How to use this calculator
Follow the three steps below. Start with the example values, replace them with your own data, and read the result together with the assumptions and limitations.
Inputs
Results
A worked example, and what the code adds to it
The same sign, calculated three ways: raw aerodynamics, ASCE 7-22, and EN 1991-1-4.
Take a solid freestanding sign, 6 m wide by 3 m high, standing on the ground (As = 18 m²), in open country, and a design wind speed of 115 mph (51.4 m/s) three-second gust — a representative ASCE 7-22 Risk Category II value over much of the US interior.
1. What this calculator returns
The tool computes stagnation pressure and multiplies it by the coefficients you supply:
q = ½ρV² = 0.5 × 1.225 × 51.41² = 1,619 Pa F = q × Cₚ × factor × A = 1,619 × 1.4 × 1.0 × 18 = 40.8 kNThat is correct fluid mechanics and the wrong number to build from. Here is why.
2. The same sign under ASCE 7-22
ASCE 7 does not apply dynamic pressure directly. It applies a velocity pressure that already carries height, terrain, topography, directionality and elevation:
qh = 0.613 Kz Kzt Kd Ke V² (SI) qh = 0.00256 Kz Kzt Kd Ke V² (V in mph, q in psf)For this sign in Exposure C, with its top at 3 m (inside the 0–15 ft band, Kz = 0.85), flat ground (Kzt = 1.0), the directionality factor for solid freestanding signs (Kd = 0.85) and sea level (Ke = 1.0):
qh = 0.613 × 0.85 × 1.0 × 0.85 × 1.0 × 51.41² = 1,171 Pa (24.5 psf)The force then comes from Chapter 29, with the gust-effect factor and a force coefficient read from Figure 29.3-1 — for a ground-mounted sign with aspect ratio B/s = 2, Cf ≈ 1.4:
F = qh G Cf As = 1,171 × 0.85 × 1.4 × 18 = 25.1 kN (5,640 lbf)The code answer is 0.61 times the raw aerodynamic answer — and that ratio is not a coincidence. It is Kz × Kd × G = 0.85³ = 0.614 exactly. Three independent reductions, each about 15%, compound into a 39% difference. Entering raw ½ρV² with a shape coefficient and no other factors is roughly 60% conservative for a low ground-mounted element in open terrain.
3. The same sign under EN 1991-1-4
The Eurocode route differs in structure, not just in coefficients. It builds a peak velocity pressure from the mean wind and the turbulence intensity at the reference height:
qp(z) = [1 + 7 Iv(z)] · ½ρ vm²(z), vm = cr(z) co(z) vbFor terrain category II (z₀ = 0.05 m, kr = 0.19), z = 3 m, flat ground (co = 1) and a basic wind velocity vb = 26 m/s:
cr = 0.19 ln(3/0.05) = 0.778 → vm = 20.2 m/s Iv = 1/ln(60) = 0.244 qp = (1 + 7×0.244) × 0.5 × 1.25 × 20.2² = 693 PaFor signboards clause 7.4.3 gives a single force coefficient cf = 1.8:
Fw = cscd · cf · qp · Aref = 1.0 × 1.8 × 693 × 18 = 22.5 kNAnd then the clause that catches people out: for a signboard the resultant is applied at the centre of the board with a horizontal eccentricity e = ±0.25b — 1.5 m here, producing a 34 kN·m torsion about the vertical axis that a pressure-times-area calculation never reveals. For a single central post that torsion often governs the connection, not the 22.5 kN of direct force.
| Route | Velocity pressure | Coefficients applied | Force on 18 m² |
|---|---|---|---|
| This calculator (raw) | 1,619 Pa | Cₚ = 1.4 only | 40.8 kN |
| ASCE 7-22 Ch. 29 | 1,171 Pa | Kz, Kzt, Kd, Ke, G, Cf | 25.1 kN |
| EN 1991-1-4 §7.4.3 | 693 Pa | cr, co, Iv, cscd, cf | 22.5 kN + torsion |
The two code results are close, which is reassuring, but they are not computed from the same wind speed — see the next section. Comparing them directly without converting the speed convention is the single most common error made with wind-load tools.
Which wind speed belongs in the box
Averaging convention, not magnitude, is where most of the error enters.
