Last Updated on September 18, 2026
Greenhouse Heater Size Calculator (BTU by Size and Glazing)
BTU per hour = surface area (square feet) x temperature difference (degrees F) x U-value for the glazing. A 20×12 ft hoop house in single-layer polyethylene, held at 60F on a 10F night, needs about 28,915 BTU per hour (490 square feet x 50F x 1.18). The calculator below works out the area from your shape and dimensions, using U-values from published greenhouse engineering sources.
Greenhouse heater size calculator
Pick a shape, enter dimensions, choose the glazing, and set the temperature to hold against the coldest night you expect.
Length is the tunnel’s long axis, width is the floor width. Peak height is assumed to be half the width.
| Surface area | 490.1 sq ft |
| Temperature difference | 50 F (60F inside minus 10F outside) |
| U-value used | 1.18 (single layer polyethylene film, mid of a 1.10 to 1.25 range) |
| Heater size needed | 28,915 BTU per hour |
| In kilowatts | 8.47 kW of heater output (1 kW is about 3,412 BTU per hour) |
Buy a heater rated for at least this output, not the input rating some fuel heaters print on the box, since burning fuel loses some heat before it reaches the air.
The math: area x temperature difference x U-value
Every greenhouse heat loss calculation reduces to one line. Rutgers University states it as Q = U x A x (Ti – To): Q is heater size in BTU per hour, U the glazing’s heat transfer coefficient, A the exposed area in square feet (floor left out), and (Ti – To) the inside temperature wanted minus the coldest outside temperature planned for. The calculator above works out the area and the U-value for you.
Working out the surface area from the shape
Only surfaces exposed to outside air count; a shared wall or a cold frame’s wooden sides are left out.
Hoop or quonset: a half-cylinder
Radius is half the floor width. The curved surface is pi x radius x length. The two half-circle ends together make one circle, pi x radius squared. Total: pi x radius x (length + radius), assuming a true half-circle, peak equal to half the width.
Gable (even-span)
Half the width and the rise (peak minus sidewall height) form a right triangle whose hypotenuse is the rafter: square root of (half-width squared + rise squared). Roof = 2 x rafter x length. Sidewalls = 2 x sidewall height x length. Each gable end, a rectangle plus a triangle: 2 x (width x sidewall height + 0.5 x width x rise). Add roof, sidewalls and ends.
Lean-to
Shares one wall with a house or fence, so only three sides are exposed. Roof slope = square root of (width squared + rise squared), roof area = slope x length. Front wall = front height x length. Two trapezoid ends: 2 x 0.5 x (back height + front height) x width. The shared wall is not counted; it loses no heat outward.
Cold frame
Usually a low box with a glazed sash lid, so only the sloped lid counts: slope = square root of (width squared + rise squared), glazed area = slope x length.
Worked example: 20×12 ft hoop house
By hand: a 20 ft long, 12 ft wide hoop house (radius 6 ft), single poly, held at 60F on a 10F night.
| Step | Arithmetic | Result |
|---|---|---|
| Curved arc | pi x 6 x 20 | 376.99 sq ft |
| Two half-circle ends | pi x 6 squared | 113.10 sq ft |
| Total surface area | 376.99 + 113.10 | 490.09 sq ft |
| Temperature difference | 60F – 10F | 50F |
| Heater size | 490.09 x 50 x 1.18 | 28,915 BTU per hour |
| In kilowatts | 28,915 / 3,412 | 8.47 kW |
U-values by glazing, and where they come from
A U-value is BTU per hour through one square foot of covering per degree F of difference. Lower means better insulation. Three published sources give slightly different numbers, so where they differ, the table shows the range and uses the middle value, which the calculator uses.
| Glazing | Range across sources | Value used |
|---|---|---|
| Single layer polyethylene film | 1.10 to 1.25 | 1.18 (mid) |
| Double layer polyethylene film | 0.70 to 0.80 | 0.75 (mid) |
| Polycarbonate, twin wall | 0.60 (all three agree) | 0.60 |
| Single pane glass | 1.10 to 1.20 | 1.15 (mid) |
| Double pane glass | 0.70 (one source) | 0.70 |
Rutgers University gives 1.1 (single) and 0.7 (double) for glass and poly film, and 0.6 for polycarbonate twin wall, already including infiltration and radiation loss. The University of Arizona CEAC gives 1.2 for single glass or poly, 0.8 for double poly, and 0.6 for double-walled polycarbonate. UAF Extension, adapting NRAES-137, lists 1.2 for single glass, 1.25 for single film, 1.2 for single polycarbonate, 0.8 for double film and 0.6 for double polycarbonate. All three agree on twin wall polycarbonate; only Rutgers gives double glass.
What the number means in practice
The BTU per hour figure is the rate of heat you must keep adding for as long as it stays at your design low. Divide by 3,412 for kilowatts, the unit most electric heater ratings use. For fuel heaters, UAF Extension warns to size against the output rating, not the input rating printed on many propane or kerosene units, since combustion loses heat before it reaches the air. Buy a unit at or above the number given, with headroom for a colder night.
Caveats worth knowing before you buy
- Wind and infiltration add to the load. Rutgers notes these U-values already include a normal infiltration allowance for a well-built structure, but an older, leaky glass house should add about 10 percent, and a very tight double-glazed house may need more ventilation capacity for humidity.
- A sunny day changes everything. None of this accounts for solar gain. I built a high tunnel over my raised beds, and on a sunny day even in winter it heats up fast, overheating rather than underheating. This sizes the heater for the coldest, darkest hours only.
- A heater is not the only option. A cold frame or an unheated high tunnel can push the season by trapping daytime heat and cutting wind, for crops that already tolerate a light freeze. Check a frost tolerance chart first.
- This sizes a heater. It does not predict a fuel bill. Fuel or electricity use depends on run hours, how often the door opens, winter sun, and the heater’s efficiency, none of which this estimates.
Quick questions
What size heater do I need for my greenhouse?
Multiply your greenhouse’s surface area by the difference between your desired inside temperature and the coldest outside temperature, then by the glazing’s U-value. A 20×12 ft hoop house in single poly, holding 60F on a 10F night, needs about 28,915 BTU per hour.
What is a good U-value for a greenhouse covering?
It depends on the glazing. Sources put single poly or glass around 1.15 to 1.18, double poly around 0.75, double glass around 0.7, and polycarbonate twin wall around 0.6. Lower means less heat escapes per degree of difference.
Does this calculator account for wind and drafts?
The U-values include a normal infiltration allowance for a well-built structure, per Rutgers University. An older or leaky structure loses more, so add roughly 10 percent if yours is drafty or in poor repair.
Do I need a heater to extend my growing season?
Not always. A cold frame or unheated high tunnel can protect crops that tolerate a light freeze, using trapped daytime heat and wind protection. A heater matters most for a temperature the structure and sun cannot manage alone overnight.
Does this tell me how much my heating bill will be?
No. It sizes the heater in BTU per hour so you buy a unit big enough for your coldest night. Actual fuel or electricity use depends on run hours, winter sun, how often the greenhouse opens, and the heater’s efficiency.
Let the planner handle what goes in it
Once the greenhouse is warm enough, let the planner lay out what grows in it. Plan My Garden is free to start: enter your zip code and it finds your frost dates, draw your beds, pick your crops, and it places each plant at square-foot spacing with a planting schedule from those dates.
Related: How to Build an Automatic Vent for a Greenhouse or Hoop House · 5 Tips for Using a Cold Frame · Vegetable Frost Tolerance Chart · Cattle Panel Arch Calculator · Seed Starting Calendar · Charts and Tools
