Full reference for HVAC duct gauge selection, GI sheet thickness by duct size, SMACNA pressure classes, IS 655 requirements, joint types, reinforcement spacing and leakage class — India focused, updated 2026.
The key standards that govern HVAC duct fabrication and material selection in India
The Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) HVAC Duct Construction Standards — Metal and Flexible is the globally followed reference for duct gauge, joint type, reinforcement, and pressure class selection. Most Indian MEP consultants specify SMACNA compliance on commercial projects.
The Bureau of Indian Standards specification IS 655:2006 — "Specification for Metal Air Ducts" is the primary Indian standard for HVAC ductwork. It specifies sheet thickness, zinc coating class, joint types, stiffening requirements and installation tolerances for GI sheet ducts in Indian buildings.
ASHRAE 90.1 governs energy efficiency in building systems including HVAC. In India, it is referenced by green-building projects (LEED, GRIHA). It mandates maximum duct leakage rates, which directly influences the choice of joint type (S&D vs TDC/TDF) and sealing class in high-performance buildings.
India's National Building Code 2016 (Part 8 — Services) specifies fire-rated duct enclosures, smoke control ductwork material (galvanized, minimum 1.2mm) and fire damper installation. For buildings above 15m in India, GI duct fire resistance requirements supersede general HVAC gauge selection.
The British BESA DW/144 standard is widely used in India for high-specification MEP projects led by UK-based consultants. It aligns closely with SMACNA but uses metric mm thickness values directly. Indian contractors on international projects must confirm which standard governs before ordering sheet stock.
In India, the project consultant's Design Basis Report (DBR) or Material Specification typically overrides standard defaults. Always read the project specification for required standard (SMACNA/IS 655/DW144), pressure class, leakage class, GI coating class (Z275/Z350) and joint type before ordering GI coil or sheet stock.
Pressure class determines minimum gauge thickness, joint type and reinforcement spacing — the three most important HVAC duct design decisions
Standard GI sheet gauge and thickness for HVAC ducts used in India — with duct size applicability
| GI Gauge | Nominal Thickness (mm) | Base Steel (mm) | Duct Longest Side (mm) | Pressure Class | Joint Type | Typical Application India |
|---|---|---|---|---|---|---|
| 26 G | 0.50 | ~0.36 | Up to 300 mm | Low (≤ 500 Pa) | Slip & Drive | FCU branch ducts, small split AC ducts, bathroom exhaust |
| 24 G | 0.63 | ~0.50 | 301–750 mm | Low (≤ 500 Pa) | Slip & Drive / TDC | General supply/return air, AHU branch ducts, office HVAC |
| 22 G | 0.80 | ~0.66 | 751–1500 mm | Low–Medium | TDC / TDF flange | Main supply ducts, AHU plenums, hospital HVAC |
| 20 G | 1.00 | ~0.86 | 1501–2250 mm | Medium (≤ 1000 Pa) | TDC / TDF flange | Large AHU discharge, shopping mall main ducts, risers |
| 18 G | 1.25 | ~1.11 | 2251–2750 mm | Medium–High | Angle flange | Industrial supply, large plant room ducts, smoke extract |
| 16 G | 1.60 | ~1.46 | Above 2750 mm | High (≤ 2500 Pa) | Angle flange | High-pressure industrial exhaust, clean rooms, data centre HVAC |
* Nominal GI thickness includes Z275 zinc coating on both sides. Base steel thickness = total minus ~0.14mm (Z275 coating allowance). Values per IS 655:2006 and SMACNA HVAC Duct Construction Standards 4th Edition (2020). Always confirm project-specific gauge requirements in the consultant's Design Basis Report before ordering material. Use our GI Sheet Weight Calculator to get exact weight per metre of duct at any gauge.
