Greenhouse Heater Calculator

The greenhouse heater calculator estimates how much heat your greenhouse needs to maintain your target temperature during cold weather. It uses your greenhouse size and shape, covering material, outdoor winter design temperature, air leakage, and perimeter insulation to estimate heat loss.

Your result is shown in BTU/hr and the equivalent electric heating capacity in kW. The calculator also shows where most of the heat is being lost and how changes such as better covering, improved sealing, or perimeter insulation could reduce the heating requirement.

Unlike a simple greenhouse BTU calculator based only on square footage, this tool accounts for the main sources of heat loss that affect real greenhouse heater sizing. For a deeper explanation of the calculation and assumptions, see our greenhouse heating guide.

Your result is the estimated heating output needed to maintain the indoor temperature you selected under the winter conditions entered in the calculator. When choosing a greenhouse heater, look for the next available size rated at or above the calculated BTU/hr requirement.

For example, if the calculator estimates 18,400 BTU/hr, a heater rated for at least that much heat output would be appropriate. Avoid sizing a heater by greenhouse square footage alone, since covering type, outdoor temperature, air leakage, greenhouse height, and perimeter insulation can significantly change the heating requirement.

If you are comparing gas or propane heaters, check whether the manufacturer lists input BTU/hr or actual heat output. For electric resistance heaters, you can also use the calculator's kW equivalent when comparing heater capacity.

How many BTUs do I need to heat my greenhouse?

The BTU requirement depends on more than greenhouse floor area. Size, shape, covering material, the temperature difference between indoors and outdoors, air leakage, and perimeter insulation all affect heat loss. Enter those details into the greenhouse BTU calculator above to estimate the heating output you need in BTU/hr.

What size heater do I need for my greenhouse?

Choose the next available heater size rated at or above the BTU/hr result from the calculator. For propane, natural gas, or other combustion heaters, make sure you are comparing the calculator result with the heater's heat output, not simply its input rating. Manufacturer specifications should indicate both where applicable.

What outdoor temperature should I use for greenhouse heater sizing?

Use a winter design temperature rather than the coldest temperature ever recorded in your area. The ZIP-code lookup in the calculator provides a local planning value automatically. If you have a more specific design temperature for your property, you can enter it manually instead.

How does greenhouse covering affect heater size?

Covering can have a major effect on greenhouse heat loss. Better-insulated materials, such as double-layer polyethylene or multiwall polycarbonate, generally lose less heat than single-layer coverings. That means two greenhouses of the same size can require very different heater capacities depending on how they are covered.

What size heater do I need for a 10 × 12 greenhouse?

There is no single heater size that works for every 10 × 12 greenhouse. A greenhouse's height, shape, covering, winter climate, sealing, and perimeter insulation can all change the required BTU/hr. Enter the actual dimensions and conditions into the greenhouse heater size calculator above for a more useful estimate than sizing by footprint alone.

How accurate is the greenhouse heater calculator?

The calculator is designed as a planning tool for greenhouse heater sizing. It estimates heat loss through the greenhouse covering, air leakage, and perimeter, with an additional allowance when you select an exposed or windy site. Accuracy depends on the inputs and local conditions, so use the result as a sizing estimate and confirm the specifications and installation requirements of the heater you plan to use.