Greenhouse Heater Size Calculator
Size a greenhouse heater in BTUs from its dimensions, covering type and the temperature you need to hold on a cold night.
How it works
Surface area = 2(L×H) + 2(W×H) + (L×W) · BTU/hr = surface area × ΔT × U-factor · ΔT = inside − outside
Heat escapes through the greenhouse skin at a rate set by the exposed surface area, the temperature difference you want to maintain, and how well the covering insulates (its U-factor). Single-layer films and glass lose heat fastest; double poly and twin-wall polycarbonate roughly halve the loss. The result is a steady-state heat load, so choosing a heater rated a little above it gives margin for wind and the very coldest nights.
Worked example
A 12×8×7 ft single-poly greenhouse holding 55 °F when it is 20 °F outside: surface = 2(12×7) + 2(8×7) + 12×8 = 168 + 112 + 96 = 376 ft². ΔT = 35 °F. BTU = 376 × 35 × 1.15 ≈ 15,134 BTU/hr.
Frequently asked questions
What is the U-factor?
The U-factor is a heat-loss coefficient for the covering. Lower is better-insulated: single poly or glass is around 1.1–1.2, double poly about 0.7, and twin-wall polycarbonate about 0.65 — so a double layer needs roughly half the heater.
Should I add a safety margin?
Yes. This is a steady-state estimate. Sizing a heater 10–20% above the calculated load covers wind-driven losses and the coldest nights, and lets the heater cycle instead of running flat out.
How can I reduce the heat I need?
Add a layer (double poly or thermal bubble wrap), seal air leaks around doors and vents, and use thermal mass like water barrels to buffer temperature. Each step lowers the BTU requirement.
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This calculator is for educational purposes. Double-check important results before acting on them.