Wind Load Calculator
A wind load calculator finds velocity pressure with the simplified ASCE 7 form: 115 mph, Kz 1.0, Kzt 1.0, Kd 0.85 gives qz of 28.78 psf.
Wind load calculator: velocity pressure
A wind load calculator estimates the pressure that moving air puts on a flat surface. This one follows a simplified form of the ASCE 7 method and finds the velocity pressure, called qz, from the wind speed and three adjustment factors.
qz (psf) = 0.00256 × Kz × Kzt × Kd × V², where V is the basic wind speed in mph.
The 0.00256 comes from the density of air and unit conversions. The speed is squared, so a little more wind makes a lot more pressure.
The factors in plain words
Kz is the exposure coefficient. It covers how high the surface is above the ground and what surrounds the building. Open country lets the wind hit harder than a neighborhood of trees and houses.
Kzt is the topographic factor. It is 1.0 on flat ground, and it rises for a building near the top of a hill or ridge.
Kd is the directionality factor. The strongest wind seldom blows from the worst direction, and for buildings the code commonly uses 0.85.
Worked examples
Example 1, the default values. V = 115 mph, Kz = 1.0, Kzt = 1.0, Kd = 0.85.
- Speed squared: 115 × 115 = 13,225
- qz: 0.00256 × 1.0 × 1.0 × 0.85 × 13,225 = 28.78 psf
Example 2, a lower exposure. V = 115 mph, Kz = 0.85, Kzt = 1.0, Kd = 0.85.
- qz: 0.00256 × 0.85 × 0.85 × 13,225 = 24.46 psf
Example 3, a lower wind speed. V = 90 mph, Kz = 0.85, Kzt = 1.0, Kd = 0.85.
- qz: 0.00256 × 0.85 × 0.85 × 8,100 = 14.98 psf
How speed changes the pressure
Kz = 1.0, Kzt = 1.0, Kd = 0.85.
| Wind speed | qz |
|---|---|
| 90 mph | 17.63 psf |
| 100 mph | 21.76 psf |
| 115 mph | 28.78 psf |
| 130 mph | 36.77 psf |
| 150 mph | 48.96 psf |
Going from 90 to 130 mph is a 44% jump in speed, but the pressure doubles, from 17.63 to 36.77 psf.
How exposure changes the pressure
At 115 mph with Kzt = 1.0 and Kd = 0.85, using Kz values that the standard lists for a height of about 15 ft.
| Exposure | Kz | qz |
|---|---|---|
| B (suburban, wooded) | 0.57 | 16.40 psf |
| C (open terrain) | 0.85 | 24.46 psf |
| D (flat, coastal) | 1.03 | 29.64 psf |
Check the standard for the edition your building department uses, since tables and definitions are updated over time.
From qz to a design load
The velocity pressure is the base. To get the pressure on a wall, roof, or sign, the full method multiplies qz by a gust effect factor and a pressure coefficient that depend on the shape, the opening sizes, and the location on the building, such as a corner or an edge. Roof edges and corners can see strong suction. That is why the final design loads vary across a building and why connections between the roof, walls, and foundation matter so much.
Finding the inputs
Ask your building department for the design wind speed and the exposure category that apply to the address. They know the adopted code. For Kzt, flat sites use 1.0, and a site near the crest of a hill needs a calculated value. The exposure and height give Kz.
Limits of this tool
- It stops at velocity pressure.
- It leaves out the ground elevation factor used in newer editions.
- It does not choose pressure coefficients or compute forces.
- It is not a substitute for the code’s complete procedure.
Wind design affects safety. A licensed engineer or designer should check any structure that people will use.
Common mistakes
- Using a gust speed from the weather report as the design speed.
- Entering Kd as 1.0 for a building when the code lists 0.85.
- Treating qz as the load on a roof panel.
- Ignoring uplift at the roof edges.
Related calculators
The snow load calculator covers the other roof load, the occupancy load calculator deals with people, and the column buckling calculator checks a post’s capacity. The deck load calculator helps with outdoor structures.
Frequently asked questions
What is velocity pressure (qz) and why is it not the final wind load?
Velocity pressure is the pressure of moving air on a flat surface, before the building shape matters. The real design pressure on a wall or roof also multiplies by gust and pressure coefficients that depend on the shape and the part of the building. This tool stops at qz.
Where do I find the basic wind speed (V) for my location?
Your local building department sets it, usually from the wind speed maps in the adopted code. Many jurisdictions also list the design speed on their permit pages. Use the number they give, and do not guess.
What do Kz, Kzt, and Kd represent?
Kz adjusts for height above ground and the terrain around the building. Kzt adjusts for hills and ridges that speed up the wind. Kd accounts for the fact that wind rarely hits from the worst direction, and for buildings it is commonly 0.85.
Can I use this qz value to size a beam, wall, or connection?
No. It is a first step for learning and for a quick comparison. Real design needs the full code method and a licensed engineer or designer for structural elements.
Why do I get a different result from a newer code edition?
Newer editions of the standard add a ground elevation factor, Ke, and change wind speed maps. This tool leaves out Ke, so results at high elevations can differ slightly. Check the edition your building department uses.
Estimates only. Verify quantities with your supplier before purchasing.