Greenhouse Heater BTU Sizer: Winter Heat Loss & Insulation Sizer
Undersizing a greenhouse heater leaves plants exposed to frost on the coldest nights, while oversizing wastes fuel and money all season long. This calculator estimates total surface area, temperature differential and glazing insulation value to recommend an accurately sized heater in BTU/hr and watts.
You need roughly a 13,915 BTU/hr (4,078W) heater to hold 60°F when it's 20°F outside.
| Measure | Value |
|---|---|
| Dimensions (W×L×H) | 10×12×7 ft |
| Surface area | 446 sq ft |
| Glazing U-value | 0.65 |
| Delta T | 40°F |
| Wind exposure factor | 1.20 |
| Required BTU/hr | 13,915 |
| Required watts | 4,078 |
- 1.
Surface_Area = 2×Length×Height + 2×Width×Height + Roof_Area - 2.
Delta_T = Target_Inside_Temp − Lowest_Outside_Temp - 3.
Required_BTU = Surface_Area × U_Value × Delta_T × 1.20
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
Formula & step-by-step maths
- U_Value
- Glazing material insulation rating
- Delta_T
- Inside minus outside temperature
- 1.20
- Wind exposure safety factor
Why glazing material changes everything
Single-pane glass has a U-value around 1.1, meaning it loses heat almost twice as fast as 6mm twin-wall polycarbonate (U≈0.65). Choosing better glazing can cut required heater capacity nearly in half for the same structure.
Understanding delta T (temperature differential)
The bigger the gap between your target inside temperature and the coldest expected outside temperature, the more heat escapes every hour. Always size for your coldest realistic night, not the seasonal average, to avoid crop loss during cold snaps.
The wind exposure safety factor
The 1.20 multiplier in this formula accounts for wind-driven heat loss through gaps, door seals and increased convective loss on exposed structures — greenhouses in open, windy locations may need an even higher factor.
Propane vs electric heater sizing
Propane heaters are rated directly in BTU/hr, while electric heaters are rated in watts; this calculator provides both units so you can compare running costs and equipment options directly regardless of fuel type.
U-values by common greenhouse glazing
| Glazing type | U-value | Relative heat loss |
|---|---|---|
| Single pane glass | 1.1 | Highest |
| Double poly film | 0.7 | Medium |
| 6mm twin-wall polycarbonate | 0.65 | Medium-low |
| 8mm twin-wall polycarbonate | 0.55 | Low |
| 16mm triple-wall polycarbonate | 0.4 | Lowest |
| Double pane glass | 0.5 | Low |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: greenhouse width 7.8 ft, covering / glazing material "Single Glass (U=1.1)"
On the lower / more conservative end. You need roughly a 20,320 BTU/hr (5,955W) heater to hold 60°F when it's 20°F outside.
Example 2: greenhouse width 10 ft, covering / glazing material "6mm Twin-Wall Polycarbonate (U=0.65)"
A typical middle-of-the-road setup. You need roughly a 13,915 BTU/hr (4,078W) heater to hold 60°F when it's 20°F outside.
Example 3: greenhouse width 13 ft, covering / glazing material "Double Poly Film (U=0.7)"
On the higher / more demanding end. You need roughly a 17,788 BTU/hr (5,213W) heater to hold 60°F when it's 20°F outside.
Quick answers about the Greenhouse Heater BTU
What exactly does the Greenhouse Heater BTU work out?
Undersizing a greenhouse heater leaves plants exposed to frost on the coldest nights, while oversizing wastes fuel and money all season long. You enter greenhouse width, greenhouse length, ridge/wall height and covering / glazing material (plus 2 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 6 fields: greenhouse width, greenhouse length, ridge/wall height, covering / glazing material, target night inside temp and lowest expected outside temp. Nothing else is needed and nothing is stored.
How is it calculated — why glazing material changes everything?
Single-pane glass has a U-value around 1. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for greenhouse Heater BTU?
The bigger the gap between your target inside temperature and the coldest expected outside temperature, the more heat escapes every hour. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "U-values by common greenhouse glazing" table on this page tell me?
It is the reference range this tool works against — 6 rows from "Single pane glass" (1.1) up to "Double pane glass" (0.5). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which covering / glazing material should I pick?
The dropdown offers 3 choices — Single Glass (U=1.1), 6mm Twin-Wall Polycarbonate (U=0.65) and Double Poly Film (U=0.7). Pick the one that matches your real situation rather than the one you would like to be true; covering / glazing material usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Greenhouse Heater BTU runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Greenhouse Heater BTU leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
With twin-wall polycarbonate and a 40°F delta T, a 10x12x7ft greenhouse typically needs around 8,000-12,000 BTU/hr — use this calculator with your exact temperatures for a precise figure. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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