Greenhouse Heater Calculator

Size a greenhouse heater in BTU/hr from surface area, temperature swing, and glazing.

Inputs
sq ft
°F
Results update as you type — no submit
ResultsLIVE
Heater output
13,200 BTU/hr
≈ 3.87 kW
Heat loss3,869 W
GlazingSingle glass / single poly
Rule-of-thumb sizing — add a margin for wind and air leaks, and use a full heat-loss analysis for a large or costly build.
Formula
loss = 300 sq ft × U × 40°F = 13,200 BTU/hr
How it works

A greenhouse loses heat through its skin, so the heater has to replace exactly what escapes. The surface-area method captures that with one equation: heat loss equals the exposed glazing area times a heat-loss coefficient for the covering times the temperature difference you want to hold between inside and outside. Match the heater's output to that loss and the greenhouse stays warm on the coldest design night.

Two inputs drive the answer. The exposed surface area is the total of the walls and roof — every square foot facing the cold — not the floor. The temperature difference is how far above the outside low you want to keep the interior; a greenhouse held at `50°F` when it is `10°F` out is working against a `40` degree swing. Glazing sets the coefficient: single glass or single poly leaks fastest, while double poly and twin-wall polycarbonate cut the loss by a third or more.

The result comes in BTU per hour, the unit heaters are rated in, alongside a kilowatt figure for electric units. Add a margin for wind, air leaks, and a safety cushion, and remember this is a rule-of-thumb estimate — a full heat-loss analysis is more precise for a large or costly build.

Worked example
Exposed area `300 sq ft` · hold `40°F` above outside · glazing `single glass`
U-factor ≈ `1.10` BTU/hr·ft²·°F
heat loss = 300 × 1.10 × 40 = about `13,200` BTU/hr (about `3.87 kW`)
Questions about greenhouse heat
Multiply exposed surface area by the glazing U-factor and by the temperature difference you want to maintain. A 300 sq ft single-glass house holding a 40 degree rise needs roughly 13,200 BTU/hr — then add a margin for wind and leaks.
SOURCES — University extension greenhouse-heating references (UMass, UGA, Virginia Tech) · NIST SI conversions
Rule-of-thumb sizing — verify for a large build
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