Complete fabrication specification for rectangular GI ducts by size — used on Indian commercial & industrial HVAC projects
| Duct Longest Side | GI Gauge | Thickness (mm) | Transverse Joint | Max Joint Spacing | Stiffener / Reinforcement | Max Pressure (Pa) |
|---|---|---|---|---|---|---|
| 0 – 300 mm | 26G | 0.50 | Slip & Drive (S/D) | 2400 mm | None required | 250 Pa |
| 301 – 450 mm | 24G | 0.63 | Slip & Drive (S/D) | 2400 mm | None required | 250 Pa |
| 451 – 600 mm | 24G | 0.63 | Slip & Drive (S/D) | 2400 mm | Cross-break panels | 250 Pa |
| 601 – 750 mm | 24G | 0.63 | TDC / S-Drive | 2400 mm | Cross-break panels | 500 Pa (TDC) |
| 751 – 900 mm | 22G | 0.80 | TDC flange | 2400 mm | Angle 25×25×3 mm @1200mm | 750 Pa |
| 901 – 1200 mm | 22G | 0.80 | TDC flange | 2400 mm | Angle 25×25×3 mm @1200mm | 750 Pa |
| 1201 – 1500 mm | 22G | 0.80 | TDF flange | 2400 mm | Angle 38×38×3 mm @1200mm | 750 Pa |
| 1501 – 1800 mm | 20G | 1.00 | TDF flange | 2400 mm | Angle 38×38×3 mm @1200mm | 1000 Pa |
| 1801 – 2250 mm | 20G | 1.00 | TDF flange | 1200 mm | Angle 50×50×5 mm @600mm | 1000 Pa |
| 2251 – 2750 mm | 18G | 1.25 | Angle flange | 1200 mm | Angle 50×50×5 mm @600mm | 1500 Pa |
| Above 2750 mm | 16G | 1.60 | Angle flange | 1200 mm | Angle 65×65×6 mm @600mm | 2500 Pa |
* Specifications represent standard Low–Medium pressure construction per SMACNA 4th Ed. (2020) and IS 655:2006. TDC = Transverse Duct Connector (Roll-formed flange). TDF = Duct Flange (flanged TDC with gasket). Angle flange = MS or GI angle bolted flange. Cross-break = diagonal pressed indentations in sheet panel to prevent oil-canning. For High Pressure or special applications, refer to SMACNA Tables directly. GI coating class Z275 minimum for all standard HVAC applications; Z350 for coastal/marine environments.
Spiral and straight-seam round GI duct gauge selection by diameter — India HVAC reference 2026
| Round Duct Diameter | GI Gauge | Thickness (mm) | Joint Type | Max Pressure (Pa) | Typical Use India |
|---|---|---|---|---|---|
| Up to 150 mm | 26G | 0.50 | Slip socket / beaded | 500 Pa | FCU spigots, bathroom exhaust, small extract |
| 151 – 300 mm | 24G | 0.63 | Slip socket / drive slip | 500 Pa | Branch supply air, circular diffuser feeds |
| 301 – 600 mm | 24G | 0.63 | Beaded slip / TDC | 750 Pa | Spiral duct main supply, AHU connections |
| 601 – 900 mm | 22G | 0.80 | TDC / flange | 750 Pa | Main AHU discharge, large return air |
| 901 – 1200 mm | 22G | 0.80 | Angle flange | 1000 Pa | Plant room main circular ducts, risers |
| 1201 – 1500 mm | 20G | 1.00 | Angle flange | 1000 Pa | Industrial circular supply, large smoke extract |
| Above 1500 mm | 18G | 1.25 | Angle flange | 1500 Pa | Large industrial ventilation, tunnel fans |
* Spiral-wound round ducts (mechanically formed seam) can use one gauge lighter than straight-seam fabricated ducts at the same diameter due to the inherent stiffness of the spiral seam. Minimum gauge for any round HVAC duct is 26G per SMACNA. IS 655 does not distinguish between spiral and straight-seam — apply the straight-seam table conservatively unless SMACNA is specified. Verify with project consultant on large-diameter spiral ducts.
Zinc coating class, base steel grade and IS 277 compliance requirements for HVAC GI sheet selection in India
Joint type determines leakage class, pressure rating, and installation speed — choose correctly for your pressure class
Max Pressure: 250–500 Pa (Low Pressure only). Use: Small ducts up to 750mm, branch supply & return air, FCU ducting. Sealant: Duct mastic sealant mandatory on all S/D joints. India: Most widely used jointing method in Indian residential and light commercial HVAC. Note: Do not use above 500 Pa — joints open under higher pressure.
