Polytunnel Calculator — Cover Area & Growing Space

Buy a flat sheet sized over the hoops plus a fixing allowance both ways. The sheet width = over-hoop arc (π × half the span) + the fixing allowance; the sheet length = tunnel length + tunnel span (to fold over and seal both ends) + the fixing allowance. Trenching adds 2m total (1m each side); a base rail adds 1m total (0.5m each side). For a 6m × 3m trenched tunnel that is roughly 6.7m × 11m ≈ 74 m² before waste.

A 3m × 6m (10ft × 20ft) polytunnel is ideal for most home gardens. It gives about 14 m² of growing space after a central path is accounted for. Hoops are usually spaced 1.5-1.8m apart, so lengths come in multiples of that.

Good-quality 720-gauge (180 micron) UV-stabilised polythene carries a 5-year warranty and commonly lasts 5-7 years, sometimes 8-10 with care. Budget covers may need replacing every 3-4 years. Anti-fog and thermal types last longest.

The cover wraps over the curved hoops (the arc is longer than the flat span), folds over the two domed ends, and buries into a trench or clamps to a base rail. All of that is extra material beyond the footprint, which is why a 6m × 3m tunnel needs roughly 70-80 m² of polythene, not 18 m².

In most UK gardens, polytunnels under 2.5m high do not need planning permission. Check with your local authority if in doubt, especially in conservation areas or on agricultural land.