| Scenario | Tunnel length | Bed width | Hoop height | Spacing | Hoops | Total hoop material | Cover size (L x W) |
|---|---|---|---|---|---|---|---|
| Spring greens bed | 6.00 m | 1.20 m | 0.90 m | 1.00 m | 7 | ~25.0 m | ~6.8 m x ~4.1 m |
| Windy site, closer hoops | 8.00 m | 1.50 m | 1.00 m | 0.75 m | 12 | ~49.0 m | ~8.8 m x ~4.8 m |
| Compact raised bed | 3.00 m | 0.90 m | 0.65 m | 1.00 m | 4 | ~10.0 m | ~3.8 m x ~3.0 m |
C ~= pi [ 3(a+b) - sqrt((3a+b)(a+3b)) ]
The hoop arc length is taken as C/2.
N = ceil(L / S) + 1
- Choose your unit system, then enter tunnel length.
- Enter bed width and expected hoop height.
- Set hoop spacing based on wind and rigidity.
- Add end overhang for tying or closing ends.
- Add side bury allowance for sealing edges.
- Enter your available cover roll width.
- Press Calculate and review the results above.
- Download CSV or PDF for saving and sharing.
Field measurements vary. Add a small safety margin for cuts, knots, and anchors.
Planning tunnel length and hoop geometry
Accurate hoop tunnel planning starts with the covered bed length and the shape of each hoop. This calculator estimates hoop arc length using a half-ellipse approximation that matches many low tunnel arches made from conduit, wire, or fiberglass rods. By combining hoop geometry with spacing along the bed, you can forecast frame material and avoid mid-build shortages. It keeps unit conversions consistent when field notes mix systems.
Selecting hoop spacing for performance
Hoop spacing controls strength and crop clearance. Wider spacing reduces hoop count, but it increases fabric flutter and stress on anchors in gusty sites. Tighter spacing improves rigidity and helps keep cloth or film from sagging onto foliage. Use the spacing input to test scenarios for your conditions and hoop stiffness. Record chosen spacing before buying hoops and cover.
Cover width, roll choice, and seams
Cover sizing is often the most costly decision, so width matters as much as length. The cover length adds end overhang for tying, clipping, or sealing. The cover width adds side bury allowance for soil burial, sandbags, or clip rails. Enter your roll width to see whether one sheet is sufficient or seaming is likely.
Allowances that prevent installation issues
Allowances are practical, not optional. End overhang should account for folds, knots, and the space taken by end hoops. Side bury allowance should reflect your anchoring method and the soil you can mound without tearing fabric. If you use ridge poles or baseboards, keep extra margin so the cover still reaches the ground securely. In wet soil, anchors may shift.
Turning outputs into a purchase plan
Use the results section to translate measurements into purchasing and cutting lists. The hoop arc length guides per-hoop cuts, while total hoop material supports bulk ordering. Cover area helps compare products sold by square units, and the roll length estimate aids budgeting. Always add a contingency for trimming and field adjustments during installation. When roll width is short, plan overlap seams.
1) What arch shape does the calculator assume?
It models each hoop as half of an ellipse using a standard circumference approximation. This fits many low tunnels better than a perfect semicircle, especially when hoops are pushed wider at the base or flattened by clips.
2) How do I choose hoop spacing?
Start with 0.6 to 1.2 m in windy areas and 1.0 to 1.5 m in sheltered gardens. Reduce spacing for flexible hoops, heavy covers, or snow risk, and increase spacing only when the frame remains rigid.
3) Why add side bury allowance?
Burying or weighting the edges improves sealing, reduces flapping, and protects plants from drafts. The allowance ensures the cover still reaches the ground after draping over the hoops and accounts for folds and anchor points.
4) Does the roll width check affect calculations?
The required cover width is geometry plus allowances. If your roll is narrower, the tool flags it and estimates extra roll length assuming you seam two strips. For precise seam plans, measure overlap and adjust allowances accordingly.
5) Can I use feet instead of meters?
Yes. Select feet and enter every measurement in feet. The calculator converts internally and returns results in the same unit, including cover area expressed as square feet.
6) Should I add a safety margin?
Yes. Add 5 to 10 percent for trimming, tying, and field adjustments. If you expect strong wind or frequent handling, increase the margin and consider stronger anchoring methods to protect the cover edges.