Wind Load Pressure Calculator (Signs, Fences, Walls)
Design wind pressure from wind speed with velocity pressure, exposure/height factor and force coefficient — psf and kPa, total force on a sign, fence, solar array or wall, overturning moment at the base, and the post/footing implications.
How it works
q_z = 0.00256 × K_z × K_zt × K_d × V² (psf, V in mph); K_z = 2.01 (z/z_g)^(2/α) with z ≥ 15 ft; design pressure p = q_z × G × C_f; force F = p × A; moment M = F × height to centroid
Wind pushes on a surface with a pressure that grows with the square of the wind speed: doubling the speed quadruples the load, which is why hurricane-zone design speeds matter so much. ASCE 7 builds the design pressure in stages — the velocity pressure from the speed, adjusted upward for height (wind is faster and gustier off the ground) and for open terrain (exposure C or D versus sheltered B), reduced slightly by a directionality factor, then multiplied by a gust factor and a shape coefficient that is about 1.3 for a solid sign or fence and much lower for porous or round objects. Multiplying by area gives the force, and the force times the height of the area’s centroid gives the moment that tries to lever the posts out of the ground — the number that sizes the posts, their embedment and the footings.
Worked example
A solid 8 × 6 ft sign with its centre 12 ft up in open terrain at 115 mph: Kz 0.85, velocity pressure 24.4 psf, design pressure 24.4 × 0.85 × 1.3 = 27.0 psf. That is 1,300 lb on 48 ft² and a 15,500 lb·ft overturning moment — 650 lb of shear and 7,800 lb·ft of bending per post for a two-post sign.
Frequently asked questions
Which wind speed do I use?
The ultimate (strength-level) 3-second gust from ASCE 7 hazard maps for your risk category (or the local code’s basic wind speed). If your code gives service-level speeds (older ASCE 7-05 style, ~90 mph inland), the pressure formula applies a load factor of 1.6 instead.
Does a fence really see the full pressure?
A solid fence does; lattice, chain-link or 50%-open slats see roughly half or less (Cf 0.5–0.8 on the gross area), and gaps at the bottom reduce it further.
Related calculators
Simplified ASCE 7 procedure for preliminary sizing of signs, fences and similar solid structures; buildings, topographic effects and hurricane-coast designs require the full standard and an engineer.