Wind Load Pressure Calculator (ASCE 7)
The ASCE 7 velocity pressure formula converts basic wind speed into a design pressure for use in determining wind loads on buildings and structures. The velocity pressure qz (in lb/ft^2) at height z is: qz = 0.00256 * Kz * Kzt * Kd * V^2. This pressure is then multiplied by pressure coefficients (Cp for MWFRS, GCp for components) and the exposure factor Ke (= 1.0 at sea level) to obtain design pressures on specific surfaces. This calculator computes qz from user-input parameters, providing the foundation for complete ASCE 7 Chapter 27 and Chapter 30 wind pressure calculations.
ASCE 7 velocity pressure formula
qz = 0.00256 × Kz × Kzt × Kd × V²
Where V is in mph and qz is in lb/ft^2 (psf). The constant 0.00256 derives from the air density at standard sea-level conditions (rho/2 in imperial units). Multiply qz by appropriate pressure coefficients (Cp, GCp) to obtain design pressures on specific surfaces.
Using the velocity pressure in design
- For MWFRS (Main Wind Force Resisting System): design pressure p = qz * G * Cp - qi * (GCpi), where G is the gust factor and GCpi is the internal pressure coefficient.
- For components and cladding: design pressure p = qz * (GCp) - qi * (GCpi).
- The gust factor G = 0.85 for rigid buildings (natural frequency greater than 1 Hz); flexible buildings require dynamic analysis per ASCE 7 Section 26.11.
- ASCE 7-22 introduced Ke (ground elevation factor, Section 26.9); for most sites at or near sea level, Ke = 1.0 and does not change this formula.
Frequently asked questions
What is velocity pressure qz in ASCE 7?
Velocity pressure qz (psf) is the dynamic pressure at height z above ground, computed as qz = 0.00256 * Kz * Kzt * Kd * V^2, where V is the basic wind speed in mph. This is the fundamental pressure from which design pressures on walls, roofs, and components are derived using additional pressure coefficients.
What is the exposure category factor Kz?
Kz accounts for terrain roughness and height above ground. ASCE 7 Table 26.10-1 gives Kz for Exposures B, C, and D at various heights. Exposure B (urban/suburban with obstructions) gives the lowest values; Exposure D (flat open coastal terrain) the highest. At 30 ft: B = 0.70, C = 0.85, D = 1.03.
What is the topographic factor Kzt?
Kzt accounts for speed-up effects on isolated hills, ridges, or escarpments. For flat terrain with no topographic features, Kzt = 1.0. For sites near hill crests, Kzt can be significantly greater than 1.0 and must be computed per ASCE 7 Section 26.8.
What is the wind directionality factor Kd?
Kd reduces wind load to account for the reduced probability that the maximum pressure occurs simultaneously with the maximum wind direction. For MWFRS of buildings, Kd = 0.85. For components and cladding, Kd = 0.85. This factor is already embedded in ASCE 7 load combination factors.
How do I find the basic wind speed V for my site?
ASCE 7 Figures 26.5-1A through 26.5-1C provide basic wind speed maps for the continental US and territories. For Risk Category II buildings, the 700-year MRI map applies. V ranges from about 85 mph in most interior areas to over 180 mph in coastal Florida and hurricane-prone regions.
Official sources
- American Society of Civil Engineers: ASCE 7-22 Minimum Design Loads for Buildings and Other Structures, Chapter 26-30.
- International Building Code (ICC): IBC 2021 Chapter 16 Structural Design.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.