How to use this calculator
Start by selecting your primary greenhouse use and checking off the types of plants you want to grow. Enter the number of seedling trays and pots you expect to have at any given time. Choose whether you will use raised benches or ground beds, pick your preferred aisle width, and select your climate zone.
The calculator recommends a standard greenhouse size that fits your needs, then breaks down the bench layout, growing surface area, plant capacity, heating requirements in BTU, exhaust fan size in CFM, foundation type, and estimated cost ranges for three common glazing materials.
Choosing the right greenhouse size
The most common mistake is underestimating how much space you need. Walkways, doors, staging areas, and equipment (heaters, fans, supply shelves) all consume floor space that cannot be used for plants. A good rule of thumb: only 60 to 70 percent of a greenhouse footprint is usable growing area when you account for aisles and structural clearances.
Standard kit sizes are designed around common lumber and panel dimensions, so sticking with those sizes keeps costs down. Custom widths and lengths often require additional framing and waste more material.
Frequently asked questions
What size greenhouse do I need for a backyard garden?
For most backyard gardeners, an 8x12 (96 sq ft) greenhouse provides a great balance of growing space and cost. It fits two bench rows with a center aisle and can hold 30 to 50 potted plants or 8 to 12 standard seedling trays at a time.
How many BTU do I need to heat a greenhouse?
A general rule is 1.0 to 1.5 BTU per square foot per degree of temperature difference you need to maintain. For example, a 100 sq ft greenhouse in a cold climate with a 60 degree temperature difference needs roughly 6,000 to 9,000 BTU. Insulated glazing like double wall polycarbonate can reduce that by 30 to 40 percent.
Is polycarbonate or glass better for a greenhouse?
Polycarbonate panels are lighter, more impact resistant, and provide better insulation (especially twin wall panels), making them the most popular choice for hobby greenhouses. Glass transmits slightly more light and lasts longer, but it is heavier, more fragile, and significantly more expensive. Polyethylene film is the most affordable option but needs replacing every 2 to 4 years.
Do I need a foundation for a greenhouse?
Yes. Small greenhouses (under 8x10) can sit on a leveled gravel pad or pressure treated timber frame. Larger structures benefit from a concrete perimeter foundation or concrete piers. A proper foundation keeps the greenhouse level, anchors it against wind, prevents moisture from wicking up, and extends the life of the frame.