Greenhouse Ventilation Calculator
Estimates the fan capacity (CFM) required to maintain a target temperature inside a greenhouse based on its footprint and the temperature differential with outside air. Use it when sizing exhaust fans or designing a new ventilation system.
Last updated: September 2026
Formula below · 2 sources (usda.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Summer greenhouse fans are sized per square foot of floor: the standard design rate is 8 CFM per sq ft at sea level (about one air change per minute in a house averaging 8 ft high), which holds the inside about 7°F above outside air on a sunny day. Hot climates and high solar loads use 10-12 CFM per sq ft. The calculator multiplies floor area by the design rate you choose: CFM = length × width × designRate × max(1, (outsideTemp − insideTemp) / 10). The last factor adds airflow when the outside air is 10°F or more hotter than your target; ventilation alone can never cool below the outside temperature, so in that case you also need evaporative pads, fogging or shade cloth, and the extra airflow is only a rough allowance for a pad-and-fan system. Increase airflow about 4% per 1,000 ft of elevation.
How to use
Example: A greenhouse is 100 ft long × 30 ft wide (3,000 sq ft). The desired inside temperature is 80 °F and the outside temperature is 95 °F. Design rate = 8 CFM per sq ft (standard). Step 1 — Temperature factor: max(1, (95 − 80)/10) = 1.5. Step 2 — Multiply: 100 × 30 × 8 × 1.5 = 36,000 CFM. You would need fans totalling about 36,000 CFM, plus evaporative pads to get the inside below outside temperature. On a day when outside air is cooler than your target, the factor is 1 and the requirement is 24,000 CFM.
Frequently asked questions
How many air exchanges per minute does a greenhouse need in summer?
The widely accepted standard is one complete air exchange per minute during peak summer heat. This means fan capacity in CFM should approximately equal the greenhouse volume in cubic feet. In very hot or humid climates, growers sometimes target 1.5–2 exchanges per minute. Insufficient exchange allows heat and humidity to accumulate, promoting fungal diseases and heat stress. The design airflow rate in this calculator (8, 12 or 15 CFM per sq ft) lets you match the climate and solar load.
What is the difference between natural and mechanical greenhouse ventilation?
Natural ventilation relies on ridge vents and side openings to create a convective chimney effect, moving warm air upward and out. It is low-cost and energy-free but depends on wind and temperature differentials, making it unreliable during calm, hot days. Mechanical ventilation uses motorized exhaust fans paired with inlet louvers to force a controlled air exchange regardless of outside conditions. Most commercial greenhouses use mechanical systems as primary cooling and reserve natural ventilation as a supplement, especially above 5,000 sq ft.
Why does the temperature difference between inside and outside air affect required fan size?
Each cubic foot of air removes heat in proportion to how much it warms up inside the house (about 1.08 BTU per hour per CFM per °F). When outside air is much cooler than your target, a modest airflow removes the solar heat; when the allowed rise is small, you need much more air. If outside air is hotter than the target, ventilation alone cannot reach it and evaporative cooling or shading is required; this calculator adds airflow in that case through its max(1, (outside − inside)/10) factor as a rough pad-and-fan allowance.