velocity correction | 1 | Compared with climate |
Fan count is rounded up to a whole fan, so installed capacity can show a substantial margin.
Ventilation alone cannot cool greenhouse air below the outdoor dry-bulb temperature.
If the fan rating already includes the actual installed static pressure, use 100% delivery to avoid double derating.
How to use this calculator
- Measure the ventilated length from the intake end to the exhaust-fan end.
- Choose the summer design height and elevation band that match the house.
- Enter the peak inside light level and the temperature rise you can tolerate.
- Use a fan CFM rating at the expected static pressure, then derate it if installed restrictions are not included.
- Compare the fan count, intake area, and seasonal stages before selecting equipment.
How the exhaust fan size is calculated
Forced greenhouse ventilation is usually sized from floor area and a summer design height, then adjusted for conditions that make the fans work harder. This calculator starts with the simple summer airflow rate:
Qb = length * width * design height
The design height is 8 ft for the common summer rate or 10 ft when a grower wants extra capacity for hotter conditions. The calculator then applies the controlling correction factor:
F = max(elevation * light * temperature, short-house velocity)
- Elevation comes from the published adjustment table through 10,000 ft.
- Light is at least 1, and increases when inside peak light is above 5,000 foot-candles.
- Temperature is at least 1, and increases when the allowed intake-to-fan rise is below 7 F.
- Short-house velocity is at least 1, and equals
10 / sqrt(length)for short air paths.
Required delivered airflow is Q = Qb * F. Because a fan may not deliver its catalog airflow after shutters, screens, belts, dirt, wind, and obstructions are added, the tool divides by the installed-delivery fraction to show the required nameplate airflow. It also multiplies the candidate fan rating by that same fraction, rounds Q / delivered CFM per fan up to a whole fan, and reports the installed capacity and margin.
What moves the answer most
The biggest drivers are floor area, design height, temperature rise, and the actual delivered CFM of the fan. A smaller allowed temperature rise raises airflow quickly because the same heat gain must be removed with less warming of the air stream. Higher light levels and high elevations also increase the estimate. Fan count can change abruptly because two and a small fraction of a fan still requires three fans.
Intake opening and seasonal stages
The intake check uses circular fan face area as a planning proxy. One fan face area is pi * (diameter / 24)^2 square feet, because diameter is entered in inches and the radius is needed in feet. The calculator multiplies total fan face area by 1.25 for clear intake area, then divides by the louver free-area fraction to estimate the gross opening to buy or frame.
Winter and spring/fall stages are based on the uncorrected 8 ft summer volume rate. The winter stage is 25% of that value for humidity control, and the intermediate stage is 75%. These staged values are planning numbers, not thermostat settings.
What this calculator leaves out
This estimate does not model crop density, evaporative pads, fan curves, exact static pressure, local wind, shade systems, glazing heat gain, or control sequencing. It also cannot show cooling below outdoor dry-bulb temperature; for that, growers usually need evaporative cooling, shade, misting, or a different climate-control design.
Worked example
For a 100 ft by 30 ft greenhouse using the 8 ft standard height, the base summer airflow is 100 * 30 * 8 = 24,000 CFM. At 0-1,000 ft elevation, 5,000 foot-candles, and a 7 F allowed rise, the correction factor is 1.00, so required delivered airflow stays 24,000 CFM.
A 12,000 CFM candidate fan derated to 90% delivers 10,800 CFM. The calculator rounds 24,000 / 10,800 up to 3 fans, giving 32,400 CFM installed capacity and a 35.0% margin. Three 36 in fans need about 26.5 sq ft of clear intake area, or 37.9 sq ft of gross louver opening at 70% free area. The winter stage is 6,000 CFM and the spring/fall stage is 18,000 CFM.
Common questions
How many CFM does my greenhouse exhaust fan need?
Start with length times width times the summer design height, then adjust for elevation, light intensity, allowed temperature rise, and short-house velocity. The result is required delivered CFM, which is the airflow the fans need to move after installed losses.
Should greenhouse fan sizing use floor area or greenhouse volume?
This method uses floor area times an 8 ft or 10 ft design height, not the full peaked roof volume. That convention matches common greenhouse ventilation guidance and keeps the rate tied to the occupied crop zone and summer heat removal.
Why does allowable temperature rise change the required fan size?
A lower allowed rise means each cubic foot of air is permitted to pick up less heat before it leaves the house. To remove the same heat load, the fan system has to move more air.
How large should the intake shutter be?
This calculator estimates clear intake area as 1.25 times total fan face area. It then divides by louver free area, so a 70% free-area louver needs a larger gross opening than its clear airflow area.
Should I use one large fan or several staged fans?
Several fans usually give better seasonal control because the system can run one fan for mild weather and more fans during peak summer. The calculated fan count is a capacity requirement; final staging should also consider controls, redundancy, noise, and available louver area.
Can ventilation make a greenhouse cooler than the outdoor air?
Plain exhaust ventilation cannot cool below outdoor dry-bulb temperature. It can limit the rise above outdoor air; cooling below outdoor air requires another process such as evaporative cooling, shade, or misting.