It’s the coldest night of the year, the seedlings you started in February are inside, and the little heater you bought on a guess is running nonstop — and losing. You don’t need a bigger guess. You need a number.
This calculator gives you that number: what size heater your greenhouse actually needs on the coldest night. It uses the same heat-loss math university Extensions publish — how much heat escapes through your covering, how much sneaks in through gaps, and how much leaks out around the base. Type in your house, your covering, and your coldest night, and it answers in BTU per hour (how much heat escapes every hour) and watts (the size of heater that replaces it).
Use the calculator
Measure your greenhouse in feet, then work down the list. Nothing here is pre-filled for you — your crop and your climate decide the temperatures.
A lean-to — one side against a heated building — isn’t covered here. The shared wall changes the math, and no verified method for it was found.
For a round-top house, the width is the diameter of the curve.
The height of the straight walls, from the ground to the eaves. For a round-top house sitting straight on the ground, enter 0.
Gable houses only — the height from the ground to the peak. It has to sit at or above the sidewall height. For a round-top house, leave this blank; the curve sets the height.
The U-value is how fast heat escapes through one square foot of that covering for each degree of difference — higher means leakier. Single poly is the common hobby-house case, so it’s the default.
Your crop’s nighttime minimum — the temperature the heater has to hold, not the daytime high.
The coldest night your area usually sees. The city table below lists 99% design lows — only about 1 night in 100 gets colder than that number.
Older and looser houses swap their inside air for cold outside air more often each hour. The calculator uses the high end of your range — the cautious number — and shows you the full range in the answer.
Heat leaks out around the base where the walls meet the ground. An insulated base loses about half as much as a bare one.
Leave blank for a calm night. Wind strips heat faster, so windy nights need a bigger number. The table steps every 5 mph — your number rounds up to the next step, and the table tops out at 35 mph.
A heat curtain only helps while it’s closed — in a snowstorm most growers open theirs, so don’t count on the curtain alone. With the box ticked, the calculator uses 0.5, the cautious end of the 0.3–0.5 curtain range.
Your answer will appear here.
How the math works
Heat leaves a greenhouse three ways, and this calculator adds up all three. First, heat escapes straight through the covering — the walls and the roof. A single layer of poly lets heat through faster than double poly or double-wall polycarbonate, and a bigger house with more surface loses more. Second, cold air sneaks in through gaps around doors, vents, and glazing bars, and warm air slips out the same way. Older and looser houses leak more. Third, heat leaks out around the base, where the walls meet the ground. Wind makes the first two worse, so a windy night needs a bigger heater than a calm night at the same temperature.
The calculator works it out in three steps. It measures your house — the surface area of the walls and roof, and the volume of air inside — from the dimensions you typed. It multiplies each kind of heat loss by the gap between the temperature you want inside and the coldest night outside. Then it adds the three losses together and converts the answer into watts, the number on the heater’s box. One watt is 3.412 BTU per hour, so watts are just the same answer in heater-sized units.
Two things the calculator doesn’t do: it doesn’t add a safety margin for you, and it doesn’t pick a heater. The number is the number. When you shop, buy the next size up — heaters come in steps, and a heater running flat out all night is a heater with no room for a colder night than the one you planned for.
The formulas, in plain words
Heat through the covering equals the surface area, times how leaky the covering is, times the temperature gap, times the wind factor. Heat from air leaks equals 0.02, times the temperature gap, times the volume of the house, times how many times the air swaps each hour, times the wind factor. Heat around the base equals the base factor, times the perimeter of the house, times the temperature gap — wind doesn’t change this one. Add the three together and you have the total heat loss in BTU per hour. Divide by 3.412 and you have watts.
Covering heat loss (U-values)
The U-value is how fast heat escapes through one square foot of covering for each degree of difference. Arkansas Extension publishes the precise numbers below; Rutgers publishes rounded versions of the same table, shown in the right-hand column. The calculator uses the precise numbers.
| Covering | U-value | Rounded version |
|---|---|---|
| Single glass | 1.13 | 1.1 |
| Double glass | 0.65 | 0.7 |
| Single polyethylene film | 1.15 | 1.1 |
| Double polyethylene film | 0.70 | 0.7 |
| Fiberglass | 1.00 | — |
| Double-wall polycarbonate | 0.65 | 0.6 |
| Double-wall acrylic | 0.65 | 0.6 |
| Concrete block, 8-inch | 0.51 | 0.5 |
| Concrete block with foam urethane | 0.13 | — |
| Poured concrete, 6-inch | 0.75 | — |
The block and concrete rows are for kneewalls — the low solid wall some houses have under the glazing. The calculator takes one covering for the whole house, so a house with a block kneewall under glass needs the math done material by material, as in the Arkansas example below.
