Use this calculator to estimate the minimum BTU output your greenhouse heater needs to maintain your target temperature in cold weather.
This is a baseline estimate. Wind, drafts, insulation gaps, and how tightly your greenhouse is sealed can all increase your actual heating needs.
A few things to keep in mind before choosing a heater
This calculator gives you a baseline, not a guarantee. Your actual heating load can be higher if your greenhouse has wind exposure, air leaks around doors, vents, or panels, or if you open it frequently in cold weather.
If you’re between heater sizes, it’s usually safer to size a little above the minimum rather than right at it.
Q&As About the Greenhouse Heater Calculator
What information do I need to use this calculator?
For the most useful result, you’ll need four things:
- The total exterior surface area of your greenhouse (roof and sides, not floor space)
- The R-value or U-value of your glazing
- The inside temperature you want to maintain
- The coldest outside temperature you expect, or your USDA zone as a rough alternative
Should I use outside temperature or USDA zone?
Use your actual minimum outside temperature when you can. USDA zone is only a rough estimate.
What surface area should I enter?
Use the total exterior surface area — the roof and sides exposed to the outside, not the floor footprint. If you don’t have that number yet, use our Greenhouse Surface Area Calculator first, then come back and enter it here.
What’s the difference between R-value and U-value?
R-value measures how well a material resists heat loss — higher is better. U-value measures how easily heat passes through — lower is better.
You only need one of these to run the calculation.
Here are some general reference points, though you should always use the manufacturer’s published specs when available:
- Single-pane glass: R-0.9 to R-1.0
- Twin-wall polycarbonate: R-1.5 to R-2.0
- Double-pane glass: R-2.0 to R-2.5
- Triple-wall polycarbonate: R-2.0 to R-3.0
What does BTU mean?
BTU stands for British Thermal Unit. Here it refers to how much heat a heater can produce per hour.
Is this the exact heater size I need?
No – it’s a starting point. Real conditions like wind, drafts, and air leaks can increase your heating load beyond the estimate.
Need Help Choosing a Greenhouse Heater?
This calculator gives you a solid starting point, but real greenhouse heating needs can vary quite a bit based on climate, glazing, wind exposure, and how your structure is sealed.
If you want help comparing heater options or figuring out what makes sense for your setup, we’re happy to help.