Wind Load Calculator
Estimate wind pressure on a wall, sign, or surface from the design wind speed (mph), the exposure and height factor Kz, and a force coefficient. The tool returns velocity pressure and design pressure in psf and the total force on an area — a screening estimate, not a stamped design.
Example: with Design wind speed (mph) 115 · Exposure / height factor Kz Exposure C, ≤15 ft (0.85) · Force / pressure coefficient Wall, net windward + leeward (1.3) · Projected area (ft², optional) 100 → Velocity pressure qz: 28.78 psf.
- Design wind pressure37.41 psf
- Total force on area3,741 lb (16.64 kN)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Wind pressure grows with the square of wind speed: qz = 0.00256·Kz·V². Double the wind and the load quadruples — the core reason storms escalate so quickly.
How wind becomes pressure
Moving air carries kinetic energy, and when it hits a surface that energy converts to pressure. The ASCE 7 velocity-pressure equation captures it as qz = 0.00256·Kz·V², with V the design wind speed in mph and Kz an exposure factor that grows with height and openness of the terrain. The 0.00256 constant bakes in air density; the squared speed is why a jump from 90 to 130 mph roughly doubles the load.
Turning velocity pressure into an actual force needs a coefficient for the shape and how wind wraps around it — higher for a flat wall catching both a push and a suction, lower for a streamlined roof. Real code design layers on gust effects, directionality, internal pressure, and component-versus-cladding rules that this screening tool leaves out, so treat the number as a sanity check and have a licensed engineer run the full ASCE 7 analysis for anything you build.
How it’s calculated
Velocity pressure qz = 0.00256·Kz·V² (psf, V in mph), the ASCE 7 form with air density folded into 0.00256. Design pressure P = qz·Cf, where Cf is a combined force/pressure coefficient (about 1.3 net for a wall, 1.2 for a freestanding sign, 0.9 for a roof). Total force F = P·area; kN = lb × 0.00444822.
A simplified screening estimate only. It omits gust-effect factor G, directionality Kd, topographic factor Kzt, internal pressure, and component-and-cladding provisions. Do not use it for permit or construction; have a licensed engineer perform the full ASCE 7 design.
Velocity pressure qz at Kz = 0.85 (psf)
| Wind speed | qz (psf) | Wall pressure at Cf 1.3 |
|---|---|---|
| 90 mph | 17.6 | 22.9 |
| 110 mph | 26.3 | 34.2 |
| 115 mph | 28.8 | 37.4 |
| 130 mph | 36.8 | 47.8 |
| 150 mph | 49.0 | 63.7 |
Computed with qz = 0.00256·0.85·V²; Exposure C at ≤15 ft. Screening estimate.
Common mistakes
- Using a basic wind speed for the wrong risk category or an outdated map; codes specify the design speed.
- Leaving Kz at a low value for a tall structure — the factor rises substantially with height.
- Forgetting that a wall feels both windward push and leeward suction, which the net coefficient accounts for.
- Treating this screening number as a code-compliant design instead of a first-pass estimate.
Frequently asked questions
What is the wind load formula?
Velocity pressure qz = 0.00256 × Kz × V², with V the design wind speed in mph and Kz an exposure-and-height factor. Design pressure is qz times a force coefficient, and total force is pressure times area.
What does the 0.00256 constant mean?
It folds in the density of air so that a wind speed in mph produces pressure in pounds per square foot. It comes directly from the ASCE 7 velocity-pressure equation.
Why does wind load rise so fast with speed?
Pressure depends on the square of wind speed, so doubling the speed quadruples the load. A modest increase in design wind speed is a large increase in force.
Can I use this for a building permit?
No. This is a screening estimate that omits gust, directionality, topographic, and internal-pressure factors. Any real structure must be designed to the full ASCE 7 procedure by a licensed engineer.