All-in values — a different method
One guide publishes U-values that already include average air leaks inside them. Those numbers look handy, but they must never be combined with a separate air-leak calculation — that would count the leaks twice.
| Covering | All-in U-value |
|---|---|
| Single-layer poly, polycarbonate, or glass | 1.25 |
| Double-layer poly | 0.8 |
| Double-wall polycarbonate | 0.7 |
| Triple-wall polycarbonate | 0.6 |
Air leaks (infiltration)
Air leaks are measured in air changes per hour — how many times the air inside the house is swapped for cold outside air in an hour. Rutgers gives a range for each kind of house, and advises taking the largest number in the range to stay on the safe side. That’s what the calculator does, and it shows you the full range in the answer.
| House type and condition | Air changes per hour |
|---|---|
| New, glass | 0.5–1.0 |
| New, double poly | 0.75–1.5 |
| Old, glass, good condition | 1.0–2.0 |
| Old, poor condition | 2.0–4.0 |
Arkansas Extension gives the same idea as single numbers per frame and glazing type, as a cross-check: metal frame with glass 1.08, wood and steel with glass 1.05, wood with glass tight 1.00, wood with glass moderately tight 1.13, wood with glass loose 1.25, wood with fiberglass 0.95, metal with fiberglass 1.00, metal with double glass 0.70, metal with single poly 1.00, metal with double poly 0.70.
Base and wind
Heat loss around the base depends on whether the base is insulated. Wind speeds up the heat escaping through the covering and through gaps, but not the heat leaking around the base — so the wind factor applies to the first two losses only.
| Base | Base factor |
|---|---|
| Uninsulated | 0.8 |
| Insulated | 0.4 |
| Wind speed | Wind factor |
|---|---|
| 15 mph or less | 1.00 |
| 20 mph | 1.04 |
| 25 mph | 1.08 |
| 30 mph | 1.12 |
| 35 mph | 1.16 |
On a 25-mph night, the small house in the first worked example below needs 12,340 BTU/hr before the base is counted — up from 11,426 on a calm night.
Coldest nights by city (99% design temperatures)
Rutgers publishes the coldest night to plan for, city by city, for December through February. A 99% design temperature means only about 1 night in 100 — roughly 29 hours across the winter — gets colder than the number shown. Pick your nearest city as a starting point, then use your own coldest night if you know it. Burlington, Vermont is left out of this table: its printed figure doesn’t fit the cities around it, so it stays out until it can be checked.
| City | Design low |
|---|---|
| Bangor, ME | −11°F |
| Concord, NH | −8°F |
| Albany, NY | 0°F |
| Worcester, MA | 0°F |
| Columbus, OH | 0°F |
| Pittsburgh, PA | 1°F |
| New Haven, CT | 3°F |
| Newark, NJ | 10°F |
| Atlanta, GA | 18°F |
| Portland, OR | 18°F |
| Tucson, AZ | 28°F |
| San Diego, CA | 43°F |
Fuel energy and efficiency
A heater’s rated output is the heat it delivers; what it burns is more, because no heater turns all its fuel into heat. The table below gives the energy in each fuel and the steady-state efficiency — the share that becomes useful heat while the heater is running. Seasonal efficiency runs lower. No prices here; those change too fast to print.
| Fuel | Energy content | Efficiency |
|---|---|---|
| Electricity | 3,412 BTU per kWh | 98% |
| Natural gas | 100,000 BTU per therm | 82% |
| Propane | 91,333 BTU per gallon | 78% |
| Oil (#2) | 138,690 BTU per gallon | 78% |
| Kerosene | 135,000 BTU per gallon | 80% |
To work out what a heater burns, divide the heat loss by the efficiency. A house losing 12,290 BTU/hr needs a gas heater fed about 14,988 BTU/hr of gas (12,290 ÷ 0.82) — roughly 0.15 therms an hour.
