Greenhouse planning
Greenhouse Fan CFM Calculator
Estimate the exhaust-fan airflow needed to exchange the air in a greenhouse, then add practical capacity for screens, louvers and warm weather.
What the CFM result means
The design result is the airflow the exhaust system should deliver after the entered allowance. Greenhouse volume multiplied by the desired air exchanges per minute gives the starting CFM. Screens, shutters, guards and wind add resistance, so the number printed on a fan at free air may not be the airflow it delivers after installation.
Compare candidate fans at the expected static pressure, not only at zero pressure. Penn State notes that shutters, guards and other system resistance reduce delivered airflow. UGA Extension also recommends providing adequate inlet opening so replacement air can enter without excessive restriction.
Measure average height carefully
For a simple gable greenhouse, average height is approximately the eave height plus half the roof rise. Irregular lean-to and gothic structures are better divided into simple shapes. The calculator is intended for summer exchange planning; winter minimum ventilation and internal circulation are separate decisions.
Read the greenhouse ventilation guide and compare the result with the greenhouse heater calculator.
Frequently asked questions
Is one air change per minute always required?
No. It is a useful warm-weather starting point, but solar load, crop density, shading and climate affect the required rate.
Should I add two fan ratings together?
Yes, when both fans operate together under comparable installed conditions.
Does a circulation fan count as exhaust CFM?
No. A circulation fan moves air inside the house but does not necessarily replace it with outdoor air.
Technical reference: UGA Cooperative Extension greenhouse heating, cooling and ventilation guidance.