Mode 1 — Velocity pressure q from wind speed
Mode 2 — Wind force F on a surface
Mode 3 — Inverse
3a. Wind speed V from pressure q
3b. Pressure q from known force F
3c. Area A from known force F
Mode 4 — Scenario comparison + reference tables
Add cases (e.g. exposure B vs D, or 100 vs 140 mph) then click Calculate — the table shows q for each case side by side.
Kz reference table (computed from the ASCE 7 formula)
Kz = 2.01·(z/zg)2/α; below 15 ft the 15 ft value is used. α/zg: B = 7.0/1200 ft; C = 9.5/900 ft; D = 11.5/700 ft. The printed ASCE table rounds this formula to 2 decimals — some cells may differ by 0.01 due to rounding.
| Height (ft) | Exposure B | Exposure C | Exposure D |
|---|---|---|---|
| 15 | 0,57 | 0,85 | 1,03 |
| 20 | 0,62 | 0,9 | 1,08 |
| 25 | 0,67 | 0,95 | 1,13 |
| 30 | 0,7 | 0,98 | 1,16 |
| 40 | 0,76 | 1,04 | 1,22 |
| 50 | 0,81 | 1,09 | 1,27 |
| 60 | 0,85 | 1,14 | 1,31 |
| 80 | 0,93 | 1,21 | 1,38 |
| 100 | 0,99 | 1,27 | 1,43 |
Kd, I, and Cp reference (summary)
| Coefficient | Value | Description |
|---|---|---|
| Kd | 0,85 | Buildings (MWFRS & components) |
| Kd | 0,90 | Round chimney/tank, square chimney, pole |
| Kd | 0,85 | Freestanding solid wall, lattice tower |
| I | 0,87 | Risk category I |
| I | 1,00 | Risk category II (standard) |
| I | 1,15 | Risk category III/IV |
| Cp | +0,8 | Windward wall (typical, simplified) |
| Cp | −0,5 | Leeward wall (suction) |
| Cp | −0,7 | Side wall |
| Cp | −0,9 | Flat roof |
| Cp | +1,2 | Flat signboard |
Wind speed scale (context)
| Speed | General context |
|---|---|
| 25–31 mph | Strong breeze (Beaufort 6) |
| 39–46 mph | Gale (Beaufort 8) |
| 55–63 mph | Storm (Beaufort 10) |
| ≥ 74 mph | Hurricane threshold (Beaufort 12) |
| 105–180 mph | ASCE 7 basic design wind speeds for the US (risk category II, region-dependent) |
Mode 5 — Unit conversion
Scope boundaries — read before use
- This is NOT wind load for boat anchoring. anchoring-lab (#169) computes wind load on a boat hull to choose anchor size (marine, ABYC standard) — a totally different domain from the structural wind load on buildings on this page.
- This is NOT a wind turbine economics calculator. wind-turbine-profit (#1477) computes electricity potential and revenue from wind energy — not force on structures.
- Not a detailed roof uplift calculator. Uplift pressure uses roof zones and internal coefficients (GCpi) per ASCE 7 figures — outside the scope of this simplified version.
- Not a dynamic analysis. Flexible/tall structures require dynamic gust analysis (full gust-effect factor per ASCE 7 Chapter 26), not a fixed G = 0.85.
- Structurally related: beam-load — beam analysis (reactions, shear, moment) for distributed loads from wind calculations.
Verified examples
| Case | Input | Result |
|---|---|---|
| Basic case | V = 100 mph, Kz = Kzt = Kd = I = 1 | q = 25.6 psf (0.00256 × 100² exact) |
| With real Kz | V = 120 mph, Kz(30 ft, exposure C) = 0.9823, Kd = 0.85 | q = 30.78 psf |
| Windward wall force | q = 25.6 psf, G = 0.85, Cp = +0.8, A = 100 ft² | F = 1740.8 lbf = 7743 N |
| Leeward suction | q = 30.78 psf, G = 0.85, Cp = −0.5, A = 200 ft² | F = -2616 lbf (negative = suction) |
| Inverse: V from q | q = 25.6 psf, all coefficients = 1 | V = 100 mph |
| Unit conversion | 100 mph; 25.6 psf; 100 ft² | 44.704 m/s; 1.2257 kPa; 9.2903 m² |
All numbers above were recomputed with 50-digit precision arithmetic (oracle.py) and match this page's engine.
Methodology
Core formulas (classic ASCE 7-16 style, simplified): q = 0.00256 × Kz × Kzt × Kd × V² × I with q in psf and V in mph, and F = q × G × Cp × A. Inverse: V = √(q / (0.00256 × Kz × Kzt × Kd × I)), q = F / (G × Cp × A), A = F / (q × G × Cp).
Kz is computed from the ASCE 7-16 Table 27.3-1 footnote formula: Kz = 2.01 × (z/zg)^(2/α), with z = max(height, 15 ft); constants (α, zg): B = (7.0; 1200 ft), C = (9.5; 900 ft), D = (11.5; 700 ft). Kd values follow Table 26.6-1 (0.85 buildings; 0.90 round chimney/pole). The I factor uses ASCE 7-10 style (0.87 / 1.00 / 1.15).
