Greenhouse Size Calculator

Greenhouse growing area is the internal floor space, in square feet or square metres, minus any permanent staging, pathways and structural obstructions. A standard 6ft x 8ft greenhouse gives 48 sq ft of floor but roughly 36 to 40 sq ft of usable growing space once a central path is taken out. The calculator converts your chosen dimensions into both usable ground beds and shelved growing capacity, so you can see what you would actually be able to plant rather than what the sales listing implies.

Plant capacity comes down to spacing at maturity. Tomato cordons need roughly 3 sq ft each, which is 18-inch spacing in rows 24 inches apart. A standard UK seed tray of 14" x 9" holds 24 to 40 plugs and takes about 0.9 sq ft of bench. Peppers and aubergines want 2.5 sq ft each, and cucumbers need 4 sq ft for adequate air circulation. Staging then effectively doubles the area for smaller plants and propagation, since the same 6ft x 8ft house with staging along both long walls gains another 20 to 24 sq ft of shelf space.

Size follows ambition and budget. A hobby grower with salad, herbs and two or three tomato plants is fine in 1.8m by 2.4m, the classic 6 by 8ft, at £200 to £500. A medium garden growing tomatoes, cucumbers and peppers alongside seedlings for the borders wants 2.4m by 3m, or 8 by 10ft, at £400 to £900. Serious production needs 2.4m by 4.2m, 8 by 14ft, at £700 to £1,800. Reckon on each m² producing about 10kg of vegetables a year at peak, so an 8m² house yields roughly 80kg of tomatoes plus side crops. Allow 1.8m of height to walk about, and 2.1m if you want room above tall plants.

Frame and glazing decide both the price and the character. An aluminium frame is the cheapest at £200 to £800, light and maintenance-free if not especially handsome. A wooden frame in cedar or larch costs £500 to £3,000 and beyond, is heavier, looks far better and wants occasional treatment, but it will run past 30 years. For glazing, horticultural glass at £50 to £100 per m² is the classic, breakable and sharp-edged. Toughened safety glass costs more and is safer around children. Twin-wall polycarbonate at £20 to £40 per m² is light, insulating and prone to scratching. Aluminium with polycarbonate is the practical budget pairing, and wood with glass is the one you choose on looks.

Position is worth more than any accessory. Run the long axis east to west so the glass faces the southern sun, and keep the structure south of trees, fences and buildings, since even partial shade cuts yield by 30 to 50%. Avoid low ground where cold air collects into a frost pocket, and use a hedge or fence to the north for wind protection and insulation. A level concrete pad or compacted gravel base is not optional. Building regulations usually treat a greenhouse as permitted development if the floor area is under 25 m², the height under 4m and it stands more than 2m from the boundary.

Heating decides how much of the winter you get to use. Unheated, the house is productive from March to October. Holding it frost-free at 5°C costs £100 to £300 a year in electricity and keeps seedlings, citrus and succulents going. A cool house at a minimum of 10°C runs £400 to £800 and supports tropical plants and off-season tomatoes, while a hot house above 15°C for orchids and exotics costs £800 to £2,000. An electric fan heater is the cheapest to install at £150 to £400 but expensive to run, paraffin is cheap and smelly, propane is efficient but means changing bottles, and solar with thermal mass costs a lot to set up and nothing to run.

Yield varies enormously by crop. Tomatoes are the UK greenhouse workhorse at 10 to 15kg per plant per season, and cucumbers give 30 to 40 fruits per plant. Peppers, chillies and aubergines all want warmth and a long season, so they repay heating. Lettuce and salad crop year-round in a heated house, and herbs such as basil, parsley and coriander are easy and high-value. Citrus can summer outside and come in for the winter, and strawberries can be forced for an early crop. The quiet win is seed starting: sow tomatoes in March, plant out in June, and you gain a three-month head start over anything sown directly.