How to Calculate Greenhouse Heating Requirements

How to Calculate Greenhouse Heating Requirements

How to Calculate Required BTU Output for Your Greenhouse Heating Needs

Your greenhouse is only capable of keeping the plants inside it warm, when there is enough direct sunlight hitting the covering daily, to do so. When the sun angles away from our hemisphere, resulting in colder temperatures and wintry conditions, the sun’s rays aren’t powerful enough to keep the plants inside your greenhouse warm. If you want your plants to survive the winter chills, you will need a heater that is up to the task.

Notice: This guide was built for the purpose of educating the public, as to how these figures are actually determined. If you’re not interested, in learning the extended mathematical process,™ provides a free greenhouse BTU heating calculator and a greenhouse surface area calculator.



Hobby Greenhouse Heating ElementsTable of Contents:


Part #1: Brief Introductory Statement

Part #2: Tools Required

Part #3: Calculating Equations and Formulas

Step #1 – Calculating Surface Square Area Footage

FREE Square Foot Calculator Tool

Step #2 – Calculating Temperature Expectations and Goals

Step #3 – Calculating Celsius into Fahrenheit

Step #4 – Calculating Greenhouse Coverings Heat Loss Values

Step #5 – Calculating British Thermal Units Formula

FREE BTU Calculator

Step #6 – Using BTU Heating Values During Buying & Building Processes

Part #4: Closing Statement



Part #1: Brief Introductory Statement



Introduction to Greenhouse Heating Elements:

Heaters aren’t given in size ranges. A larger unit doesn’t automatically mean more heat output. Heaters are measured by their output of BTUs. This number reflects the size of an area that a heater is capable of keeping warm. If you want a heater that will beat back the toughest winter freeze, you will need to do some calculations.

Greenhouse Temperature Management Heat Calculations

Part #2: Tools Required


Finding the right BTU output for your greenhouse is an easy task.

You will need a measuring tape, paper, something to write with, a calculator, and possibly a second set of hands and a ladder, depending on your greenhouse’s size.


What Do You Need:

  1. Measuring Tape (Required)
  2. Paper (Required)
  3. Writing Utensil (Required)
  4. Calculator (Required)
  5. Additional Help (Depends upon size)
  6. Ladder (Depends upon size)

How to Calculate Greenhouse Heating Requirements

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Part #3: Calculations Equations, and Formulas


Step #1: How to Calculate Greenhouse Surface Area Sq. Footage

First, you need to find the total square feet of your greenhouse’s surface areas.

The equation for finding the square feet of a building with a sloped roof isn’t that difficult. On your paper, write down these letters, leaving space for measurements after each: H, L, W, R and S


For each of these letters, find the following measurements and write them beside the appropriate spot.

  • H – Height of the greenhouse wall from the ground to the lowest point of the roof.
  • L – Length of the longest greenhouse wall.
  • W – Length of the shortest greenhouse wall.
  • R – Height from the ground to the highest point of the greenhouse roof.
  • S – Length from the highest roof point to the lowest roof point, measured along one eave.


Note: For the purposes of this explanation, the * symbol will mean you should multiply. Remember, anything that appears inside (…) needs to be the first math done in an equation.


Equation #: 1

First, plug the appropriate measurements into this equation: 2*(H+S)+L


Equation #: 2

Next, work the equation: (R+H)* W


Equation #: 3

Add those two results together, and you have your surface area in square feet.

Write this final calculation down in an unused corner of your paper.


Greenhouse Square Foot Calculator Tool: Input Your Values & Hit Enter!


 Length - In Feet greenhouse diagram
 Width - In Feet
 Height – In Feet
 Height of Sidewall – In Feet
 Area of Greenhouse – This number is used to calculate the minimum BTU output needed for your greenhouse.



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Step #2: Temperature Expectations and Goal Calculations

Next, write down the temperature, in Fahrenheit, that you want your greenhouse to be at. Below that, write down the coldest temperature your climate experiences. Now, subtract the two. If your coldest temperature is a negative reading, add the two instead. Record the difference in Fahrenheit down by your square feet measurement.

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Step #3: Converting Celsius to Fahrenheit Calculations

You will need to use Fahrenheit numbers for this math:

Equation #: 1

To convert Celsius to Fahrenheit quickly, double the Celsius reading, then subtract 1/10th of its value. Add 32.

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Step #4: How to Find the Greenhouse Cover Heat Loss Values

The third thing you will need to know is the heat loss value of your covering material.

Some of the most popular types of greenhouse coverings are listed here, with their heat loss value. If your covering isn’t on this list, you will need to look up the heat loss value for yourself.

