Glass wind load calculator (IS 875)
Design wind pressure pz = 0.6 × Vz², where Vz is the IS 875 basic wind speed for your city adjusted for terrain and height (k2). Pick a city, terrain and building height to get an indicative pressure in kN/m² — and a first-cut toughened glass thickness for your panel span.
Indicative pre-design aid only. Assumes k1 = k3 = k4 = 1.0, no area or combination factors, and a four-side-supported toughened panel. Final glazing design must be done by a structural engineer or facade consultant to IS 875 (Part 3) and IS 16231 for your actual site.
How the estimate works
IS 875 (Part 3) starts from a basic wind speed Vb — a mapped 3-second gust speed at 10 m height in open terrain, between 33 and 55 m/s across India. The design wind speed is Vz = Vb × k1 × k2 × k3 × k4, and the design pressure is pz = 0.6 × Vz² (pz in N/m² when Vz is in m/s).
This calculator simplifies deliberately, and says so:
- k1 (probability / design life), k3 (topography) and k4 (cyclonic importance) are taken as 1.0 — a common pre-design assumption for ordinary inland buildings on level ground.
- k2 comes from IS 875 Table 2, taken at the conservative top of your height band.
- The pressure shown is pz itself — no area-averaging (Ka), combination (Kc) or directionality (Kd) reductions, and no local cladding coefficients (Cpe/Cpi), which an engineer would apply.
- The thickness suggestion assumes a four-side-supported toughened panel governed by its short span.
IS 875 basic wind speed zones
The IS 875 (Part 3) wind map divides India into six basic wind speed zones. Coastal and cyclone-exposed regions sit at the top of the range; the sheltered interior south sits at the bottom. Confirm your exact site against the map — zone boundaries matter near coasts and hills.
| Basic wind speed (Vb) | Example cities |
|---|---|
| 33 m/s | Bengaluru, Mysuru and the interior south Deccan |
| 39 m/s | Ahmedabad, Pune, Kochi, Coimbatore, Nashik, Ranchi |
| 44 m/s | Mumbai, Hyderabad, Nagpur, Surat, Vadodara |
| 47 m/s | Delhi, Jaipur, Lucknow, Kanpur, Chandigarh, Patna |
| 50 m/s | Chennai, Kolkata, Bhubaneswar, Visakhapatnam, Guwahati |
| 55 m/s | Darbhanga and cyclone-prone pockets of the east coast and the north-east |
How terrain category changes the pressure
Terrain category describes how much the surroundings slow the wind before it reaches your glass:
- Category 1 — exposed open terrain (sea front, flat open land): nothing slows the wind, so pressures are highest.
- Category 2 — open land with scattered obstructions under 10 m (farmland, town outskirts): the reference exposure for the wind map.
- Category 3 — built-up suburban or city terrain with buildings up to about 10 m: what most Indian city sites actually are.
- Category 4 — dense city centres with tall, closely packed buildings: the most sheltered at low level, and the hardest to justify claiming.
Moving one category towards open terrain typically raises the design pressure at low heights by roughly 10–30% — and shelter can only be claimed if it exists in every wind direction. The k2 values this tool uses (IS 875 Table 2, top of each height band):
| Building height | Terrain 1 | Terrain 2 | Terrain 3 | Terrain 4 |
|---|---|---|---|---|
| Up to 10 m | 1.05 | 1.00 | 0.91 | 0.80 |
| 10–20 m | 1.12 | 1.07 | 1.01 | 0.80 |
| 20–50 m | 1.20 | 1.17 | 1.12 | 1.10 |
Indicative toughened thickness chart
The chart the calculator uses — indicative values for a four-side-supported toughened panel, read by panel short span (the smaller of width and height) against design pressure. Larger spans, higher pressures, two-side or point-fixed support, and deflection-sensitive applications all need an engineered check instead.
