Calculator
Example Data Table
| Scenario | Shape | Anchor spans | Clearance | Recommended sail | Approx. fabric area |
|---|---|---|---|---|---|
| Patio corner | Rectangle | 4.8 m × 3.6 m | 0.30 m | 4.2 m × 3.0 m | 12.60 m² |
| Small seating | Square | 4.0 m × 4.0 m | 0.25 m | 3.5 m × 3.5 m | 12.25 m² |
| Triangular garden | Triangle | 5.0 m, 4.2 m, 3.8 m | 0.30 m | 4.4 m, 3.6 m, 3.2 m | 5.61 m² |
Formula Used
Triangle: Heron’s formula: s = (a + b + c)/2, Area = √(s(s−a)(s−b)(s−c)).
FabricCost = FabricAreaWithWaste × CostPerArea, then add the fixed hardware cost.
How to Use This Calculator
- Choose your measurement units and sail shape.
- Measure anchor-to-anchor spans between fixing points.
- Set corner clearance based on your hardware length.
- Enter a small sag percent and a height difference.
- Optionally add waste and cost values for budgeting.
- Press Submit to view results above the form.
- Download CSV or PDF for installers and suppliers.
Site measurements and setbacks
Start with anchor-to-anchor spans, measured level between fixing points for accuracy. Record both directions and confirm diagonals to catch skew. If your anchors are on posts, note their offsets from edges so you can keep planting beds shaded without blocking access. For patios, allow 150–300 mm clearance from walls for airflow and cleaning access. If planting beds sit under the sail, keep at least 600 mm open perimeter for pruning paths and irrigation lines.
Clearance and hardware allowance
The calculator subtracts clearance at both ends of each side. Typical clearance ranges from 250–400 mm (10–16 in) depending on turnbuckle length and connection rings. Longer hardware gives more tension range but needs more room, so the recommended sail dimension often runs 8–15% smaller than the anchor span.
Sag and tension planning
A modest sag target improves shade uniformity and reduces flutter. For woven HDPE sails, 1–4% sag per side is common: a 4.0 m edge at 2.5% targets about 0.10 m mid-span sag. If your sail is waterproof, keep sag lower and increase slope instead to avoid ponding. After installation, re-tension after 7–14 days as fabric relaxes.
Slope and drainage performance
Use the height difference to create reliable runoff. A 300 mm drop over a 4.0 m reference length is a 7.5% slope, which typically sheds light rain well. In leafy gardens, choose a steeper slope to move debris off the fabric and reduce staining near seams. Aim to keep the lowest corner clear of walkways where drips can splash.
Material area, waste, and budgeting
Fabric area is calculated from the recommended dimensions and then expanded by the waste allowance. Use 2–6% waste for standard hems and corner patches, and more if you expect complex trimming. Budgeting improves when you price per area and add a fixed hardware line for posts, plates, and fasteners. Comparing the coverage ratio helps you decide whether to increase span, reduce clearance, or select a different shape for the same shaded footprint.
FAQs
What clearance should I enter for each corner?
Use the full hardware “take-up” space from sail corner to anchor point. Many installs fall between 0.25–0.40 m (10–16 in). If you have longer turnbuckles or eye plates, increase clearance to keep tensioning range.
Why does my triangle report an invalid shape?
The three sides must satisfy the triangle rule: each side must be shorter than the sum of the other two. Re-measure, check units, and confirm you used straight-line distances between the same anchor points.
How should I choose the sag percent?
For most tensioned shade fabrics, start near 2–3%. Increase slightly if flutter appears, and reduce if you need a flatter look. Waterproof fabrics usually need lower sag with more slope to avoid ponding.
What slope values work for drainage?
A practical target is 5–10% slope along a main run, created by lowering one corner. Heavier rain, leafy debris, or waterproof fabric may benefit from the higher end of that range.
Does this calculator replace wind-load engineering?
No. It estimates sizing, area, and basic slope. Wind loads depend on local conditions, exposure, post strength, and fixings. For large sails or high-wind zones, use engineered posts and follow supplier load guidance.
How do the CSV and PDF downloads behave?
After a successful calculation, downloads use the latest result stored in your session. Run the calculator again to update the export. If you open the download links first, you will receive a message indicating no saved result.