Worked examples
A small hobby house. An 8-by-10-foot gable house, 6-foot walls, ridge 2 feet above the eaves, single poly, holding 60°F inside against a 30°F night outside — a 30-degree gap. The walls and roof come to 321.4 square feet: side walls 2 × (10 × 6) = 120, end walls 2 × (8 × 6 + half of 8 × 2) = 112, roof 2 × (10 × 4.47) = 89.4. The volume is 560 cubic feet. Heat through the covering: 321.4 × 1.15 × 30 = 11,090 BTU/hr. Air leaks at one air change an hour: 0.02 × 30 × 560 × 1.00 = 336 BTU/hr. Heat around the 36-foot base, uninsulated: 0.8 × 36 × 30 = 864 BTU/hr. Total with the base: 12,290 BTU/hr. Without the base: 11,426 BTU/hr, or about 3,350 watts — more than one standard 1,500 W portable heater. (You may see 10,890 BTU/hr quoted elsewhere for a house this size; that figure comes from an unstated surface area. With the dimensions written out above, the total is 11,426.)
A big glass house, Arkansas’s own example. A 40-by-100-foot A-frame, glass-glazed, metal frame, tight construction, 8-foot walls with 2 feet of concrete-block kneewall, gable 8 feet from eave to peak, holding 60°F against 0°F outside — a 60-degree gap, calm night. Glazed surface 8,000 square feet, block wall 560 square feet, volume 48,000 cubic feet. Heat through the glass: 8,000 × 1.13 × 60 = 542,400 BTU/hr. Heat through the block wall: 560 × 0.51 × 60 = 17,136 BTU/hr. Air leaks at 1.08 changes an hour: 0.02 × 60 × 48,000 × 1.08 = 62,208 BTU/hr. Total: 621,744 BTU/hr, or about 182,200 watts. This is also how a mixed-material house is done — each material worked out on its own, then added together.
A round-top house, Arkansas’s own example. A 30-by-100-foot Quonset in double poly, holding 60°F against 0°F outside — a 60-degree gap, calm night. Surface 5,409 square feet, volume 35,325 cubic feet. Heat through the covering: 5,409 × 0.70 × 60 = 227,178 BTU/hr. Air leaks at 0.70 changes an hour: 0.02 × 60 × 35,325 × 0.70 = 29,673 BTU/hr. Total: 256,851 BTU/hr, or about 75,300 watts. Our calculator’s own round-top math lands a touch higher — 5,419 square feet and 35,343 cubic feet — because it measures the curve itself rather than the flat covering width. The answers agree within a fraction of a percent.
A hoophouse with the all-in shortcut. A 30-by-100-foot gothic hoophouse, 5,010 square feet, double poly, holding 60°F against −10°F outside — a 70-degree gap. Using the all-in U-value of 0.8: 5,010 × 0.8 × 70 = 280,560 BTU/hr. No air-leak term is added on top — the 0.8 already includes it.
What a double-poly upgrade saves. Take the small hobby house from the first example and swap single poly for double poly. Heat through the covering falls to 321.4 × 0.70 × 30 = 6,750 BTU/hr. Air leaks for a new double-poly house run 0.75 to 1.5 changes an hour, so 252 to 504 BTU/hr. Total: 7,002 to 7,254 BTU/hr — about 2,130 watts at the cautious end, roughly 36% below the single-poly house.
What the base costs. On the small hobby house, the 36-foot base loses 0.8 × 36 × 30 = 864 BTU/hr uninsulated, and 0.4 × 36 × 30 = 432 BTU/hr insulated. Insulating the base cuts that loss in half.
Where the sources disagree
Different guides print slightly different numbers for the same covering. Here’s what we used, and where the others differ. Single-layer coverings print anywhere from 1.1 to 1.25 — mostly rounding, except one guide’s 1.25, which already counts average air leaks inside it. We use the precise 1.13 for glass and 1.15 for single poly, and never add air leaks on top of an all-in number. Double-wall polycarbonate prints as 0.6 in one guide and 0.65 in two others; we use 0.65. Double glass prints as 0.7 in one guide and 0.65 in two others; we use 0.65, and one standards table’s copy of this number came through garbled, so we don’t quote it at all. One guide credits a heat curtain with cutting heat loss sharply, while another warns curtains get opened in snowstorms — our curtain option uses the cautious end of that range and says so. Texas A&M prints a much larger air-leak number than the other three sources — big enough to make leaks look ten times larger than the heat through the walls — so we use 0.02, the number Rutgers and Arkansas agree on. Some guides give air leaks as a range for the house’s age and condition and others as one fixed number per frame type; we use the ranges, default to the high end, and show the fixed numbers above as a cross-check. Rutgers sizes to the 99% design temperature — only about 1 night in 100 gets colder — while one standards body uses a slightly warmer 97½% figure; ours is the colder, more cautious one. One UMass page about fan shutters prints a number its own worked example doesn’t use, so we don’t use that page’s method at all, and we don’t quote the stray number.
