About the Greenhouse Heater Size Calculator
This greenhouse heater calculator estimates the heater size needed to hold your greenhouse at a set temperature on the coldest night you expect. It works out the exposed surface area of the walls and roof from the greenhouse shape — gable, hoop house (quonset) or lean-to against a building — multiplies it by the heat-loss coefficient (U-value) of your covering and by the temperature difference, and returns the heat loss in BTU per hour and the heater rating to buy.
It is useful for hobby growers overwintering tender plants, market gardeners starting transplants in late winter and anyone choosing between an electric, propane or natural-gas heater. Double layers of film or multiwall polycarbonate lose far less heat than single glass or single poly, so the tool also shows how much your covering choice matters.
The result is a design heat loss at the minimum outside temperature you enter, so use a realistic cold-night low for your area. The U-values are typical published figures for conduction through the covering. Air leakage (infiltration) is not included and typically adds roughly 5–25% depending on how tight the structure is, with old, drafty or windy-site greenhouses at the high end, so most growers add headroom. Floor losses are ignored.
With the default inputs, the heater size needed is 12,150 BTU/hr. Change any value above to recalculate instantly.
How to use the greenhouse heater size calculator
- 1Choose the greenhouse shape and enter its length, width and heights in feet.
- 2Select your covering material, or enter a custom U-value.
- 3Enter the minimum temperature you need inside — around 45–50°F for overwintering, 60°F+ for seedlings.
- 4Enter the coldest night-time temperature you expect outside.
- 5Set heater efficiency and read the BTU/hr rating to buy, adding headroom for windy sites.
Formula and method
Heat loss Q in BTU per hour equals the exposed surface area A (walls plus roof, in square feet) times the covering’s overall heat-transfer coefficient U (BTU per hour per square foot per °F) times the difference ΔT between the inside temperature you want and the coldest outside temperature. For a gable house the area is the four walls, the two triangular gable ends and the two roof slopes; a hoop house is a half-cylinder with two half-circle ends; a lean-to omits the wall shared with the building.
The heater rating is the heat loss divided by heater efficiency, because vented fuel heaters send part of their input up the flue. One kilowatt of electric heat delivers 3,412 BTU per hour, and a gallon of propane holds about 91,500 BTU. The U-values cover conduction only; air leaking in through gaps adds load that grows with the number of air changes per hour, so drafty or windy sites need extra capacity on top of this figure.
- Q
- Heat loss, BTU per hour
- A
- Exposed surface area of walls and roof, sq ft
- U
- Heat-loss coefficient of the covering, BTU/hr·ft²·°F
- ΔT
- Inside temperature − coldest outside temperature, °F
Worked examples
10 × 12 ft gable greenhouse, double poly, 50°F inside at 10°F outside
Walls (264 sq ft), gable ends (30 sq ft) and the two roof slopes (about 140 sq ft) total 434 sq ft. At U 0.7 and a 40°F difference the heat loss is 434 × 0.7 × 40 ≈ 12,150 BTU/hr, about 3.6 kW of electric heat.
12 × 20 ft single-poly hoop house with a vented propane heater
The half-cylinder roof (π × 6 × 20 ≈ 377 sq ft) plus two half-circle ends (113 sq ft) is 490 sq ft. At U 1.15 and a 45°F difference the loss is about 25,360 BTU/hr; an 80%-efficient vented heater must be rated near 31,700 BTU/hr input.
6 × 10 ft polycarbonate lean-to
Against a house wall, only the sloped roof (67 sq ft), the 5 ft front wall (50 sq ft) and two trapezoid ends (78 sq ft) lose heat — 195 sq ft. At U 0.6 and 35°F difference the loss is about 4,100 BTU/hr, which a 1.2–1.5 kW electric heater can cover.
Frequently asked questions
How many BTUs do I need to heat a greenhouse?+
Multiply the exposed surface area by the covering’s U-value and by the temperature rise you need. A 10 × 12 ft double-poly greenhouse kept at 50°F on a 10°F night needs about 12,000 BTU/hr; the same house in single glass needs about 19,100.
What U-value should I use for my greenhouse covering?+
Typical figures are about 1.1 for single glass, 1.15 for single polyethylene, 0.7 for double glass or air-inflated double poly, 0.6 for 8 mm twin-wall polycarbonate and about 0.5 for 16 mm triple-wall polycarbonate, in BTU/hr·ft²·°F.
Is an electric heater big enough for a small greenhouse?+
A standard 1,500 W plug-in heater delivers about 5,100 BTU/hr, which is enough for a small, well-sealed greenhouse with a modest temperature difference. Larger houses or colder climates usually need 240 V electric, propane or natural gas.
Do I need to vent a propane greenhouse heater?+
Unvented heaters release combustion gases, moisture and ethylene inside the greenhouse, which can damage plants and be dangerous to people. Vented heaters are safer for regular use; always follow the manufacturer’s ventilation and carbon-monoxide guidance.
How can I reduce greenhouse heating costs?+
Seal gaps, add a second layer of film or bubble insulation, use an internal thermal curtain at night, insulate the north wall and foundation, and keep the setpoint only as high as your plants require. Each of these reduces A, U or ΔT in the heat-loss formula.