Polytunnel cover calculator: estimate a roof sheet (UK)

Estimate the geometry, not an order size

Enter the tunnel width, length and hoop height, then an allowance for each sheet edge. The calculator models the hoop as the upper half of an ellipse, running from one ground edge to the other. It assumes no separate straight side walls.

It estimates one roof sheet. It does not include separate end panels, doors, seams, fittings or model-specific fixing details. A real frame may have a different profile.

Do not use this result as a maker’s ordering formula. Check the instructions for your exact frame and confirm the sheet dimensions with the supplier before ordering. The allowance you enter is not a recommended burial depth.

What the allowance changes

The same entered allowance is added at both sides of the curved width and at both ends of the tunnel length. Entering 0.5 m therefore adds 1 m to each sheet dimension. Whether that allowance suits a particular fixing system is outside this calculation.

For an illustrative semicircular hoop 3 m wide and 1.5 m high, the curved distance is about 4.71 m. With a 0.5 m edge allowance, the estimated sheet width is about 5.71 m. A tunnel length of 6 m gives an estimated sheet length of 7 m. These example inputs are not fitting instructions.

The calculation

Let a be half the tunnel width and b be the hoop height. The model describes the upper half of the ellipse as x = a cos(t), y = b sin(t), for angles from 0 to π.

The curved distance is the integral of √(a² sin²(t) + b² cos²(t)) over that range. The calculator estimates it using Simpson’s rule with 1,000 intervals.

Sheet width = estimated curved distance + twice the entered edge allowance.

Sheet length = tunnel length + twice the entered edge allowance.

Displayed lengths are rounded to two decimal places. They are geometry estimates, not material ordering dimensions. The method is an original calculation checked against the implementation; manufacturer formulas and frame compatibility have not been verified.