Greenhouse Sizing Calculator
Sizing a greenhouse correctly means knowing four numbers: the floor area you have to plant, the internal air volume your ventilation must move, the glazing surface that loses heat, and the airflow a fan needs to deliver your target air changes per hour. This calculator models a standard gable greenhouse as a rectangular box topped by a triangular-prism roof and returns each figure from your length, width, wall height, and ridge height. All four formulas are exact geometry with no estimated constants, so they apply to any greenhouse of that common shape.
Greenhouse sizing formulas
Floor area = length * width
Box volume = length * width * wall height
Roof volume = length * width * (ridge - wall) / 2
Volume = box volume + roof volume
Roof slope = sqrt((width / 2)^2 + (ridge - wall)^2)
Glazing = 2*(length*wall) + 2*(gable triangle + wall rectangle end) ...
Airflow (CFM) = volume * air changes per hour / 60
Glazing sums the two long side walls, the two gable ends (each a wall rectangle plus a triangle), and the two roof slopes. Airflow converts air changes per hour to cubic feet per minute by dividing by 60 minutes.
Greenhouse sizing context
- Floor area sets how many benches and plants fit; allow aisles for access.
- Internal volume drives ventilation and heating sizing.
- Glazing surface is the area through which most heat is lost or gained.
- Air changes per hour for greenhouses commonly target a high exchange rate in summer; enter your design value.
- A gable (even-span) greenhouse is modelled here; hoop houses use different geometry.
Greenhouse sizing: frequently asked questions
How do I calculate greenhouse floor area?
Floor area is length multiplied by width. For a 12 ft by 8 ft greenhouse that is 96 square feet of growing space. Floor area is the starting point for planning bench layout, plant spacing, and ventilation requirements.
How is greenhouse volume calculated for a gable-roof structure?
Model the greenhouse as a rectangular box up to the eaves plus a triangular prism for the roof. Box volume is length times width times wall height; roof volume is length times width times (ridge height minus wall height) divided by 2. The total is the sum, which this calculator computes for you.
Why does greenhouse volume matter for ventilation?
Ventilation fans and vents are sized to move a target number of air changes per hour. Knowing the internal volume lets you size a fan: required airflow equals volume times air changes per hour. Adequate air exchange controls temperature and humidity.
What glazing area should I expect?
Glazing area is the area of the covering surfaces: the two side walls, two gable ends, and the two roof slopes. This calculator sums them so you can estimate covering material and the surface through which heat is lost.
Are these formulas exact?
Yes. They are geometric formulas for the area and volume of a box plus a triangular-prism roof, which is the standard shape of a gable greenhouse. They contain no empirical constants, so the results are exact for the dimensions you enter.
Official sources
- USDA National Agricultural Library: Greenhouse and controlled-environment horticulture resources.
- National Institute of Standards and Technology: Weights and measures unit definitions.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.