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BTU Calculator

Greenhouse Temperature Management and Control: Calculating Heat Requirements

How Do You Determine the Optimal Temperature for a Greenhouses BTU Output?


  • This calculator is designed to give an estimate of the BTU (British Thermal Units) heating needs of a greenhouse.
  • Simply enter the area to be heated and select the climate adjustments.
  • This is to be used for a general guideline only.
Greenhouse Area - The total square feet of exposed surface area of your greenhouse not including the floor (this is not length * width). To find out the area your greenhouse, click here.
Coldest Outside Temperature - You will want to enter the lowest temperature expected for your area. Not sure? You can click here for the USDA plant hardiness zones which provide an average annual temperature rating for areas within the United States.
Maximum Inside Temperature - This is the highest temperature you want to maintain in your greenhouse.

Heat Loss Value - Check the table at the bottom of the page to find the heat loss value for the covering used on your greenhouse. Some values may vary with manufacturer. If you know the R-value of your covering, you can convert it to a heat loss value using this formula: Heat loss value = 1 / R-value.


Resulting Minimum BTU - This is the minimum BTU output your heater should have. Most heaters use a BTU input rating. BTU output = (heater efficiency) * (BTU input).
This result is: Area x (Max - Min) x Heat Loss

Covering Insulation (R) Values for Polycarbonate, Polyethylene, Glass, and Fiberglass:
Covering MaterialR ValueCovering MaterialR Value
3 mm Glass (single layer)0.954 mm Twin-wall Polycarbonate1.43
Two layers of Glass (insulated)2.006 mm Twin-wall Polycarb
8 mm Twin-wall Polycarb1.614 mm Polyethylene
10mm Twin-wall Polycarb1.896 mm Poly0.87
16 mm Triple-wall Polycarb2.506 mm Poly double layer (inflated)1.43
Fiberglass / Polycarb (single layer)0.8311 mm woven Poly0.95
R value: A unit used to measure the effectiveness of thermal insulation.
A higher insulation value indicates greater heating and cooling efficiency.


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