ConverterHub

Greenhouse Heating Calculator

Enter the greenhouse area and target temperatures to size your heater.

How it is calculated

The heater size is the heat the greenhouse loses through its covering at the coldest expected moment: BTU/hr = area × ΔT × 1.1, with ΔT = inside − outside. area is the covering (glazing) surface in square feet, inside is the temperature you want to hold and outside is the lowest outdoor temperature you plan for, both in °F. The factor 1.1 is the fixed single-layer covering loss factor used by this tool.

Watts are then BTU/hr ÷ 3.412. The inputs are imperial whatever the page language. If the inside temperature is not higher than the outside one, or the area is zero or negative, there is no result. Double glazing, insulation and wind are not adjusted for, so treat the figure as a starting size.

Worked example

A greenhouse has 600 sq ft of covering. You want 65 °F inside when it falls to 20 °F outside. The temperature difference is 65 − 20 = 45 °F. Heat loss is 600 × 45 × 1.1 = 29,700 BTU per hour, so the tool shows 29,700 BTU/hr. Dividing by 3.412 gives about 8,705 W, which is the electric heater power that would match it.

Try this example →

Common mistakes

  • Entering the floor area instead of the surface area of the walls and roof. The loss happens through the covering, so floor area usually underestimates it.
  • Typing temperatures in °C. The tool works in °F, so 20 °C must be entered as 68 °F.
  • Using the average winter temperature as the outside value. Size for the coldest night you expect, not the typical one.
  • Using square metres. The area field is in square feet; multiply square metres by 10.764 first.
  • Putting the target below the outside temperature, which means cooling. The tool then gives no result.

About this calculator

Sizing a greenhouse heater starts with the heat that escapes through its glazing: multiplying the surface area by the temperature difference between inside and outside gives the base heat loss, and multiplying that by a standard glazing factor of about 1.1 (accounting for the extra losses typical of single-layer glass or plastic covering) gives the heater output needed, usually expressed in BTU/hr, to hold a target temperature.

This method is used by gardeners, greenhouse growers, and hobbyist horticulturists deciding what size heater to buy before winter — undersizing leaves plants exposed to frost during cold snaps, while oversizing wastes money on both the unit and its running cost. It's the same basic heat-loss logic professional greenhouse and grow-room designers use, simplified to the inputs a home grower actually has on hand.

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