On this page
Use this greenhouse ventilation calculator to turn your inside dimensions into three bounded numbers: floor area, approximate volume, and a UMass Extension summer exhaust reference. It also converts that reference to equivalent ACH and can check nominal intake area from stated fan-opening geometry. It does not select equipment or predict cooling, humidity, or condensation.
Calculate your greenhouse ventilation reference
Bounded summer reference
Greenhouse ventilation calculator
Enter inside dimensions in feet. Required results use a rectangular-volume approximation. Optional fields stay blank until exact current data is available.
UMass Extension
Source date
August 11, 2026
Results
Candidate airflow comparison
- Candidate equivalent ACH
- Comparison with this reference, not delivered-performance verification
Minimum nominal intake-area check based on stated fan-area geometry
- Total stated fan-area geometry proxy
- Minimum nominal intake-area check based on stated fan-area geometry
- in² / ft²
Copyable calculation record
Record includes formulas, inputs, source date, and limitations.
Fallback: use A = length × width, V = length × width × average height, Qref = 8 × A, and ACHref = Qref × 60 ÷ V. Example: 12 ft × 8 ft × 8 ft gives 96 ft², 768 ft³, 768 CFM, and 60 ACH equivalent.
How this calculator handles the numbers
The required calculation is deliberately narrow. Floor area is A = length × width in square feet. Approximate declared volume is V = length × width × average height in cubic feet. The summer exhaust reference is Qref = 8 × A in CFM, using UMass Extension’s Ventilation for Greenhouses reference for an average-size greenhouse.
Equivalent ACH is ACHref = Qref × 60 ÷ V. ACH means air changes per hour in this arithmetic; it is not a selectable universal target. At an average inside height of 8 ft, the reference displays as 60 ACH, or about one air change per minute. A different height changes the conversion because declared volume changes; it does not change floor-area CFM reference.
Optional intake-area check
When you know stated opening geometry, the tool applies minimum nominal intake area = 1.25 × total stated fan area. Round openings use π × diameter² ÷ 4; rectangular openings use width × height. Results show square inches first and square feet second.
This is a minimum nominal intake-area check based on stated fan-area geometry, not verified free area or installed compliance. Fan blade diameter, outer housing dimensions, a product photo, or an unknown cutout does not establish stated opening geometry. Screens, shutters, louvers, obstructions, air path, fan curve, and static pressure can still reduce delivered airflow. UMass Extension notes that inlet louver area should be at least 1.25 times fan area and that restrictions can reduce airflow; treat the calculator result as a screening number, not a pass/fail result.
Before relying on result
- Use candidate CFM only from exact current manufacturer or certified data at expected system conditions. A free-air listing is not delivered greenhouse airflow.
- Average height makes volume approximate for peaked, arched, or uneven roofs. Use one declared average height only as an approximation.
- Summer ventilation need varies with height, weather, solar radiation, outside temperature, and greenhouse tightness. This tool does not model those conditions.
- Check current manufacturer instructions, equipment marking or listing, local requirements, location suitability, required clearances, electrical work, and structural modifications separately. Seek qualified help when any point is uncertain.
What this calculator cannot calculate
- Cooling result or heat load.
- RH, humidity, or condensation.
- Evaporative cooling.
- Fan selection, fan-curve performance, or electrical suitability.
- Clearance, mounting, roof cutting, structural changes, or code compliance.
For whole-system context, read the greenhouse ventilation guide. For source interpretation, fan-data checks, and the next sizing decisions, use the greenhouse fan sizing guide.