Greenhouse Heater Size Calculator
Use this free greenhouse heater size calculator to find the heater size your greenhouse needs. Enter your dimensions, glazing type, and the temperatures you want to hold, and get an instant estimate in BTU/hr and watts — plus a recommended heater size to shop for.
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Estimates based on floor area, temperature difference, and glazing type. Always round up to the next common heater size for the coldest nights.
How to use this calculator
Start by measuring your greenhouse's length and width in feet — the defaults show a common 10 by 12 ft hobby greenhouse. Then pick the glazing type that matches your walls: single-layer poly film for a basic hoop house, double-layer inflated poly for a serious hobby house, glass or twin-wall polycarbonate panels, or insulated panels if yours is built for winter growing.
Next, enter the temperature you want to hold inside — 60°F suits most seedlings and tender plants, 50°F works for overwintering hardy crops. Then enter the coldest outdoor temperature you expect on a winter night, not the average low. Results update instantly as you change any input.
The results show your estimated heat need in BTU/hr and the equivalent wattage, plus the recommended heater size: the next common electric heater wattage at or above your calculated need. Shop for that size (or the gas BTU equivalent), and use a thermostat so it cycles off when the setpoint is reached.
How it works
The calculator uses the standard greenhouse heat-loss formula: BTU/hr = floor area (sq ft) × ΔT (°F) × heat-loss factor. The heat-loss factor depends on your glazing because glazing is where most heat escapes: 1.1 for single poly film, 0.8 for double poly, 0.7 for glass or polycarbonate panels, and 0.5 for insulated panels. For example, a 10×12 ft greenhouse (120 sq ft) holding 60°F against a 30°F night (ΔT = 30) with single poly needs 120 × 30 × 1.1 = 3,960 BTU/hr.
To convert to electric heater sizing, it divides BTU/hr by 3.412, since 1 watt of electric heat equals 3.412 BTU/hr. So 3,960 BTU/hr ÷ 3.412 ≈ 1,161 watts. It then rounds up to the next common heater size — in this case a 1,200-watt heater — because sizing up a notch covers colder-than-expected nights.
Assumptions (read before you buy): this formula estimates heat loss for the whole structure from floor area and a single glazing factor — it does not correct for wind-driven infiltration through gaps, so a drafty greenhouse needs more. It also assumes steady nighttime conditions with no solar gain. Treat every result as an estimate and round up to the next heater size; a thermostat will prevent overheating.
Frequently asked questions
What size heater do I need for my greenhouse?
Multiply your floor area in square feet by the temperature difference (desired inside temp minus coldest outside temp) and by a heat-loss factor for your glazing: 1.1 for single poly film, 0.8 for double poly, 0.7 for glass or polycarbonate panels, 0.5 for insulated panels. For example, a 10x12 ft single-poly greenhouse with a 30°F difference needs about 3,960 BTU/hr. Enter your numbers in the calculator above for your exact result.
How many watts is 3,960 BTU/hr?
Divide BTU/hr by 3.412 to get watts. 3,960 BTU/hr ÷ 3.412 ≈ 1,161 watts, so a 1,200-watt heater would cover it. The calculator above does this conversion automatically and suggests the next common heater size up.
Should I buy a bigger heater than the calculator suggests?
Yes — it is safer to round up to the next common heater size rather than down. A slightly oversized heater warms the greenhouse faster on the coldest nights and cycles off via its thermostat, while an undersized one runs constantly and may not hold temperature. The calculator above recommends the next size up automatically.
Which glazing type loses the most heat?
Single-layer polyethylene film loses heat fastest, with a heat-loss factor of 1.1 — roughly twice as much as insulated panels (0.5). Double poly film (0.8) and glass or twin-wall polycarbonate (0.7) fall in between. If you heat through a cold winter, upgrading from single to double poly can cut your heating bill significantly.