Greenhouse Heating Calculator

A greenhouse creates a controlled growing environment by trapping solar heat, protecting plants from frost, and extending the growing season. Sizing a greenhouse correctly ensures adequate space for your plants while making heating and ventilation manageable.

Key formulas: Floor Area = Length × Width Growing Capacity = Floor Area × Bench Efficiency Factor Heating Load (BTU/hr) = Surface Area × U-value × ΔT Ventilation Rate (CFM) = Floor Area × 1.25 (minimum for summer cooling)

What each variable means:

• Bench Efficiency Factor — the fraction of floor space usable for growing benches. Standard: 0.65–0.75 (paths and structural elements take the rest).
• U-value — heat transmission coefficient of the glazing material. Lower = better insulation. Single glass: U≈1.0; double-wall polycarbonate: U≈0.57; triple-wall poly: U≈0.41.
• ΔT (Delta T) — the temperature difference between desired interior temp and the coldest expected exterior temp (°F or °C).
• Surface Area — total area of walls and roof (not floor). For a simple lean-to or hoop house, add all exposed surfaces.
• CFM (Cubic Feet per Minute) — ventilation airflow rate. A greenhouse needs complete air exchange every 1–2 minutes in summer to prevent overheating.

Common greenhouse sizes and crops:

• 6×8 ft (48 sq ft) — hobby starter; 12–15 tomato plants or 50+ seedling trays
• 8×12 ft (96 sq ft) — serious hobby; small vegetable production
• 12×24 ft (288 sq ft) — small commercial; 100+ plants
• 30×100 ft (3,000 sq ft) — commercial scale

Worked example: Greenhouse: 12 × 20 ft = 240 sq ft floor. Desired temp: 60°F minimum; coldest night: 10°F. ΔT = 50°F. Surface area (walls + roof, simplified): ~600 sq ft. Using double-wall polycarbonate (U = 0.57): Heating load = 600 × 0.57 × 50 = 17,100 BTU/hr — a 20,000 BTU propane heater is sufficient. Ventilation: 240 × 1.25 = 300 CFM minimum fan size for summer. Growing capacity = 240 × 0.70 = 168 sq ft of bench space.