Greenhouse BTU Calculator: Heat Loss & Sizing

A greenhouse BTU calculator estimates the heating capacity needed to keep a greenhouse at a target temperature, based on its surface area, glazing type, and the difference between outside and inside temperatures. You enter the greenhouse dimensions, the type of covering (glass, polycarbonate, or film), and your desired inside temperature; the calculator returns the BTU per hour required. This value tells you the minimum heater output needed for the coldest expected outside temperature.

Formula

The standard greenhouse heat-loss formula is:

BTU/hr = Surface Area (sq ft) × U-factor × Temperature Difference (°F)

Where:

• Surface Area = the total exposed area of the greenhouse (walls + roof), in square feet. For a simple rectangular structure with a flat roof, this is 2 × (length × height) + 2 × (width × height) + (length × width). The roof area is approximated as the floor area for a flat or gently sloped roof.
• U-factor = the heat transfer coefficient of the glazing material, in BTU/(hr·sq ft·°F). Common values: single glass ≈ 1.13, double polycarbonate ≈ 0.70, single film ≈ 1.0, double film ≈ 0.65.
• Temperature Difference = the desired inside temperature minus the coldest expected outside temperature, in °F.

The result is the steady-state heat loss. It assumes no heat from the sun, no wind, and no infiltration (air leaks).

Worked Example

Using the default values from the calculator:

• Length = 20 ft, Width = 10 ft, Height = 8 ft
• Temperature difference = 30°F
• Glazing = single glass (U-factor = 1.13)

Step 1: Calculate surface area.

Walls: 2 × (20 × 8) = 320 sq ft (long walls)
2 × (10 × 8) = 160 sq ft (short walls)
Roof: 20 × 10 = 200 sq ft
Total = 320 + 160 + 200 = 680 sq ft

Step 2: Multiply by U-factor and temperature difference.

680 × 1.13 × 30 = 23,052 BTU/hr

So the calculator shows 23,052 BTU/hr (rounded to 23,052). This is the heater output needed to maintain a 30°F rise above outside temperature.

Why the Shape and Glazing Matter Most

Heat loss scales with surface area, not floor area. A tall, narrow greenhouse loses more heat than a low, wide one with the same footprint. The glazing material is the second biggest factor: single glass loses almost twice as much heat as double-film plastic. Polycarbonate panels (twin-wall) are a middle ground, offering good insulation with decent light transmission. If you are planning a new greenhouse, choosing a more insulating cover can cut heating costs by 30–50% compared to single glass.

How to Use the Calculator for Real Sizing

Follow these steps to get a practical heater size:

1. Measure the greenhouse exterior dimensions: length, width, and height to the eaves (not the peak, unless the roof is steep).
2. Find the coldest winter night temperature in your region (use a local climate chart or weather service data).
3. Decide the minimum temperature your plants need (e.g., 50°F for many tropicals, 40°F for hardy greens).
4. Subtract the outside temperature from the inside target to get the temperature difference.
5. Select the glazing type that matches your greenhouse.
6. Run the calculator to get the BTU/hr requirement.

Then add a safety margin: most growers add 10–20% to the calculated value to account for wind, air leaks, and cold spots. If the heater is undersized, it will run continuously and still not reach the target temperature on the coldest nights.

Common Mistakes and Misconceptions

A frequent error is using the floor area instead of the surface area. A 10×10 ft greenhouse with 6-ft walls has about 340 sq ft of surface, not 100 sq ft – that triples the heat requirement. Another mistake is ignoring the temperature difference: using the average winter temperature instead of the coldest night. If you size for an average of 20°F but the coldest night is -10°F, your heater will be 30°F short. Also, the formula assumes no solar gain. On sunny days, the greenhouse may heat up significantly, but the heater must handle the worst-case night, not the daytime average.

Some people confuse BTU (British Thermal Unit) with watts. One BTU/hr ≈ 0.293 watts. A 5,000 BTU/hr heater is about 1,465 watts. Electric heaters are often rated in watts; gas and propane heaters in BTU/hr. Always convert units before comparison.

