Greenhouse Size Calculator

Greenhouse growing area is calculated as the internal floor space in square feet or square metres, minus any permanent staging, pathways, and structural obstructions. A standard 6ft x 8ft greenhouse provides 48 sq ft of total floor area but approximately 36-40 sq ft of usable growing space after accounting for a central path. The calculator converts your chosen dimensions into both usable ground beds and shelved growing capacity.

Plant capacity depends on species spacing requirements at maturity. Tomato cordons need roughly 3 sq ft per plant (18-inch spacing in rows 24 inches apart). Seedling trays (standard UK seed tray is 14" x 9") hold 24-40 plugs and require about 0.9 sq ft of bench space each. Pepper and aubergine plants need 2.5 sq ft each, while cucumber plants require 4 sq ft for adequate air circulation.

Staging and shelving effectively double your growing area for smaller plants and propagation. A 6ft x 8ft greenhouse with staging along both long walls gains an additional 20-24 sq ft of shelf space. The calculator factors in whether you plan ground-level beds for tall crops (tomatoes, cucumbers) or multi-tier staging for compact plants (herbs, lettuce, seed trays), providing a combined plant count estimate.

What size greenhouse do you need? Hobby grower (salad, herbs, 2-3 tomato plants): 1.8m × 2.4m (6×8ft) — £200-£500. Medium garden (tomatoes, cucumber, peppers + seedlings for borders): 2.4m × 3m (8×10ft) — £400-£900. Serious productive: 2.4m × 4.2m (8×14ft) — £700-£1,800. Coverage: each m² produces ~10kg vegetables/year peak season. Sample 8m² greenhouse: 80kg/year tomatoes + side crops. Height: 1.8m minimum to walk comfortably; 2.1m better for tall plants like tomatoes.

Aluminium vs wooden vs polycarbonate. Aluminium frame: cheapest (£200-£800), light, no maintenance, less attractive. Wooden frame (cedar, larch): premium (£500-£3,000+), heavier, beautiful, needs occasional treatment — 30+ year life. Glazing: horticultural glass (sharp edges, breakable but classic, £50-£100/m²); toughened safety glass (premium, kid-safe, expensive); twin-wall polycarbonate (£20-£40/m², lightweight, insulating, scratches). UK choice: aluminium + polycarbonate best for budget + practicality; wood + glass for aesthetic.

Greenhouse positioning — south-facing rules. Long axis east-west for maximum southern sun exposure. South of all obstructions (trees, fences, buildings) — even partial shade reduces yield 30-50%. Avoid frost pockets (low ground accumulates cold air). Wind protection: hedge or fence to north helps insulation. Foundation: level concrete pad or compacted gravel base — essential, not optional. Greenhouse Building Regulations: usually permitted development if under 25 m² floor area, under 4m height, more than 2m from boundary.

Greenhouse heating — UK winter productivity. Unheated: usable March-October. Frost-free (5°C): year-round seedlings, citrus, succulents — £100-£300/year electricity. Cool (10°C minimum): tropical plants, off-season tomatoes — £400-£800/year. Hot (15°C+): orchids, exotic — £800-£2,000/year. Heating options: electric fan heater (cheapest install, £150-£400 to buy; expensive to run); paraffin (cheap install, smelly); propane (efficient but bottle changes); solar + thermal mass (free running cost, expensive setup).

Greenhouse productivity — what to grow. Tomatoes: highest UK yield (10-15kg per plant per season). Cucumbers: 30-40 fruits per plant. Peppers/chillies: long season starting from heat. Aubergines: tropical, need warmth. Lettuce/salad: year-round if heated. Herbs (basil, parsley, coriander): high-value, easy. Citrus (lemons, limes): bring indoors for winter or heated. Strawberries: forcing for early crop. Seed starting: massively extends garden season — start tomato seeds March, plant outdoor June for 3-month head start over direct-sown.