  • Glass, single pane – 1.05
  • Glass, double pane – .50
  • Plastic sheeting – varies by brand
  • Plastic films – 1.20


Next: Once you know your covering’s heat loss value, write it down alongside your square feet and temperature differences in the corner.

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Step #5: Calculation of British Thermal Units

Finally, it is time to calculate the BTUs you will need in a heater. The equation for this is, square footage multiplied by temperature difference, multiplied by heat loss.


Equation #: 1

Square Feet * Temperature Difference * Heat Loss Value

BTU Calculator Tool: Input Your Values & Hit Enter!


  • 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 Material R Value Covering Material R Value
3 mm Glass (single layer) 0.95 4 mm Twin-wall Polycarbonate 1.43
Two layers of Glass (insulated) 2.00 6 mm Twin-wall Polycarb 1.54
8 mm Twin-wall Polycarb 1.61 4 mm Polyethylene 0.83
10mm Twin-wall Polycarb 1.89 6 mm Poly 0.87
16 mm Triple-wall Polycarb 2.50 6 mm Poly double layer (inflated) 1.43
Fiberglass / Polycarb (single layer) 0.83 11 mm woven Poly 0.95
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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Step #6: Shopping for Heaters using Formula Results

Take this number with you when you are searching for a greenhouse heater. Only purchase a heater that meets this minimum BTU output, but you don’t need to go much over it, either.

1500 Watt Electric Greenhouse Heater Comfort Zone Picture

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Part #4: Closing Statement




Whitney @GardeningFun Segura⇔ This article should help even beginning gardeners, to determine the proper greenhouse temperature management and control regulations, to implement. Prior to buying a greenhouse, I highly recommend doing the calculations, to understand and fully comprehend the cost, difficulty, and to create a much better planned outdoor greenhouse structure. If you have any questions or comments, please feel free to post them below, and we would be happy to help iron out any difficulties, you might run into along the way.

Whitney Segura

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This post was written by:

EarthCare Greenhouses - who has written 5 posts on EarthCare Greenhouse Gardening Advice Blog.

EarthCare greenhouses is a family owned company based out of Youngsville, Louisiana USA. EarthCare provides extremely unique, high quality, and very affordable hobby greenhouse gardening supplies & accessories, available at retail and wholesale pricing. The company strives to produce a customer and visitor experience, that creates better gardeners and thus better gardens, across the world.

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4 Responses to “How to Calculate Greenhouse Heating Requirements”

  1. Greg Johnson Says:

    Excellent guide and tool, I have yet to find a tutorial on greenhouse temperature management, which included a full definition of how to calculate these figures, in addition to a live tool that will give both square ft area and BTU output.

    – Greg

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  2. Laura from Garden Girls Alliance Says:

    Combined with the article on how to trap heat inside the structure and this post, it really does give one almost a perfect picture of how heating works within green houses of all sorts, and the tools to look at the generalized picture, and not just the small thing, like having the required heating output for your heaters, but not having the actual knowledge to understand what that means for your plants and that may cause a lack of actionable items, for those who do not have a good understanding.

    The team at EarthCare did a great job on this post, so I thank you guy’s for that, I will be forwarding this to my horticulture friends.
    > Laura (Co-Founder @ the Garden Girls Alliance)

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  3. Marion Says:

    I am guilty that I was once of those people count this information as redundant and this portion is of no value or significant effect.

    But now, after reading this article I fully understand now the value of learning the BTU calculation. Wow! It is really significant and one of the most important things to consider while building, buying and even maintaining a greenhouse garden. Heating and assuring the required and minimum/maximum temperature is so important!

    I thought it was so difficult to determine the BTU, thinking it needs a more technical and jargon computations and calculation, but I was really wrong, it is simple and vital to grow our plants and veggies in a greenhouse in right heating temperature

    By also using BTU calculator it makes the calculation hassle-free! Thanks EarthCare Greenhouses staff for this sensible and helpful article.

    Noted and bookmarked. :)

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  4. Butch Says:

    Really useful guide and tools, it’s a difficult task for a person who knows nothing about heat needs, or even the knowledge that they must find out these values, much less knowing how to actually go and do that.

    This is great, however, I would like to see a few improvements to the tutorial, such as calculating the surface area’s for different types and styles of greenhouses;

    For example:
    1. A-Frame Greenhouses
    2. Hoop House Greenhouses
    3. Geodesic Dome Greenhouses
    4. Quonset Greenhouses
    5. Ridge Frame Greenhouses
    6. Gothic Greenhouses
    7. Lean-to Greenhouses

    Also, a monthly heating cost calculator tool would be nice to see. Overall, this was a really good guide, however, if you guy’s would make the additions that I have suggested above, this would be an ultimate and killer page.

    Good luck,
    – Butch

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