How much ventilation does a greenhouse need?

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.

What types of greenhouse vents are there?

Roof vents (most effective as hot air rises), louvre vents (side panels), automatic vent openers (wax cylinder type), and doors used as vents.

When should I ventilate my greenhouse?

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.

Should I use automatic vent openers?

Yes, they are highly recommended. Autovent openers use a wax cylinder that expands with heat. They cost about £20 each and need no electricity.

Is a fan needed in addition to vents?

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.

GardenCalcs

Ventilation Calculator — Greenhouse Vent Requirements

Work out ventilation requirements for greenhouses and polytunnels.

Length

Length of the greenhouse footprint along the eaves (gutter to gutter end)

Width

Width of the greenhouse footprint (gable end span)

Height

Average internal height from floor to ridge — used for air-volume context, not the vent sizing

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

Related Calculators

Greenhouse Size Calculator

Work out the ideal greenhouse size for your growing plans.

Cold Frame Calculator

Calculate cold frame size and capacity for hardening off plants.

Grow Light Calculator

Work out grow light wattage and coverage for indoor growing.

Seed Starting Calculator

Calculate seed quantities with germination rates and succession sowing.

Propagation Calculator

Work out propagator capacity and timing for cuttings and seeds.

Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.