| Panel short span | ≤ 1.0 kN/m² | ≤ 1.5 kN/m² | ≤ 2.0 kN/m² | ≤ 3.0 kN/m² |
|---|---|---|---|---|
| Up to 900 mm | 6 mm | 6 mm | 6 mm | 8 mm |
| 901–1200 mm | 6 mm | 8 mm | 8 mm | 10 mm |
| 1201–1500 mm | 8 mm | 8 mm | 10 mm | 12 mm |
| 1501–1800 mm | 10 mm | 10 mm | 12 mm | 12 mm |
Spans above 1800 mm or pressures above 3.0 kN/m² are outside this indicative chart and always need engineered design per IS 875 (Part 3) and IS 16231.
Worked example — Mumbai facade panel
A glazed panel at 15 m height on a building in a built-up Mumbai suburb (terrain category 3), panel short span 1200 mm. Basic wind speed for Mumbai Vb = 44 m/s; k2 for terrain 3 in the 10–20 m band = 1.01 (with k1 = k3 = k4 = 1.0). So Vz = 44 × 1.01 = 44.4 m/s, and pz = 0.6 × 44.4² ≈ 1,185 N/m² ≈ 1.18 kN/m² (about 121 kgf/m²). At a 1200 mm span, the indicative chart suggests 8 mm toughened glass for a four-side-supported panel — a starting point for the facade engineer to verify, not a final specification.
Frequently asked questions
How is wind load on glass calculated in India?
Per IS 875 (Part 3), take the basic wind speed Vb for your location from the wind map (33–55 m/s), convert it to a design wind speed Vz = Vb × k1 × k2 × k3 × k4 (k2 accounts for terrain and height), then compute the design wind pressure pz = 0.6 × Vz² in N/m². The glass thickness is then chosen so the panel can resist that pressure over its span — typically checked per IS 16231.
What is the basic wind speed for my city?
IS 875 (Part 3) divides India into six basic wind speed zones: 33, 39, 44, 47, 50 and 55 m/s. For example, Bengaluru is 33 m/s, Pune and Ahmedabad 39 m/s, Mumbai and Hyderabad 44 m/s, Delhi and Jaipur 47 m/s, and Chennai, Kolkata and Visakhapatnam 50 m/s. Always confirm your exact site against the IS 875 wind map — zone boundaries matter near coasts and hills.
What glass thickness do I need for wind load?
It depends on the design wind pressure, the panel span and how the panel is supported. As an indicative guide for four-side-supported toughened glass, spans up to about 1200 mm at moderate pressures (around 1–1.5 kN/m²) typically land at 6–8 mm, while larger spans or higher pressures push towards 10–12 mm. Treat any chart value as a starting point — the final thickness must come from an engineered check.
Can I use this calculator for a final facade design?
No. This tool takes k1, k3 and k4 as 1.0, uses banded k2 values, ignores area-averaging and combination factors, and assumes a four-side-supported panel. A real design must follow IS 875 (Part 3) and IS 16231 in full — including support conditions, deflection limits, openings and cyclonic factors — and be signed off by a structural engineer or facade consultant.
Does toughened glass take more wind load than annealed glass?
Yes — toughened (tempered) glass has a much higher design bending strength than annealed glass of the same thickness, which is one reason facades and large windows use it. Its stiffness is unchanged though, so deflection under wind is the same; large panels can still need extra thickness to control deflection even when strength is adequate.
What are terrain categories in IS 875?
Terrain category describes how much the surroundings slow the wind: Category 1 is exposed open terrain (sea front, open flats), Category 2 is open land with scattered low obstructions, Category 3 is typical built-up suburban or city terrain, and Category 4 is dense city centres with tall, closely packed buildings. More open terrain means higher design pressure at the same height.
Need the glass supplied?
Tell us the sizes and spec — we'll connect you with verified glass suppliers near you for a free, no-obligation quote.
Request a free quotation →Also see: glass thickness guide, weight calculator, Indian glass standards.