Greenhouse Sizing Calculator

Greenhouse sizing depends on what you want to grow, how many plants you need, and your available space. Getting the right size prevents overcrowding and ensures adequate light and air circulation.

Space requirements by plant type:

Standard greenhouse sizes:

• Small (6x8 ft / 1.8x2.4 m): 48 sq ft. Good for hobby gardeners with 20-40 plants.
• Medium (8x12 ft / 2.4x3.7 m): 96 sq ft. Room for benches and a good variety.
• Large (10x16 ft / 3.0x4.9 m): 160 sq ft. Serious growing with walkways.
• Extra Large (12x20 ft / 3.7x6.1 m): 240 sq ft. Near-commercial capacity.

Important considerations:

• Subtract 30% of floor space for walkways and work areas.
• A second bench level adds about 80% more growing area, not a full 100%. The lower shelf sits in the shade of the upper one, so it suits seedlings, cuttings and shade-tolerant greens rather than anything you expect to fruit.
• Minimum recommended width is 6 feet (1.8 m) to allow a center walkway.
• Orient the long side east-west for maximum winter sun exposure in the Northern Hemisphere.
• Ventilation: at least 20% of the floor area should be openable vents or windows.

When no standard size fits

The four sizes above top out at 240 sq ft, which is roughly 60 tomato plants once walkways are taken out. Past that you are not shopping for a bigger garden greenhouse, you are into commercial structures: gutter-connected ranges, or several polytunnels run side by side. A 12 ft span is about the widest a hobby frame goes before the roof needs a centre post, so growth past that point happens in length, in bays of about 2 ft.

A caution on the per-plant figures

The space allowances in the table are bench footprints for a mature plant, and they assume you are growing to full size. Tomatoes at 4 sq ft each is a trained cordon on a string, not a bush allowed to sprawl, which wants closer to 9. Seedling trays at half a square foot each is the tray footprint, so a 1020 tray of 72 cells counts as one unit, not 72.