Design wind speed Vz and design wind pressure pz per IS 875 (Part 3):2015.
h/b = — (enter b) · f = — (enter T)
Pressure coefficients and net design pressures
Rectangular clad building with a duopitch roof: wall Cpe from Table 5, roof Cpe from Table 6 (interpolated on the roof angle), Cpi from Cl 7.3.2. Net pressure (Cpe − Cpi)·pd, Cl 7.3.1, with the sign of Cpi that governs. h above is the height to the eaves.
1/2 < h/w ≤ 3/2 (h/w = 0.83), 3/2 < l/w < 4 (l/w = 1.67), Cpi = ±0.2 (openings under 5 % of wall area, Cl 7.3.2.1).
| Surface | Cpe | Cp,net | kN/m² |
|---|---|---|---|
| Wall A | 0.70 | 0.90 | 0.941 |
| Wall B | -0.30 | -0.50 | -0.523 |
| Wall C | -0.70 | -0.90 | -0.941 |
| Wall D | -0.70 | -0.90 | -0.941 |
| Roof EF | -0.80 | -1.00 | -1.045 |
| Roof GH | -0.60 | -0.80 | -0.836 |
| Surface | Cpe | Cp,net | kN/m² |
|---|---|---|---|
| Wall A | -0.50 | -0.70 | -0.732 |
| Wall B | -0.50 | -0.70 | -0.732 |
| Wall C | 0.70 | 0.90 | 0.941 |
| Wall D | -0.10 | -0.30 | -0.314 |
| Roof EG | -1.00 | -1.20 | -1.255 |
| Roof FH | -0.60 | -0.80 | -0.836 |
Local wall coefficient near the vertical edges: Cpe = -1.1 → -1.359 kN/m² for cladding and fixings (use Kd = 1.0 for local pressures, Cl 7.2.1). Roof local coefficients, overhangs and other roof shapes: see Table 6 and Cl 7.3.3.
Reference only. Computes design wind speed Vz = Vb·k1·k2·k3·k4 (Cl. 6.3), pz = 0.6·Vz² and pd = Kd·Ka·Kc·pz ≥ 0.7·pz (Cl. 7.2) per IS 875 (Part 3):2015. k1 from Table 1 (interpolated between zone speeds), k2interpolated from Table 2 (to 500 m); Kd = 1.0 in cyclone-affected regions (Cl. 7.2.1). Buildings with h/b > 5 or a natural frequency below 1 Hz must also be examined for dynamic effects (Cl. 10.1). Pressure coefficients from Tables 5 and 6 and Cl 7.3.2 cover rectangular clad buildings with flat or duopitch roofs; other shapes, local roof zones, gust/dynamic response factors and load combinations still need the code — verify against the code and a qualified engineer.
How the Wind Pressure Calculator works
This calculator turns a basic wind speed Vb into the design wind speed Vz and the design wind pressure at a given height, following IS 875 Part 3:2015. You can pick Vb for ten Indian cities (for example Mumbai 44 m/s, Delhi 47 m/s, Chennai 50 m/s, Bengaluru 33 m/s) or enter your own value.
Vz is Vb multiplied by four factors. k1, the risk coefficient, comes from Table 1 for the class of structure and is interpolated between the basic wind speed columns. k2, the terrain and height factor, is interpolated linearly from Table 2 for terrain categories 1 to 4 up to 500 m; heights below 10 m use the 10 m value. k3, the topography factor, is entered by you between 1.00 and 1.36. k4 is 1.0 outside the cyclonic coastal belt, and 1.30, 1.15 or 1.0 inside it depending on the structure (Cl 6.3.4).
The wind pressure is pz = 0.6 Vz² in N/m². The design wind pressure is pd = Kd × Ka × Kc × pz, but not less than 0.7 pz (Cl 7.2). Kd is 0.90 for buildings or 1.00 for circular forms, and is forced to 1.0 in cyclone-affected regions (Cl 7.2.1). Ka (area averaging) and Kc (combination factor) are entered between 0.8 and 1.0. The page shows when the 0.7 pz floor governs.
