YielixGreenhouse Fan Sizing Calculator: Required CFM and Number of Exhaust Fans
Calculate the required ventilation capacity in CFM or m³/h for your greenhouse based on volume and air exchange rate. This fan sizing calculator follows the NGMA standard of 1 CFM per cubic foot for peak summer ventilation. Enter your greenhouse dimensions and fan capacity to determine how many fans you need for adequate cooling and air circulation.
Measure Crop Conditions Before Changing Airflow
Fan capacity describes an equipment maximum, not the condition at a leaf. Use the VPD calculator to compare canopy temperature and humidity before and after a ventilation change, and the DLI calculator to keep canopy light and lamp runtime measurements visible. When a ventilation schedule would release enriched air, review it with the CO₂ enrichment calculator rather than assuming airflow, light and CO₂ have one interchangeable setting.
Inputs Explained
- Greenhouse Length
- The length of your greenhouse in feet or metres. Used with width and height to calculate total volume.
- Greenhouse Width
- The width (span) of your greenhouse in feet or metres.
- Eave Height
- The height at the sidewall (eave), not the ridge peak. For gutter-connected ranges, use the gutter height. Using average height (midpoint of eave and ridge) also gives a reasonable estimate.
- Air Exchange Rate (ACH)
- Air changes per hour. The NGMA standard is 60 ACH (1 CFM/ft³) for peak summer cooling. Winter ventilation typically needs only 2-4 ACH.
- Fan Capacity
- The rated CFM or m³/h of each individual fan you plan to install. Used to calculate how many fans are needed.
How This Calculator Works
Worked Example
30 x 100 ft greenhouse, 10 ft eave height, 60 ACH target, fans rated at 10,000 CFM each
- 1. Calculate volume
30 x 100 x 10 = 30,000 ft³
- 2. Calculate required CFM
30,000 ft³ x 60 ACH / 60 = 30,000 CFM
- 3. Determine fan count
30,000 CFM / 10,000 CFM per fan = 3.0 → 3 fans needed
- 4. Convert to metric
30,000 CFM x 1.699 = 50,970 m³/h
30,000 CFM required (50,970 m³/h), 3 fans at 10,000 CFM each
How to Interpret Your Results
| Condition | What It Means |
|---|---|
| Fan count equals 1 | A single fan may create dead spots. Consider two smaller fans spaced evenly across the end wall for more uniform airflow. |
| Fan count rounds up significantly | You have excess capacity, which provides a safety margin for hot days and fan maintenance. This is acceptable. |
| Required CFM exceeds available fan capacity | You may need to combine multiple fan sizes or consider pad-and-fan evaporative cooling to supplement. |
Common Mistakes to Avoid
Using ridge height instead of eave height
Use the sidewall (eave) height for volume calculation, not the peak ridge height. The average of eave and ridge is also acceptable.
Sizing fans for winter ventilation rates
Always size fans for the 60 ACH summer peak. Use variable-speed controllers or staged fan operation for winter at 2-4 ACH.
Comparing fan CFM ratings at different static pressures
Ensure all fan capacities are rated at the same static pressure (typically 0.05 or 0.10 inches water column) for a fair comparison.
Related Calculators
Greenhouse VPD Calculator
Calculate greenhouse air and leaf vapor pressure deficit (VPD) in kPa from air temperature, relative humidity and optional measured leaf temperature using the FAO-56 equation.
Daily Light Integral (DLI) Calculator
Calculate greenhouse daily light integral (DLI) in mol/m²/day from canopy PPFD, lamp hours and measured natural light, or solve the supplemental lighting hours needed to reach your target.
Fertigation Injector & Stock Tank Calculator
Calculate soluble fertilizer mass, stock concentration and selected-nutrient outlet ppm from your product analysis, source-water contribution and stock-to-final-solution injector ratio.
Greenhouse Heating Cost Calculator
Estimate seasonal greenhouse heating costs from cover material U-value, glazing area, heating degree days, and fuel type.
Greenhouse ROI Calculator
Calculate greenhouse return on investment, payback period, and year-by-year cumulative cash flow with a break-even chart.
CO2 Enrichment ROI Calculator
Calculate CO2 enrichment annual costs, break-even yield increase requirement, and net ROI range for greenhouse operations.
Irrigation Water Requirement Calculator
Calculate crop water needs from ET₀, crop coefficient, and irrigation efficiency
Water Cost Calculator
Calculate seasonal irrigation water and energy costs per hectare or acre
Related Guides
VPD Temperature and Humidity Explained
Use air and leaf temperature with relative humidity to interpret greenhouse vapor pressure deficit, verify leaf conditions, and spot a possible condensation signal.
DLI, PPFD and Lighting Hours
Measure canopy light, convert supplemental PPFD and run time into DLI, and test whether a crop-specific target fits a practical photoperiod.
Greenhouse Heating Costs: How to Estimate Your Winter Energy Bill
Learn how heating degree days, cover material U-values, and fuel type determine your greenhouse seasonal heating costs.
CO2 Enrichment for Greenhouses: Costs, Benefits, and Break-Even Analysis
Why plants respond to elevated CO2, which crops benefit most, equipment options, and how to calculate your break-even yield increase.