What this number doesn’t cover
This tells you how much heat your greenhouse loses on its coldest night — not how often to run the heater, what fuel to buy, or whether your wiring can carry it. It doesn’t know your shade, your snow, or how drafty your door really is, so treat the answer as a planning number and buy the next size up.
If your temperature gap tops 70 degrees, the Rutgers guide adds 8% for every 5 degrees over 70, and 4% for every 5 mph of wind over 15 — an 80-degree gap on a 25-mph night works out to 1.24 times the base number. This calculator doesn’t add that automatically, so treat its answer as a floor on those nights and size up.
Getting the heat where it needs to go
A heater that warms one corner and leaves the far end cold is a heater that wastes fuel. In a small freestanding house — up to about 60 feet long — a unit heater on its own usually does the job. Bigger houses need help moving the warm air: perforated poly tubes running down the house, or horizontal-airflow fans 12 to 20 inches across, spaced 40 to 50 feet apart, pushing air at 50 to 100 feet a minute. Getting the temperature even matters more than it looks — every degree you can turn the thermostat down saves about 3% on fuel. As a rule of thumb for ventilation, total fan capacity runs about 8 cubic feet per minute for each square foot of floor, and horizontal-airflow fans about 2. Those are ventilation numbers, not heater numbers.
Check the fan shutters before winter. A 48-inch shutter with bent blades or sticky hinges can lose about 10,500 BTU an hour — more through air leaks than through the shutter itself. Cover any shutter you don’t need in winter; double bubble wrap with a shiny face adds about R-3.
Where to go next
If you haven’t bought the house yet, our walk-in tunnel greenhouse review covers what 12 by 7 feet of growing room really holds and where the wind limits are. If you’re shopping for the heater itself, our 1,500 W greenhouse heater review walks through what that size does and doesn’t do — run your numbers here first, then see whether 1,500 watts is anywhere near enough. And for the nights when the forecast turns on you, how to protect your plants from frost covers the quick moves that buy you a night.
Run your numbers before the first hard freeze, buy the next size up, and the coldest night of the year becomes a night your heater was ready for.
Sources
Rutgers NJAES — A.J. Both, “Greenhouse Energy Efficiency (Heating),” Farm Energy IQ Training Program (NE-SARE grant ENE13-128), fetched in full Nov 7, 2026: https://archive.org/download/diy-greenhouse/Greenhouse%20Energy%20Efficiency.pdf
Arkansas Extension — “Heating Systems,” Section 5: Calculating Greenhouse Heat Requirements, fetched in full Nov 7, 2026: https://greenhouse.hosted.uark.edu/Unit05/Section05.html
Texas A&M AgriLife Extension — “Greenhouse Heating Requirements,” fetched in full Nov 7, 2026: https://aggie-horticulture.tamu.edu/ornamental/greenhouse-management/greenhouse-heating/
UMass Amherst CAFE — “Greenhouse Heat Distribution” (John W. Bartok Jr., 2013), fetched in full Nov 7, 2026: https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/greenhouse-heat-distribution
UMass Amherst CAFE — “Fan Shutter Heat Loss in Greenhouses” (2011), fetched in full Nov 7, 2026: https://ag.umass.edu/greenhouse-floriculture/fact-sheets/fan-shutter-heat-loss-in-greenhouses
National Greenhouse Manufacturers Association — “Heat Loss” standard (adopted April 15, 1983), fetched in full Nov 7, 2026: https://ngma.com/wp-content/uploads/2018/05/Heatloss2010.pdf
John W. Bartok Jr., Greenhouse Management — “Basic greenhouse engineering calculations” (Dec 29, 2015), fetched in full Nov 7, 2026: http://www.greenhousemag.com/article/basic-greenhouse–engineering-calculations/
