GardenCalcs.uk
Greenhouse Size Calculator — What Size Greenhouse Do I Need
Work out the ideal greenhouse size for your growing plans.
How many plants you want growing at once — count mature plants, not seed trays
Pick your main crop so the right floor allowance applies (0.3 m² tomato, 0.5 m² cucumber, 0.05 m² seedling). Choose Custom to set your own space per plant.
Multiplier for paths and staging area (default 1.3 = 30% extra)
Width to fit length against — use 2.4m for standard UK greenhouses
How We Calculate This
The calculator multiplies the number of plants by the space each type needs: tomatoes 0.3 m², peppers 0.25 m², cucumbers 0.5 m², seedlings 0.05 m², and general plants 0.15 m².
A 30% allowance is added for paths, staging, and working space. The result is fitted to a standard 2.4m width (the most common UK greenhouse width) with length rounded up to the next 0.5m.
The per-plant figures are practical floor allowances that bundle in immediate access space, not pure grid spacing. For reference, RHS spaces cordon tomatoes 45-60cm apart (one per 30-45cm pot, two per growbag), so 0.3 m² per plant leaves working room rather than packing plants edge to edge.
Frequently Asked Questions
Allow roughly 0.3 m² of floor space per cordon tomato (RHS spacing is 45-60cm apart in growbags or 30-45cm pots, plus room for access). Ten plants therefore need about 4-5 m² of floor, so an 8x6ft (2.4x1.8m) greenhouse is the practical minimum — it holds 8-10 plants with staging both sides. A 6x4ft (1.8x1.2m) only suits 4-6 plants. Step up to 8x10ft for comfortable working room around ten plants.
The most popular sizes in the UK are 6x4ft (1.8x1.2m), 8x6ft (2.4x1.8m), and 10x8ft (3x2.4m). The 8x6ft is the most versatile for hobby gardeners.
Typically 25-35% of floor area goes to paths and staging. This calculator adds a 30% allowance for paths, access, and staging areas.
Almost always yes. Most gardeners wish they had gone bigger. Extra space is useful for overwintering, seed starting, and unexpected additions.
In the UK, a freestanding greenhouse should ideally run east-west to catch maximum winter sun. Lean-to greenhouses are best on south or west-facing walls.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.