Hoop House Calculator (hoops, pipe length & greenhouse film size)

Understanding the hoop house calculator — concept and principle

This calculator sizes a hoop house (polytunnel): the number of hoops, the length of pipe for each hoop and in total, the size of the cover sheet, the area of each end wall and the growing floor. Gardeners and small farmers use it to order pipe and film for a season-extension tunnel.

Semicircle hoops: with the default inputs the result is 70.8 m of pipe (cover 7.48 × 11.2 m).

How it is calculated

Hoop radius R = (w²/4 + h²) ÷ (2h), for width w at the ground and height h at the centre.
Hoop angle θ = 2 × atan2(w/2, R − h); arc length = R × θ (a semicircle gives π × w/2).
Hoops = ceil(length ÷ spacing) + 1.
Pipe per hoop = arc + 2 × ground legs.
Cover sheet = (arc + 2 × extra) by (length + 2 × extra).
End wall area = R² × (θ − sin θ) ÷ 2; floor area = w × length.

The principle

Each hoop is treated as an arc of a circle passing through both feet and the top. Knowing the chord (the width) and the rise (the height), geometry gives the radius, the angle the arc spans and so its length. When the height is half the width, the arc is a semicircle. The end wall is a circular segment, the area between the arc and its chord. The cover must reach over the arc and continue into the ground or onto battens on both sides, and also cover the ends, which is why extra is added in both directions.

Worked example

Default inputs: width 4 m, height 2 m, length 10 m, spacing 1.2 m, ground legs 0.4 m, extra film 0.6 m.

R = (4 + 4) ÷ 4 = 2 m; height = half the width, so a semicircle with θ = π.
Arc = 2 × π = 6.28 m; pipe per hoop = 6.28 + 0.8 = 7.08 m.
Hoops = ceil(10 ÷ 1.2) + 1 = 9 + 1 = 10; total pipe = 10 × 7.08 = 70.8 m.
Cover sheet = 6.28 + 1.2 = 7.48 m wide by 10 + 1.2 = 11.2 m long = 83.8 m².
End wall = 2² × π ÷ 2 = 6.28 m²; floor = 4 × 10 = 40 m².

Each point is the calculator re-run with only “Hoop spacing (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Hoop spacing (m)	Hoops needed
0.3	35
0.6	18
0.9	13
1.2	10
1.5	8
1.8	7
2.4	6

How to read the result

Buy film in the next stock size up; greenhouse film is sold in standard widths and lengths. Add pipe for a ridge pole, purlins, door frames and bracing, which are not included. Closer hoop spacing makes the frame stronger against wind and snow.

Common mistakes

Setting the height above half the width, which a simple round hoop cannot reach.
Forgetting the pipe driven into the ground.
Ordering film that only just covers the arc, with nothing to bury or fix.
Leaving out end-wall materials.

Keep in mind: Hoop houses must withstand wind and snow loads. Anchor hoops firmly, follow local planning rules, and get professional advice for large or permanent structures.

Frequently asked questions

How long is each hoop for a 4 m wide tunnel?

For a 2 m tall semicircle, about 6.28 m of arc plus the ground legs.

How many hoops do I need?

Length divided by spacing, rounded up, plus one; 10 hoops for 10 m at 1.2 m spacing.

How wide should the cover film be?

The arc length plus enough on each side to bury or fix it, here about 7.5 m.

Sources: Circular-arc geometry (chord and sagitta, arc length, segment area), e.g. D. Zwillinger (ed.), CRC Standard Mathematical Tables and Formulas, 33rd ed. (2018)

Checked 9 Nov 2026. Circular-arc geometry (radius from chord and sagitta, arc length R·θ, segment area R²(θ − sin θ)/2): D. Zwillinger (ed.), CRC Standard Mathematical Tables and Formulas, 33rd ed. (2018), §4 (Geometry).

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