Greenhouse Plastic Film Calculator
Size the poly film for a hoop or tunnel greenhouse — cover width over the arch, roll length and the end-wall sheets.
How it works
Arch length ≈ ½ × ellipse perimeter (semi-axes span/2 and height) · Cover width = arch + 2 × anchor · Cover length = length + 2 × anchor · Total = (roof + 2 end walls) × (1 + waste%)
A single sheet of greenhouse film drapes over the hoops from one anchor point, up and over the ridge, and down to the other side — so the roll width you need is the curved length of the arch, not the flat span. The arch is estimated as half the perimeter of an ellipse whose half-width is the span/2 and whose height is the peak. Add anchor allowance on every edge to bury or batten the film, and buy a UV-stabilized greenhouse-grade poly so it lasts several seasons instead of shredding in one.
Worked example
For a 20 ft tunnel with a 10 ft span and 7 ft peak, the arch is about 19 ft; adding 2 ft of anchor per side gives a 23 ft cover width and a 24 ft cover length — roughly 552 sq ft. The two curved end walls add about 110 sq ft, and 10% waste brings the total to about 728 sq ft.
Frequently asked questions
How thick should greenhouse plastic be?
Most tunnels use 6-mil UV-stabilized greenhouse film, which typically lasts about four years. Thinner painters plastic degrades within a season under sunlight, so it is false economy for a permanent structure.
Why is the cover wider than the greenhouse span?
Because the film follows the curve of the hoops, not the flat ground. The distance over the arch is always longer than the span, and you still need extra on each side to anchor or bury the edges.
Do I need separate film for the ends?
Usually yes. The curved end walls are cut and fastened separately from the main roof sheet, which is why this estimate adds them in — plan doors and vents into those end panels.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.