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Greenhouse 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.

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

Based on: NGMA standard: Required CFM = Volume (ft³) x ACH / 60; Fan count = ceiling(Required CFM / Fan capacity)
Best for: Sizing mechanical exhaust fans for greenhouse summer cooling and year-round ventilation
Check locally: Verify fan ratings at the static pressure appropriate for your system; derate for altitude above 1,000 ft
Units supported: Imperial (ft, CFM), Metric (m, m³/h)

Worked Example

30 x 100 ft greenhouse, 10 ft eave height, 60 ACH target, fans rated at 10,000 CFM each

  1. 1. Calculate volume

    30 x 100 x 10 = 30,000 ft³

  2. 2. Calculate required CFM

    30,000 ft³ x 60 ACH / 60 = 30,000 CFM

  3. 3. Determine fan count

    30,000 CFM / 10,000 CFM per fan = 3.0 → 3 fans needed

  4. 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

ConditionWhat It Means
Fan count equals 1A single fan may create dead spots. Consider two smaller fans spaced evenly across the end wall for more uniform airflow.
Fan count rounds up significantlyYou have excess capacity, which provides a safety margin for hot days and fan maintenance. This is acceptable.
Required CFM exceeds available fan capacityYou 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.

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Frequently Asked Questions