Limitations of the Formula

This calculator is a simplified model. It does not account for:

• Wind speed – wind increases heat loss by removing the insulating air layer next to the glazing. A windy site can require 20–30% more heat.
• Infiltration – gaps around doors, vents, and panels let warm air escape. Older greenhouses with poor seals lose more heat.
• Thermal mass – water barrels, concrete floors, or soil store heat during the day and release it at night, reducing the heater load.
• Roof shape – a peaked roof has slightly more surface area than a flat roof, but the difference is small for typical greenhouse slopes.
• Heater efficiency – the BTU rating is the output, not the input. A 80% efficient heater needs more input fuel than its output rating.

For a precise design, use a professional greenhouse heat-loss calculator that includes wind and infiltration factors, or consult a greenhouse supplier with your specific dimensions and climate data.

Alternatives to BTU/hr: Watts and Kilowatts

If you plan to use an electric heater, convert BTU/hr to watts: divide by 3.412. For the example above, 23,052 BTU/hr ÷ 3.412 ≈ 6,756 watts (6.8 kW). Electric heaters are typically sold in 1,500 W (about 5,120 BTU/hr) increments, so you would need a 7,500 W heater or several smaller units. Gas and propane heaters are usually rated directly in BTU/hr, so the calculator output can be used as-is. When comparing fuels, remember that electric resistance heat is 100% efficient at the point of use, while gas heaters are 80–90% efficient, so the input fuel rating will be higher than the output BTU rating.

Frequently Asked Questions

What is a BTU in greenhouse heating?

BTU stands for British Thermal Unit – the amount of heat needed to raise one pound of water by one degree Fahrenheit. In greenhouse context, BTU/hr is the rate of heat output. One BTU/hr is roughly 0.293 watts. The calculator gives you the BTU/hr needed to offset heat loss through the greenhouse envelope.

How do I convert BTU/hr to watts?

Divide BTU/hr by 3.412. For example, 10,000 BTU/hr ÷ 3.412 ≈ 2,931 watts. This is useful when comparing electric heaters, which are rated in watts. Conversely, multiply watts by 3.412 to get BTU/hr.

Should I size for the coldest night or the average?

Size for the coldest night you expect to face. If you use an average, the heater will be undersized on the coldest nights, and plants may suffer frost damage. Many growers add a 10–20% safety margin to the calculated value to cover wind and leaks.

Does the calculator work for a hoop house or tunnel?

Yes, with a caveat. For a hoop house, the surface area is roughly the arc length of the hoop times the length. A simple approximation is to treat the hoop as a half-cylinder: surface area ≈ (π × width/2) × length + (width × length) for the end walls. The U-factor for single or double film applies. This calculator uses a rectangular approximation, so results are approximate for curved structures.

Why does my heater need to be bigger than the calculator says?

Because the formula assumes no wind, no leaks, and no thermal mass. Real greenhouses lose heat through gaps, wind strips away the insulating air film, and if the greenhouse is empty of water barrels or soil, it has little thermal storage. Adding 20–30% is common practice. Also, if your heater is located in a cold corner, you may need more output to distribute heat evenly.

Can I use this for a cold frame or small greenhouse?

Yes, but for very small structures, the ratio of surface area to volume is high, so heat loss per square foot is larger than a big greenhouse. The formula still works, but you may need to add extra margin for edge effects and door openings. For a cold frame (a low box with a glass lid), measure the lid area and the side areas, and use the same U-factor approach.

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in length | Length | ft | 20 | 0
in width | Width | ft | 10 | 0
in height | Height (to eaves) | ft | 8 | 0
in delta_t | Temperature difference (inside - outside) | °F | 30 | 0
sel glazing | Glazing type | Single glass=1.13, Double polycarbonate=0.7, Single film=1.0, Double film=0.65
out btu | Required heat output | BTU/hr | (2 * length * height + 2 * width * height + length * width) * glazing * delta_t | 0
out area | Total surface area | sq ft | 2 * length * height + 2 * width * height + length * width | 0