Wind speed is meaningless without its averaging period, and the two major code families use different ones:
- ASCE 7 uses a 3-second gust at 10 m in Exposure C, taken from the hazard maps by risk category. Since ASCE 7-10 these are strength-level speeds, so the resulting load is already factored for LRFD; ASD uses 0.6W.
- EN 1991-1-4 uses a 10-minute mean at 10 m in terrain category II. The gust is reintroduced later, through the turbulence term [1 + 7Iv].
The ratio between a 3-second gust and a 10-minute mean at the same site is roughly 1.4 to 1.5 in open terrain. Because pressure goes with the square of speed, substituting one for the other changes the answer by a factor of about two:
(1.45)² ≈ 2.1So a Eurocode basic velocity of 26 m/s entered into a gust-based calculation under-predicts the pressure by roughly half; a 3-second gust fed into a mean-wind formulation doubles it. Both errors are silent — the arithmetic is right and the number is wrong.
Rule for this calculator: it applies ½ρV² to whatever you enter, with no averaging assumption of its own. If you are working to ASCE 7, enter the 3-second gust and expect a conservative answer. If you are working to a Eurocode, enter the peak velocity at the reference height — not vb — or the result will be low by about a factor of two.
Pressure and force coefficients: where the numbers come from
Representative values with their governing source. Read the actual figure or table for your geometry — these are for orientation, not for design.
| Surface | Coefficient | Typical value | Source |
|---|---|---|---|
| Windward wall, enclosed building | Cp | +0.8 | ASCE 7 Fig. 27.3-1 |
| Leeward wall (L/B ≤ 1) | Cp | −0.5 | ASCE 7 Fig. 27.3-1 |
| Side wall | Cp | −0.7 | ASCE 7 Fig. 27.3-1 |
| Solid freestanding sign, ground-mounted | Cf | 1.2 – 1.8 | ASCE 7 Fig. 29.3-1 (by B/s and s/h) |
| Signboard, all cases | cf | 1.8 | EN 1991-1-4 §7.4.3 |
| Free-standing wall, mid-length zone | cp,net | ≈ 1.2 – 1.7 | EN 1991-1-4 §7.4.1 |
| Free-standing wall, windward end zone | cp,net | above 2 | EN 1991-1-4 §7.4.1 |
| Circular cylinder, subcritical flow | cf | ≈ 1.2 | EN 1991-1-4 §7.9 |
| Gust-effect factor, rigid structure | G | 0.85 | ASCE 7 §26.11 |
| Directionality, solid signs and walls | Kd | 0.85 | ASCE 7 Table 26.6-1 |
| Exposure coefficient, Exp. C, 0–15 ft | Kz | 0.85 | ASCE 7 Table 26.10-1 |
| Exposure coefficient, Exp. C, 100 ft | Kz | 1.26 | ASCE 7 Table 26.10-1 |
Two patterns are worth absorbing. First, end and edge zones carry far more load than the middle of a surface — both codes handle this with zone-dependent coefficients, and a single-coefficient calculation necessarily averages it away. Second, negative coefficients are not smaller loads: suction on a leeward wall, a roof edge or a canopy underside routinely governs fixings and uplift anchorage, and it acts in the direction that pulls cladding off rather than pushing it on.
When this screening calculation is not conservative
The worked example came out 60% high. Several common situations reverse that.
A screening tool that is conservative in the configuration you first tried is the most dangerous kind, because it teaches you to trust it. The reductions in the example above — Kz, Kd and G — all happened to be below 1. None of them always is.
- Height. Kz is 0.85 near the ground but 1.26 at 30 m and continues rising. Above roughly 20 m in open terrain the exposure coefficient alone cancels most of the directionality and gust reductions, and a raw ½ρV² calculation stops being conservative.
- Components and cladding. Main-frame coefficients are area-averaged. Cladding is designed to (GCp) values that reach well beyond ±2 in corner and edge zones for small effective areas — several times the main-frame figure. A panel fixing sized on Cp = 0.8 can be badly under-designed.
- Internal pressure. An enclosed building has (GCpi) = ±0.18; a partially enclosed one ±0.55. A single dominant opening on the windward face — a door left open, a window lost early in the storm — pressurises the envelope from inside and adds to the external suction on every other surface. This calculator has no internal-pressure term at all.
