Calculator
Enter your tunnel size and spacing. The form adapts to your screen.
Example data table
Use this sample to sanity-check your first run.
| Scenario | Length | Width | Rise | Spacing | Estimated hoops | Cover width | Cover length |
|---|---|---|---|---|---|---|---|
| Small backyard tunnel | 24 m | 4 m | 2.2 m | 2 m | 13 | ≈ 5.4 m | ≈ 25.5 m |
| Medium production tunnel | 30 m | 6 m | 3 m | 2 m | 16 | ≈ 7.5 m | ≈ 31.5 m |
| Long wind-prone tunnel | 45 m | 6 m | 3 m | 1.5 m | 31 | ≈ 7.5 m | ≈ 46.5 m |
Values are approximate. Use the calculator for exact outputs.
Formula used
Effective length is the tunnel length minus both setbacks.
hoops = ceil(Leff / spacing) + 1 (with end hoops)
The layout uses an even spacing so the final bay is not oversized.
The hoop is treated as a circular segment using width and rise.
θ = 2·asin(c/(2R))
arc = R·θ
Pipe per hoop adds both ground inserts, then waste.
How to use this calculator
- Choose meters or feet, then enter tunnel length and width.
- Enter rise, hoop spacing, and a realistic end setback.
- Add ground insert length, purlins, and baseboards if needed.
- Set cover drop, overhang, and double layer if applicable.
- Click calculate, then export the report for your materials list.
Why Hoop Geometry Matters
A hoop tunnel behaves like a curved beam, so small geometry changes affect material needs. This calculator treats each hoop as a circular segment defined by tunnel width (chord) and rise (sagitta). With those values it estimates radius, central angle, and arc length, then adds ground inserts. Accurate arc length helps avoid under-ordering pipe and reduces splicing.
Typical Tunnel Sizes and Spacing Benchmarks
Growers commonly build tunnels 4–9 m wide and 2–4 m high, with lengths from 12–60 m. Hoop spacing often falls between 1.2–2.5 m, tightened in windy sites and widened in sheltered areas. The tool converts your target spacing into an even layout so the last bay is not stretched, and it lists positions for marking.
Pipe Length Planning with Inserts and Waste
A practical materials list must include pipe that disappears into sleeves or ground posts. Enter insert length per side to add 2× that allowance to each hoop. For ordering, a waste factor of 5–10% is typical for offcuts, mistakes, and couplers; higher percentages may be needed when transporting long pipe or working on uneven terrain.
Purlins, Baseboards, and Wind Bracing
Longitudinal members stiffen the frame and help prevent racking. Many tunnels use three purlins: one ridge and two shoulder runs, while larger spans may add more. Optional baseboards (two full-length runs) support wiggle wire channels and protect plastic from abrasion. If your site sees gusts above 60 km/h, consider closer spacing, diagonal bracing, and better anchoring.
Cover Film Sizing and Material Notes
Cover width is estimated as hoop arc plus two side drops, giving wrap for fastening. Cover length includes end overhang for tie-down and end detailing. Single-layer film is common at 150–200 microns; double layers improve insulation but double the area requirement. Verify roll widths before ordering.
FAQs
1) How does the calculator estimate hoop length?
It models the hoop as a circular segment using width and rise, computes the arc length, then adds ground insert allowances and applies your waste percentage.
2) Why does actual spacing differ from my target spacing?
The tool evens out spacing so the frame fits the effective length after setbacks. This prevents a final bay that is too wide or too tight.
3) What hoop spacing is common for garden tunnels?
Many builds use 1.2–2.5 m spacing. Use tighter spacing for wind exposure, heavy snow risk, or thinner pipe, and wider spacing for sheltered sites.
4) Should I include end hoops?
Include end hoops when you want a hoop at each end of the effective length for stronger end bays and easier film fastening. Excluding them can reduce count but may increase bracing needs.
5) What waste percentage should I use?
A practical range is 5–10% for offcuts and handling. Increase it if pipe comes in fixed lengths, transport limits cause more joints, or your site requires extra anchoring hardware.
6) Does the cover sizing replace a supplier specification?
No. It provides a planning estimate using arc length, side drop, and overhang. Always confirm the film roll widths and recommended fastening method from your supplier.
7) Can I use feet instead of meters?
Yes. Select Feet and enter all dimensions in feet. The calculator converts internally for accuracy and returns results in the same unit system you chose.