Greenhouse Heating Calculator
Size a greenhouse heater from the glazed surface area, your target inside temperature, the coldest outside temperature, and the covering’s U-value. Calculates in your browser.
Heater size needed
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Show the math
Heat loss rises with the surface area, the covering, and the temperature gap:
BTU/hr = surface area × U-value × (inside − outside)
Surface area is the whole glazed envelope, walls plus roof. Use your design low temperature for the coldest night. Add a margin for wind and infiltration. All math runs locally in your browser.
What this does
A greenhouse heating calculator sizes a heater from the glazed surface area, your target inside temperature, the coldest outside temperature, and the covering’s U-value.
How to use it
- Enter the glazed surface area.
- Set the inside target temperature.
- Set the coldest outside temperature.
- Enter the covering’s U-value to read the BTU.
How it works
Heat loss rises with surface area, the U-value, and the gap between inside and outside. Lower U-values insulate better.
BTU/hr = area × U-value × temperature difference
Understanding your result
The BTU figure sizes a heater for your coldest expected night, so a milder location needs less. Heat loss climbs with surface area and the covering U-value, so better glazing or thermal screens cut the requirement, and it is wise to oversize a little for wind and the coldest snaps.
Example
A 400 sq ft envelope holding a 40°F gap at U-1.1 needs about 17,600 BTU/hr.
Sources & methodology
Last updated .
Frequently asked questions
What surface area do I use?
Use the whole glazed envelope (every wall and the roof) since heat is lost through all of it.
Which temperatures should I enter?
Set the inside target you want to hold and the coldest outside temperature you expect; the gap between them drives the heat loss.
What is a U-value?
It is how readily a covering loses heat, lower is better. Double poly and twin-wall polycarbonate insulate far better than single film or glass.
Is anything stored?
No. Everything is calculated locally in your browser.
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