A greenhouse heating calculator helps you determine exactly how many BTUs per hour your greenhouse needs to maintain a target temperature during the coldest months. By factoring in your greenhouse dimensions, glazing type, temperature differential, and wind exposure, this tool calculates the required heater size, estimated monthly fuel cost, and provides tips to reduce heat loss.
Greenhouse Dimensions
Total height at the tallest point
Glazing & Environment
Calculation Details
BTU/hr = Surface Area × U-value × ΔT × Wind Factor
Estimated Monthly Heating Cost
Based on running the heater approximately 12 hours/day during the coldest month
Tips for Reducing Heat Loss
Heating cost estimates assume average fuel prices and 12 hours/day operation. Actual costs vary by location, fuel prices, and weather conditions.
How to Use the Greenhouse Heating Calculator
Keeping a greenhouse warm during winter is essential for protecting plants from frost and enabling year-round growing. This greenhouse heating calculator uses the standard BTU heat loss formula to determine exactly how much heating capacity your greenhouse needs, so you can buy the right size heater without overspending or underheating.
Step 1: Enter Your Greenhouse Dimensions
Select your greenhouse shape — gable (traditional A-frame), quonset (hoop house), or lean-to (attached to a building). Then enter the length, width, wall height, and roof peak height. The calculator uses these measurements to compute the total surface area, including walls, end walls, and roof panels. For a lean-to, one long wall is treated as an attached solid wall with no heat loss.
Step 2: Choose Your Glazing and Environment
Select your glazing material from the dropdown. Each option shows its U-value — a measure of how fast heat escapes through the material. Lower U-values mean better insulation. Single glass (U=1.13) loses heat over twice as fast as triple-wall polycarbonate (U=0.47). Also set your wind exposure, desired inside temperature (55F is typical for a warm greenhouse), and the coldest outside temperature you expect.
Step 3: Review Your BTU Requirements
Click "Calculate Heating Requirements" to see the total surface area, required BTU per hour, and recommended heater size. The calculator rounds up to common heater sizes (25K, 30K, 40K, 50K, 60K, 75K, or 100K BTU) so you can easily shop for the right unit. The formula breakdown shows exactly how the BTU figure was derived.
Step 4: Compare Fuel Costs
The monthly cost section estimates your greenhouse heating expenses for three common fuel types: electric, propane, and natural gas. These estimates assume the heater runs approximately 12 hours per day during the coldest month. Actual run time depends on your climate, solar gain during the day, and insulation quality.
Reducing Greenhouse Heat Loss
The tips section provides personalized recommendations based on your inputs. Upgrading glazing, adding bubble wrap insulation, using thermal mass (water barrels), sealing air leaks, and adding row covers over plants can reduce your heating requirements by 30-50%. Even small improvements make a big difference when multiplied over an entire heating season.
Frequently Asked Questions
Is this greenhouse heating calculator really free?
Yes, completely free with no signup required. All calculations run locally in your browser — no data is sent to any server. Use it as many times as you need to compare different greenhouse configurations and heating options.
Is my data safe when using this tool?
Absolutely. Everything runs entirely in your browser using JavaScript. No greenhouse dimensions, temperatures, or personal information is transmitted anywhere. You can even use it offline once the page loads.
How do I calculate BTU for a greenhouse?
The formula is BTU/hour = Surface Area (sq ft) x U-value x Temperature Differential x Wind Factor. Surface area includes walls and roof. U-value depends on your glazing material. Temperature differential is the difference between your desired inside temperature and the coldest expected outside temperature.
What is a U-value for greenhouse glazing?
A U-value measures how quickly heat passes through a material — lower is better for insulation. Single glass has a U-value of 1.13 (poor insulation), while triple-wall polycarbonate is 0.47 (much better). Double-wall polycarbonate and double poly film are popular middle-ground options at 0.58-0.70.
What temperature should I keep my greenhouse in winter?
It depends on what you are growing. A cool greenhouse (40-45F) protects against frost for most overwintering plants. A warm greenhouse (55-60F) supports tropical plants and seed starting. A hot greenhouse (65F+) is needed for orchids and some exotic plants. Higher temperatures cost significantly more to maintain.
Which fuel type is cheapest for greenhouse heating?
Natural gas is typically the cheapest per BTU at about $1.00-1.50 per therm. Propane costs roughly $2.50-3.50 per gallon and is common where natural gas is unavailable. Electric heating is convenient but usually the most expensive option. Actual costs vary by region and energy prices.
How can I reduce greenhouse heating costs?
The most effective strategies are: upgrade glazing (double-wall polycarbonate cuts heat loss by 40% vs single glass), add bubble wrap insulation to the inside, seal all gaps and cracks, use a thermal mass like water barrels, install a circulating fan, and add row covers over plants for an extra layer of protection.