Greenhouse Heater BTU Calculator
Heating load for a greenhouse.
Enter your values
Enter the length in ft.
Enter the width in ft.
Enter the average height in ft.
Enter the inside target in °F.
Enter the coldest outside in °F.
Choose one of: Single glass (1.13), Single poly (1.20), Double poly (0.70), Twin-wall polycarbonate (0.60).
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Heater size
- 22,226 BTU/h
- In kW
- 6.51
- Surface area
- 907 sq ft
- Temperature difference
- 35 °F
- With 15% margin
- 25,560 BTU/h
Quick answer
Greenhouse heat loss is surface area times the U-value of the glazing times the temperature difference.
BTU/h = area x U x ΔT
At a glance
| What it does | Heating load for a greenhouse. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Length, Width, Average height, Inside target, Coldest outside, Glazing |
| Main output | Heater size |
| Formula | BTU/h = area x U x ΔT |
| Cost | Free — no sign-up, no download |
How to use the Greenhouse Heater BTU Calculator
- 1Enter the greenhouse dimensions and average height.
- 2Choose the glazing and enter your coldest expected night.
- 3Add 10-20% margin, and use a thermal blanket to cut the load sharply.
Inputs explained
- Length(ft)
- Enter the length in ft.
- Width(ft)
- Enter the width in ft.
- Average height(ft)
- Enter the average height in ft.
- Inside target(°F)
- Enter the inside target in °F.
- Coldest outside(°F)
- Enter the coldest outside in °F.
- Glazing
- Choose one of: Single glass (1.13), Single poly (1.20), Double poly (0.70), Twin-wall polycarbonate (0.60).
Worked example
Using the values the calculator loads with:
Inputs
- Length24 ft
- Width12 ft
- Average height8 ft
- Inside target55 °F
- Coldest outside20 °F
- GlazingDouble poly (0.70)
Results
- Heater size22,226 BTU/h
- In kW6.51
- Surface area907 sq ft
- Temperature difference35 °F
- With 15% margin25,560 BTU/h
Frequently asked questions
Vented or unvented heater?
Always vent combustion heaters — unvented units dump moisture and CO2 into the house.
Cheapest saving?
An internal thermal screen at night can cut heat loss by 30-50%.
What do I need to enter into the Greenhouse Heater BTU Calculator?
Just 6 values: length, width, average height, inside target, coldest outside, glazing. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Greenhouse Heater BTU Calculator work out the answer?
Greenhouse heat loss is surface area times the U-value of the glazing times the temperature difference. It applies the formula BTU/h = area x U x ΔT and shows the working so you can check each step by hand.
Is the Greenhouse Heater BTU Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.