Wind Load Calculator
Find velocity pressure, design wind pressure, and total wind force on a surface using the ASCE 7 velocity pressure method.
🌬️ What is Wind Load?
Wind load is the pressure and resulting force that wind exerts on a building or structure's surfaces. Structural engineers compute it using a velocity pressure method, exemplified by ASCE 7's qz = 0.613 Kz Kzt Kd V-squared formula, which converts a site's basic wind speed into a velocity pressure adjusted for height above ground, terrain exposure, local topography, and wind directionality.
That velocity pressure is then converted into a design wind pressure on a specific surface using p = qz G Cp, where G, the gust effect factor, accounts for turbulence-driven dynamic amplification, and Cp, the external pressure coefficient, captures how a particular surface, windward wall, leeward wall, side wall, or roof, responds differently to the same wind. Multiplying by the surface's projected area gives the total wind force that surface must resist.
A common point of confusion is forgetting that Cp can be negative. Leeward walls, side walls, and most roof surfaces experience suction rather than direct pressure, wind flowing past them creates a lower-than-ambient pressure that pulls outward. This calculator carries that sign through automatically, so a negative Cp correctly produces a negative design pressure and force.
This calculator computes velocity pressure, design wind pressure, and total force on a single surface from your site and exposure factors, and plots how velocity pressure grows with wind speed, following the formula's V-squared relationship rather than a straight-line proportional increase.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Windward Wall of a Mid-Rise Building
V = 45 m/s, Kz = 0.9, Kzt = 1.0, Kd = 0.85, G = 0.85, Cp = 0.8, A = 50 m²
Example 2 — Leeward Wall (Suction)
V = 55 m/s, Kz = 1.1, Kzt = 1.0, Kd = 0.85, G = 0.85, Cp = -0.5, A = 40 m²
Example 3 — Building on Elevated Terrain
V = 35 m/s, Kz = 0.85, Kzt = 1.2, Kd = 0.85, G = 0.85, Cp = 0.8, A = 30 m²
❓ Frequently Asked Questions
🔗 Related Calculators
What is velocity pressure in wind load design?
Velocity pressure qz is the dynamic pressure the wind would exert if it struck a surface head-on, adjusted for height, exposure, terrain, and wind direction. It is the starting point for computing design wind pressure on any building surface, given by qz = 0.613 Kz Kzt Kd V-squared in SI units.
What is the formula for wind velocity pressure?
qz = 0.613 x Kz x Kzt x Kd x V^2, where V is the basic wind speed in meters per second, Kz is the velocity pressure exposure coefficient, Kzt is the topographic factor, and Kd is the wind directionality factor. The 0.613 constant comes from standard air density in SI units.
What is the difference between velocity pressure and design wind pressure?
Velocity pressure qz depends only on wind speed and site/height factors. Design wind pressure p = qz x G x Cp additionally applies the gust effect factor G (dynamic amplification) and the external pressure coefficient Cp (how a specific surface, windward, leeward, or side, responds to the wind), giving the actual pressure used for structural design on that surface.
Why is the external pressure coefficient sometimes negative?
A negative Cp represents suction, wind flowing over or around a surface (leeward walls, side walls, most roof surfaces) creates a pressure lower than ambient, pulling outward on the surface rather than pushing inward. This calculator carries that sign through automatically to the design pressure and resulting force.
What is the velocity pressure exposure coefficient Kz?
Kz accounts for how wind speed increases with height above the ground and varies by exposure category (B for urban/suburban terrain, C for open terrain, D for flat unobstructed areas near large water bodies). It is read from a code table for the specific height and exposure category being checked, and increases with height.
What is the topographic factor Kzt?
Kzt accounts for wind speed-up over isolated hills, ridges, and escarpments, it equals 1.0 for flat or gently rolling terrain and rises above 1.0 as topographic exposure and slope steepness increase, following the code's topographic factor equations or charts.
What is the wind directionality factor Kd?
Kd accounts for the reduced probability that the maximum wind speed occurs in the specific direction most critical for a given structure. For most buildings under ASCE 7's main wind force resisting system provisions, Kd is commonly taken as 0.85.
What is the gust effect factor G?
G accounts for the dynamic amplification of wind loading due to turbulence and building flexibility. For rigid structures (most low- to mid-rise buildings), G is commonly taken as 0.85, more flexible or dynamically sensitive structures require a detailed gust effect factor calculation.
Does this calculator include internal pressure?
No, this calculator computes external design wind pressure on a single surface only. A complete envelope design also combines external pressure with an internal pressure coefficient GCpi, which depends on the building's enclosure classification (enclosed, partially enclosed, or open).
What units does this calculator use?
Basic wind speed is entered in meters per second, Kz, Kzt, Kd, G, and Cp are dimensionless, and area is entered in square meters. Results are shown in pascals for pressure and newtons (plus kilonewtons) for total force.