Greenhouse Heating Calculator
Estimate the BTU/hr heating output needed to maintain a greenhouse temperature during cold nights.
How to use this tool
- Enter the greenhouse interior dimensions in feet.
- Enter your target minimum inside temperature and the coldest expected outside temperature.
- Select the glazing type (material covering the greenhouse).
- The result shows surface area, temperature difference, and required heater BTU/hr output.
Size your greenhouse heater correctly to keep plants alive on the coldest nights.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Surface area (ft²) = 2(L×W) + 2(L×H) + 2(W×H)
ΔT (°F) = Inside temp − Outside temp
BTU/hr = Surface area × ΔT × U-value (glazing coefficient)
Where the glazing U-value (heat loss coefficient) is selected by glazing type, e.g. 1.2 BTU/(hr·ft²·°F) for single-layer polyethylene film.
How it works
This calculator uses the steady-state heat-loss formula: power required equals the total glazed surface area multiplied by the inside-to-outside temperature difference and the glazing's U-value (thermal transmittance). It models the greenhouse as a simple rectangular box — two floors/ceilings, two long walls, and two end walls — which slightly overestimates surface area for gable-roofed structures.
The U-value encodes how quickly heat escapes through a given glazing material; single-layer polyethylene film loses heat faster (higher U) than double-wall polycarbonate (lower U). Results represent peak design load under coldest expected conditions, so actual average consumption will be lower. Infiltration losses and thermal mass are not included.
Worked example
- Greenhouse: 20 ft long × 12 ft wide × 8 ft eave height. Inside 50 °F, outside 10 °F. Glazing U-value: 1.2.
- Surface area = 2(20×12) + 2(20×8) + 2(12×8) = 480 + 320 + 192 = 992 ft².
- ΔT = 50 − 10 = 40 °F.
- BTU/hr = 992 × 40 × 1.2 = 47,616 BTU/hr.
Heating required: 47,616 BTU/hr
Common mistakes to avoid
- Using indoor air temperature as the design temperature without accounting for the coldest night of the year — size the heater for the lowest expected overnight temperature, not the average.
- Applying a single U-value for all glazing when the roof and walls may use different materials (e.g. polycarbonate panels on the roof, glass on the sides).
- Forgetting to include floor area in surface area — an uninsulated concrete or soil floor loses heat to the ground and should be factored in for accurate BTU calculations.
Key terms
- BTU/hr
- British Thermal Units per hour — the standard US unit for heating capacity; one BTU raises one pound of water by 1 °F.
- U-value
- Thermal transmittance: the rate of heat flow through a material in BTU per hour per square foot per °F temperature difference. Higher U means more heat loss.
- ΔT (delta T)
- The temperature difference between inside and outside; the larger the gap, the more heating power is needed to maintain the set point.
- Design load
- The peak heating output sized for the coldest expected night; actual average energy use will be considerably lower throughout the season.
- Glazing
- The transparent or translucent covering of a greenhouse — glass, single-layer poly film, or double-wall polycarbonate — each with a different U-value.
Frequently asked questions
- How do I calculate greenhouse heating requirements?
- Multiply the greenhouse surface area × temperature difference (inside minus outside) × the glazing U-value. This gives BTU/hr required. Always add 20–25% as a safety margin.
- What temperature should I maintain in a greenhouse?
- For frost protection only, maintain 34–40°F. For cool-season crops (lettuce, kale), 45–55°F. For tomatoes and peppers, 60–70°F nights. Higher targets need significantly more heating.