Exact conversion constants: 1 mph = 0.44704 m/s; 1 mph = 1.609344 km/h; 1 mph = 22/15 ft/s; 1 psf = 47.8802589804 Pa; 1 lbf = 4.4482216152605 N; 1 ft² = 0.09290304 m²; 1 kgf = 9.80665 N.
What this simplified version does not cover: ground elevation factor (Ke, ASCE 7-22), enclosure classification and internal pressure coefficients (GCpi), detailed Cp zones per figures 27.3-1 through 30.x, minimum pressure, and dynamic analysis of flexible structures. Input validation: all quantities non-negative; denominators (I, Kd, Kz, Kzt in V-inverse; G·Cp·A in F-inverse) must be > 0 and Cp ≠ 0.
Frequently asked questions (FAQ)
What is the wind velocity pressure formula?
The simple formula is q = 0.00256 x Kz x Kzt x Kd x V^2 x I, with q in psf and V in mph. Example: V = 100 mph with all coefficients = 1 gives q = 0.00256 x 10000 = 25.6 psf.
How do you calculate wind force on a wall or surface?
Use F = q x G x Cp x A. Example: q = 25.6 psf, G = 0.85, Cp = +0.8 (windward wall), A = 100 ft^2 gives F = 1740.8 lbf, equivalent to 7743 N.
How do you find wind speed from a known pressure?
Invert the formula: V = sqrt(q / (0.00256 x Kz x Kzt x Kd x I)). Example: q = 25.6 psf with all coefficients = 1 gives V = 100 mph.
What is Kz and how do you determine it?
Kz is the velocity pressure exposure coefficient from building height and exposure category (B = urban, C = open, D = coastal). This calculator computes it from the ASCE 7 formula: Kz = 2.01 x (z/zg)^(2/alpha). Example: 30 ft height, exposure C gives Kz = 0.9823.
What Kd (wind directionality factor) values are commonly used?
Per ASCE 7 Table 26.6-1: 0.85 for buildings (MWFRS and components), 0.90 for round chimney/tank, square chimney, and poles/masts, and 0.85 for freestanding solid walls and lattice towers.
What is Cp and what are its values?
Cp is the external pressure coefficient: positive means pressure pushing on the surface, negative means suction. Simplified typical values: windward wall +0.8, leeward wall -0.5, side wall -0.7, flat roof -0.9. The full ASCE 7 defines Cp per zone in far more detail.
What is the importance factor I in the wind load formula?
I is the importance factor: 0.87 for risk category I, 1.00 for category II (standard), and 1.15 for categories III/IV. This follows ASCE 7-10 style; in ASCE 7-16 and newer, this factor is folded into the wind speed maps.
Which units does this wind load calculator support?
Speed: mph, m/s, km/h, ft/s. Pressure: psf, Pa (N/m^2), kPa, kgf/m^2. Force: lbf, N, kN, kgf. Area: ft^2, m^2, in^2. Example: 100 mph = 44.704 m/s; 25.6 psf = 1.2257 kPa.
Does this calculator replace structural engineer calculations?
No. This is a simplified version for educational estimates: it uses simplified q and F formulas without elevation factor, enclosure classification, and detailed Cp zones. Real structural design must use the full ASCE 7 and be signed by a licensed engineer.
How does it differ from wind load for boat anchors?
Totally different. This calculator computes structural wind load on building surfaces. Wind load for boat anchors (marine, ABYC standard) computes wind force on a boat hull to choose anchor size — see the anchoring-lab tool.
How does it differ from a wind turbine calculator?
A wind turbine calculator computes wind energy economics (how much electricity and money is produced). This calculator computes wind load/force on building structures — the physics domain and purpose differ.
Why does wind force increase with the square of speed?
Because velocity pressure q is proportional to V^2. A 120 mph wind gives (120/100)^2 = 1.44 times the pressure of a 100 mph wind — not 1.2 times. This is why slightly stronger storms feel far more destructive.
What is Kzt (topographic factor)?
Kzt is the topographic factor: 1.0 for flat terrain, greater than 1 for buildings on hills, escarpments, or ridges where wind is accelerated. ASCE 7 defines its calculation from hill slope and height.
How do you compare several scenarios at once?
Open the Scenario Comparison tab, add case rows (e.g. exposure B vs D, or 100 vs 140 mph), then click Calculate. The table shows q for each case side by side so you can see sensitivity to exposure and speed.
What is the difference between pressure q and force F?
q is pressure per unit area (psf or kPa) — the wind intensity at one height. F is the total force on a surface: F = q x G x Cp x A, in lbf or newtons. q answers 'how strong the pressure is', F answers 'how much total load'.
Does this calculator use the Ke elevation factor like ASCE 7-22?
No. This simplified version uses the classic form q = 0.00256 x Kz x Kzt x Kd x V^2 x I (ASCE 7-16 style). ASCE 7-22 adds the ground elevation factor Ke and moves Kd into the pressure equation. For official design, follow the ASCE 7 edition applicable in your region.