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Ventilation Calculator — Greenhouse Vent Requirements
Work out ventilation requirements for greenhouses and polytunnels.
Length of the greenhouse footprint along the eaves (gutter to gutter end)
Width of the greenhouse footprint (gable end span)
Average internal height from floor to ridge — used for air-volume context, not the vent sizing
Vent area as a fraction of floor area — RHS recommends 0.2 (20%); raise to 0.25-0.30 for small or south-facing greenhouses that overheat
Clear opening area of one vent (default 0.35 m² ≈ a 610 × 610 mm UK roof vent; louvre vents open less, count their open area)
How We Calculate This
The recommended vent area is 20% of the greenhouse floor area. This is a standard horticultural guideline for adequate air exchange.
A standard UK aluminium roof vent glazes a 610 × 610 mm pane (Elite ≈ 0.37 m²; Halls 6 ft ≈ 0.33 m²), so we use 0.35 m² of clear opening per vent by default. The total vents needed = Required vent area / vent opening area, rounded up to the next whole vent.
Volume is calculated as Length x Width x Height to give context for the air space being ventilated.
Frequently Asked Questions
The general rule is that total vent area should equal at least 20% of the floor area. This ensures adequate air exchange to prevent overheating in summer.
Roof vents (most effective as hot air rises), louvre vents (side panels), automatic vent openers (wax cylinder type), and doors used as vents.
Open vents when temperature exceeds 25°C. In summer, open first thing in the morning. In spring/autumn, open mid-morning and close by mid-afternoon.
Yes, they are highly recommended. Autovent openers use a wax cylinder that expands with heat. They cost about £20 each and need no electricity.
Passive ventilation (vents and doors) is usually sufficient for hobby greenhouses. A fan helps in very hot weather or if the greenhouse has limited vent openings.
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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.