- Topography. Kzt is 1.0 on flat ground but can exceed 1.5 on an escarpment or hill crest, which is a factor of 1.5 on pressure with no change in the wind map value.
- Dynamic response. Flexible or lightly damped structures — slender masts, long canopies, cable-supported elements — amplify rather than simply resist. Both codes replace the rigid gust factor with a dynamic treatment when the fundamental frequency falls below about 1 Hz.
- Vortex shedding and fatigue. A slender element can fail from repeated cross-wind oscillation at a speed far below its static design wind. Static pressure says nothing about it.
- Load combinations. The force here is a single unfactored action. Design requires it combined with dead, live, snow and seismic actions to the governing combination, and checked for overturning and sliding with the appropriate stability factors.
Use this page to get an order of magnitude, to check that a quoted figure is plausible, and to understand how the pieces fit together. The moment a result is going to be built, the governing standard replaces it — not supplements it.
Sources
- ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures — Ch. 26 (general), Ch. 27 (directional procedure, MWFRS), Ch. 29 (other structures and building appurtenances), Ch. 30 (components and cladding).
- EN 1991-1-4:2005+A1, Eurocode 1: Actions on structures — Part 1-4: General actions — Wind actions — §4 (wind velocity and velocity pressure), §7.4 (free-standing walls, parapets and signboards), together with the relevant National Annex.
- E. Simiu and R. H. Scanlan, Wind Effects on Structures — background to gust factors, averaging conventions and dynamic response.
- J. D. Holmes, Wind Loading of Structures — pressure coefficients, zone effects and the basis of code force coefficients.
- Durst conversion curves for wind-speed averaging intervals, as reproduced in the ASCE 7 commentary.
Frequently Asked Questions
Practical questions about assumptions, inputs, interpretation and limitations.
No. It is a physics-based screening calculator. It does not replace ASCE 7, Eurocode or another governing wind-loading standard, which may require terrain, height, gust, directionality, importance and pressure-zone factors.
Use the projected area normal to the wind direction for the simplified force calculation. Complex structures may require separate pressure zones or aerodynamic treatment.
Dynamic pressure is proportional to ½ρV². Therefore, before other coefficients are applied, doubling wind speed produces four times the dynamic pressure.
The pressure coefficient represents how shape, orientation and flow interaction modify the basic dynamic pressure acting on a surface. Appropriate values depend on geometry and the governing design method.
Use it for education and preliminary screening only. Final structural wind loads should be determined using the applicable code, site wind speed, exposure, height, pressure zones, load combinations and structural system.
For the worked example on this page — a low, ground-mounted sign in open terrain — the raw answer is about 63% higher than ASCE 7-22, because the exposure, directionality and gust factors each contribute roughly 15% and compound to 0.85³ = 0.61. That margin shrinks with height and disappears entirely above about 20 m, and it does not exist at all for cladding in corner zones, where code values exceed the simple calculation.
Whichever matches the coefficients you are using. This tool applies ½ρV² to the value you type with no averaging assumption of its own. ASCE 7 maps give 3-second gusts; Eurocode basic velocity v_b is a 10-minute mean. The ratio is about 1.4–1.5, so substituting one for the other changes the pressure by roughly a factor of two.
Because a single positive pressure coefficient cannot represent them. Suction on leeward walls, roof edges and canopy undersides needs negative coefficients applied zone by zone, and uplift anchorage is usually governed by those zones rather than by the windward face. Enter a negative Cₚ if you want the magnitude, but treat the direction as your responsibility.
EN 1991-1-4 §7.4.3 applies the resultant wind force on a signboard at the centre of the board with a horizontal eccentricity of ±0.25b. For a 6 m wide sign that is ±1.5 m, which on a 22.5 kN force is a 34 kN·m torsion about the vertical axis. On a single-post sign this frequently governs the post and its base connection, and no pressure-times-area calculation will show it.
Usually not much, but it is linear in the result. The 1.225 kg/m³ default is standard sea-level air at 15 °C. At 1,500 m altitude density falls by around 14%, and both codes handle this explicitly — ASCE 7 through the ground elevation factor K_e, the Eurocode through the recommended ρ in the National Annex.