How much polythene do I need for a polytunnel?

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.

What size polytunnel is best for a home garden?

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.

How long does polytunnel polythene last?

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.

Why is the cover bigger than the floor area?

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.

Do I need planning permission for a polytunnel?

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.

GardenCalcs

Polytunnel Calculator — Cover Area & Growing Space

Calculate polytunnel cover area and growing space.

Length

Tunnel length along the ridge (m). Hoops sit 1.5-1.8m apart, so pick a multiple of the hoop spacing.

Width

Span at ground level (m). This also sets the apex height, since the arch is modelled as a semi-circle (height = span ÷ 2).

Cover Fixing MethodHow the cover is anchored. Trenching buries 1m of polythene each side; a base rail needs only 0.5m each side.

How We Calculate This

Floor area = Length × Width. The cross-section is modelled as a semi-circle, so the over-hoop arc length = π × (Width / 2).

The flat polythene sheet to buy is wider and longer than the tunnel: sheet width = arc + fixing allowance, sheet length = tunnel length + tunnel span + fixing allowance (the +span folds over and seals the two domed ends). Trenching adds 2m total; a base rail adds 1m. The waste factor is then added on top.

Usable growing space subtracts a 60cm central access path from the width.

Frequently Asked Questions

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Last updated: June 2026

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