Fence Wind Load Calculator

This calculator determines the wind force on fence panels based on exposure zone and panel dimensions, then checks whether the post embedment depth and concrete collar are adequate to resist overturning.

The formula

Total force = Net wind pressure × Panel area × Permeability factor
Overturning moment = Force × Height to centre of pressure (panel height / 2)
Passive force F_p = ½ × η × K_p × γ × L² × b
Resisting moment = F_p × (2/3) × L
Safety factor = Resisting moment ÷ Overturning moment (≥ 1.5)

The foundation check uses lateral passive earth pressure(Broms’ method, BS EN 1997-1 §9) — the post is held up by soil pushing on the embedded collar face, not by bearing on its base. Here K_pis the passive earth-pressure coefficient (firm soil ≈ 3.0), γ the soil unit weight (≈18 kN/m³), L the embedment, b the collar diameter and η (≈3) the Broms 3-D wedge factor.

For a 1.83m × 1.83m solid panel in moderate exposure: force = 0.6 × 3.35 = 2.01 kN. Overturning moment = 2.01 × 0.915 = 1.84 kN·m. A 600mm deep, 300mm diameter collar in firm soil resists about 3.50 kN·m, giving a safety factor of approximately 1.9 (PASS) and a recommended embedment around 600mm — in line with the UK rule of thumb of burying about one-third of the post (minimum 600mm).

Storm-proofing tips

• Use concrete posts and gravel boards for maximum durability
• Concrete collars should be domed to shed water from the post base
• Consider hit-and-miss panels in exposed locations
• Fence-panel clips or screws prevent panels lifting out of slots