Greenhouse Heating Calculator
The greenhouse heating calculator tells you how much heat your structure loses on a cold night and therefore how big a heater you need to hold the temperature your plants require. The method is the standard conduction heat-loss relationship: the heat lost per hour equals the covered surface area multiplied by the covering's U-factor and by the temperature difference between inside and outside. Surface area is the total glazed area of walls and roof, the U-factor is how readily the covering conducts heat (about 1.1 for single glazing and roughly 0.5 to 0.7 for double layers), and the temperature difference is the gap between the temperature you want to hold and your coldest outside design low. Multiplying the three gives the heat loss in BTU per hour, which is the heater output you need to match that loss. Enter your own area, U-factor and temperature difference to size a heater, compare single against double glazing, or check whether your current heater can cope on the worst night of the year. Every figure here is computed deterministically from the formula shown in full below, with a worked example that reconciles exactly to the calculator so you can follow each step and trust the result.
Greenhouse heat loss equals area times U-factor times temperature difference: Q = A x U x dT. A 1,000 sq ft covering with a U-factor of 1.1 across a 40°F difference loses 44,000 BTU/hr, the heater output you need.
Greenhouse heat loss formula
Q = A x U x dT
Q = heat loss in BTU per hour
A = covered surface area in square feet
U = covering U-factor in BTU per hour per square foot per degree F
dT = inside minus outside temperature in degrees F
Each square foot of covering loses heat in proportion to its U-factor and to the temperature gap it spans. Multiplying area, U-factor and temperature difference gives the total conduction loss, which is the heater output needed to hold the inside temperature.
Worked example
A greenhouse has 1,000 square feet of single-glazed covering with a U-factor of 1.1. You want 60 degrees inside on a 20 degree night, a 40 degree difference.
- Area = 1,000 sq ft
- U-factor = 1.1
- Temperature difference = 60 - 20 = 40°F
- Q = 1,000 x 1.1 x 40 = 44,000 BTU/hr
The heat loss is 44,000 BTU per hour, so size the heater to at least that output. These are the calculator's default inputs, so the result above matches the widget exactly.
Heat loss at common U-factors
BTU per hour for a 1,000 square foot covering across a 40 degree difference.
| Covering | U-factor | BTU/hr |
|---|---|---|
| Single glass / film | 1.10 | 44,000 |
| Double poly | 0.70 | 28,000 |
| Twin-wall polycarbonate | 0.50 | 20,000 |
Energy and resource data: US Geological Survey (USGS).
Greenhouse heating calculator: frequently asked questions
How is greenhouse heat loss calculated?
Greenhouse heat loss is the covered surface area multiplied by the covering's U-factor and by the temperature difference between inside and outside. The result is the heat lost in BTU per hour, which is also the heating capacity needed to hold the inside temperature. Larger surfaces, leakier coverings and colder nights all raise the required heater output.
What is a U-factor?
The U-factor is the rate at which a covering conducts heat, in BTU per hour per square foot per degree Fahrenheit. A higher U-factor means more heat loss. Single-layer glass or film is around 1.1, while a double-layer poly or twin-wall covering is roughly 0.5 to 0.7. The covering manufacturer publishes the U-factor for the exact material.
What temperature difference should I use?
Use the difference between the temperature you want to hold inside and the coldest outside design temperature you need to cover. If you keep the greenhouse at 60 degrees and the coldest night is 20 degrees, the difference is 40 degrees. Sizing the heater to your local design low temperature ensures it can keep up on the worst night.
Does this include air infiltration?
No. This calculator covers conduction through the covering only, which is the dominant loss for most hobby and commercial greenhouses. Air leaks around doors, vents and seams add an infiltration loss that can be significant in a draughty structure. Add an allowance, often 10 to 20 percent, on top of the conduction figure to size the heater conservatively.
How do I convert BTU to a heater size?
The BTU per hour figure is the heater output you need. Gas and propane heaters are rated directly in BTU per hour, so pick a unit whose output meets or exceeds the loss. For an electric heater, divide BTU per hour by 3,412 to get the kilowatts required, since one kilowatt equals 3,412 BTU per hour.
Official sources
- Land, water and natural resource data: US Geological Survey (USGS). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.