If you enter the least lateral dimension b and the fundamental period T, the page also runs the Cl 10.1 check for dynamic effects. A building must be examined for dynamic wind effects when h/b is more than 5 or its first-mode frequency f = 1/T is below 1.0 Hz. Exactly h/b = 5 or f = 1.0 Hz does not trigger it. Leave either field blank to skip that criterion.
- Design wind speed
- Vz = Vb × k1 × k2 × k3 × k4
- Wind pressure
- pz = 0.6 × Vz² (N/m², Vz in m/s)
- Design wind pressure
- pd = Kd × Ka × Kc × pz, with pd ≥ 0.7 × pz
- Dynamic effects check (Cl 10.1)
- examine if h / b > 5 or f = 1/T < 1.0 Hz
Follows: IS 875 Part 3:2015 Cl 6.3 (design wind speed) · IS 875 Part 3:2015 Table 1 (k1) and Table 2 (k2) · IS 875 Part 3:2015 Cl 6.3.4 (k4, cyclonic regions) · IS 875 Part 3:2015 Cl 7.2 and Cl 7.2.1 (pz, pd and Kd) · IS 875 Part 3:2015 Table 4 (Ka) and Cl 7.3.3.13 (Kc) · IS 875 Part 3:2015 Cl 10.1 (when dynamic effects must be examined)
Worked example: design wind pressure at 20 m for a general building in Delhi
Given
- Basic wind speed Vb = 47 m/s (Delhi)
- General building, 50-year life: k1 = 1.0
- Terrain category 3, height h = 20 m
- Flat ground k3 = 1.00; not in a cyclonic belt, k4 = 1.00
- Kd = 0.90, Ka = 1.00, Kc = 1.00
Steps
- k2 for terrain category 3 at 20 m is read directly from Table 2: k2 = 1.01.
- Vz = 47 × 1.0 × 1.01 × 1.00 × 1.00 = 47.47 m/s.
- pz = 0.6 × 47.47² = 0.6 × 2253.4 = 1352 N/m².
- Kd × Ka × Kc = 0.90 × 1.00 × 1.00 = 0.90, so pd = 0.90 × 1352 = 1217 N/m².
- Floor check: 0.7 × 1352 = 946 N/m², which is less than 1217 N/m², so the floor does not govern.
Result: Vz = 47.47 m/s, pz = 1352 N/m² and design wind pressure pd = 1217 N/m² (1.217 kN/m²).
Enter the same values in the calculator above to check each step.
Frequently asked questions
What is the design wind pressure formula in IS 875 Part 3?
IS 875 Part 3:2015 Cl 7.2 gives pz = 0.6 Vz², with Vz in m/s and pz in N/m². The design wind pressure is pd = Kd × Ka × Kc × pz, and it may not be taken less than 0.7 pz. For Vz = 44 m/s, pz = 1162 N/m² and, with Kd = 0.9 and Ka = Kc = 1.0, pd = 1045 N/m².
How to calculate design wind speed Vz as per IS 875 Part 3:2015?
Multiply the basic wind speed by the four modification factors: Vz = Vb × k1 × k2 × k3 × k4. k1 is the risk coefficient from Table 1, k2 the terrain and height factor from Table 2, k3 the topography factor and k4 the cyclonic importance factor. A general building in Delhi at 20 m in terrain category 3 gets Vz = 47 × 1.0 × 1.01 × 1.0 × 1.0 = 47.47 m/s.
How do I find k2 for a height that is not in Table 2?
Interpolate linearly between the two heights either side, which is what the calculator does. For terrain category 2 at 25 m, k2 lies between 1.07 at 20 m and 1.12 at 30 m, giving 1.095. Below 10 m the 10 m value is used.
When does the 0.7 pz minimum govern the design wind pressure?
It governs when Kd × Ka × Kc is below 0.7. With Kd = 0.9 and Ka = Kc = 0.8 the product is 0.576, so for pz = 1162 N/m² the design pressure is set by the floor at 0.7 × 1162 = 813 N/m² instead of 669 N/m².
What does the wind pressure calculator not include?
It stops at the design wind pressure on a surface. It does not apply external or internal pressure coefficients, force coefficients, gust or dynamic response factors, or load combinations, and it does not calculate k3 from the site topography; you enter k3 yourself. It does tell you when Cl 10.1 requires a dynamic (gust factor) analysis, but it does not carry that analysis out.
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