Calculator inputs
Example data table
| Scenario | Structure | Wind | Soil | Anchor | Suggested anchors | Typical spacing |
|---|---|---|---|---|---|---|
| Backyard trellis | 3 m × 1.8 m, 40% solid | 45 km/h, suburban | Loam, dry | Spiral, medium | 4 | 1.00 m |
| Shade cloth frame | 6 m × 2.2 m, 80% solid | 60 km/h, open | Clay, damp | Earth auger, large | 8 | 0.86 m |
| Poly tunnel edge | 3 m width × 2.0 m height | 70 km/h, coastal | Sand, wet | Footing, medium | 6 | 1.20 m |
These examples show typical inputs and outcomes. Your local wind and soil conditions can change results significantly.
Formula used
1) Projected area is estimated from your structure dimensions.
2) Wind pressure uses an engineering approximation:
q = 0.613 × V²
where q is in N/m² and V is wind speed in m/s.
3) Wind force multiplies pressure by area and factors:
F = q × A × Cd × Exposure × Direction
4) Required resistance adds your safety factor:
R = F × SafetyFactor
5) Holding per anchor starts from a base rating and adjusts:
Holding = Base × Soil × Moisture × Type × DepthBonus
This tool provides planning estimates. For critical structures, consult local standards or a professional.
How to use this calculator
- Pick a unit system that matches your tape measure.
- Select the structure type closest to your build.
- Enter length and height; add width for tunnels.
- Set wind speed for your area and season.
- Choose exposure, soil type, and moisture condition.
- Pick anchor type, size, embed depth, and spacing.
- Use a safety factor suited to your risk tolerance.
- Press calculate and review anchors, spacing, and ratings.
Wind load estimate inputs
The calculator converts your wind speed to meters per second and applies the planning pressure relation q = 0.613 × V². For example, 45 km/h is about 12.5 m/s, giving roughly 96 N/m² before factors. Higher exposure settings increase demand by 15% to 30% to reflect open sites and ridges where gusts build quickly.
Projected area and solidness
Anchoring demand rises with projected area A. A 3.0 m by 1.8 m trellis has 5.4 m² of projected area, while a tunnel uses width × height for the wind-facing section. Solidness adjusts drag to represent open frames versus screens. A 40% open trellis reduces drag compared with an 80% shade surface that behaves more like a sail.
Soil grip and embed depth
Holding capacity begins with an anchor size baseline and is scaled by soil and moisture multipliers. Loam is treated as a 1.00 reference, while sand can be 0.60 and disturbed soil 0.50. Wet conditions reduce grip further. Embed depth adds a modest bonus, capped at about 20%, to reflect improved engagement with denser layers.
Safety factor and load distribution
Required resistance is R = F × SafetyFactor. A safety factor of 2.0 doubles the estimated wind force, helping cover gusts, installation variability, and aging materials. The tool then checks anchor count by both force and spacing. This prevents long structures from being under-anchored even when the force calculation suggests fewer points. As a quick check, if your anchor count changes sharply when you adjust wind speed by 10%, your build is near capacity. Increasing anchor size, reducing solidness, or adding bracing usually provides more benefit than only tightening straps.
Hardware selection and maintenance
The result includes a per-connection working load target so straps, turnbuckles, and fasteners can be selected above that rating. In corrosive garden environments, hot-dip galvanized or stainless hardware lasts longer. Re-tension after the first windy day, inspect seasonally, and replace frayed straps or bent stakes to keep capacity consistent.
FAQs
What wind speed should I enter?
Use the higher end of typical gusts for your season and site. If you only know sustained wind, increase it to represent gusts. Choosing a more open exposure also adds margin.
How do I pick the solidness percentage?
Estimate how much area blocks airflow. Open trellis panels often fall around 30–50%. Shade cloth, screens, and fencing commonly range from 70–100%. When unsure, choose the higher value for safer planning.
When should I use concrete footings?
Use footings when soil is soft or frequently wet, when wind is high, or when the structure is tall and solid. Footings reduce seasonal loosening and provide a strong option for permanent installations.
Why does the calculator need a safety factor?
Garden builds face gusts, imperfect installation, changing soil moisture, and material wear. A safety factor buffers these uncertainties. Many users start at 2.0 and increase it for exposed or critical structures.
Why are anchors checked by spacing as well as force?
Long frames can twist or rack if anchor points are too far apart. The spacing check ensures consistent restraint along the length, even when calculated force per anchor appears low.
What should I do if the soil is wet?
Wet soil reduces holding. Increase embed depth if possible, add more anchors, or switch to larger anchors or footings. Also place anchors in undisturbed soil and avoid recently tilled or filled areas.