Max Pressure: 750–1000 Pa (Low–Medium). Use: Ducts 750mm–1800mm, main supply ducts, AHU discharge. Gasket: Closed-cell gasket in TDC corner pieces mandatory. India: Standard specification on large commercial projects (hospitals, malls, offices). Faster installation than angle flanges — now preferred over S/D on most MEP projects above 750mm.
Max Pressure: 1000–1500 Pa (Medium). Use: Large ducts 1500–2250mm, high-rise main risers, variable air volume (VAV) trunk ducts. Sealing: Closed-cell foam gasket + sealant required. India: Specified on high-rise towers (20+ floors), major airport HVAC, metro rail station ventilation.
Max Pressure: Up to 2500 Pa (High Pressure). Use: Ducts above 1800mm, industrial exhaust, smoke extract systems, high-velocity systems. Bolting: MS bolts M8 @ max 150mm centres. India: Used in industrial plants, data centres, tunnels, and smoke extract for tall buildings per NBC 2016. Labour-intensive but rated for highest pressure class.
Leakage Class Reference — SMACNA (Duct Air Leakage)
Per-metre weight of standard rectangular GI duct sections at common gauge selections — for hanger design and transport logistics
| Duct Size (W×H mm) | GI Gauge | Thickness (mm) | Perimeter (mm) | Sheet Area/m Run (m²/m) | Weight/m Run (kg/m) | Weight kg/m² |
|---|---|---|---|---|---|---|
| 200 × 150 mm | 26G | 0.50 | 700 | 0.700 | 2.94 | 4.20 |
| 400 × 300 mm | 24G | 0.63 | 1400 | 1.400 | 7.42 | 5.30 |
| 600 × 400 mm | 24G | 0.63 | 2000 | 2.000 | 10.60 | 5.30 |
| 800 × 600 mm | 22G | 0.80 | 2800 | 2.800 | 17.64 | 6.30 |
| 1000 × 600 mm | 22G | 0.80 | 3200 | 3.200 | 20.16 | 6.30 |
| 1200 × 800 mm | 22G | 0.80 | 4000 | 4.000 | 25.20 | 6.30 |
| 1500 × 1000 mm | 20G | 1.00 | 5000 | 5.000 | 39.35 | 7.87 |
| 2000 × 1200 mm | 20G | 1.00 | 6400 | 6.400 | 50.37 | 7.87 |
| 2500 × 1500 mm | 18G | 1.25 | 8000 | 8.000 | 79.28 | 9.91 |
* Weight calculated as: Perimeter (m) × 1 m length × GI density (7.87 kg/m²/mm) × thickness (mm). GI density includes Z275 zinc coating (7.87 kg/m²/mm for GI vs 7.85 for plain MS). Use these values for hanger load calculation and transport manifests. For custom sizes, use our GI Sheet Weight Calculator.
Common compliance mistakes and best practices on Indian HVAC projects
Practical guidance for HVAC contractors, MEP engineers and QA teams working on Indian projects in 2026
GI coil for duct fabrication in India is priced per kg. Calculate the total GI weight required for your duct schedule before enquiry — use our GI Sheet Weight Calculator for each gauge zone. Add 8–12% cutting waste for rectangular ducts with fittings. This prevents under-ordering and emergency purchases at higher spot rates.
For projects above 2000m² duct area, investing in a TDC roll-forming machine (available in India from ₹80,000–₹2,50,000) pays back in reduced material cost (no angle flanges), faster installation, and better airtightness compared to angle-flanged joints. TDC is now the Indian commercial HVAC market standard for ducts 750mm–2250mm.
Order GI coil in widths that match your duct panel dimensions. Common Indian coil widths are 1000mm, 1220mm, and 1250mm. For 600mm high ducts, a 1220mm coil allows two 600mm strips with 20mm for seam — zero waste. Poor coil width selection is the biggest avoidable cost on HVAC sheet metal contracts.
Z180-coated GI coil is frequently substituted for Z275 in India — the visual appearance is identical. Request the mill test certificate (MTC) on every coil delivery and verify the coating weight value. A magnetic coating thickness gauge (₹3,000–₹8,000) on site allows quick coating spot-checks before the coil goes into the plasma cutter or shear.
GI coil stored in open, non-ventilated workshops during Indian monsoon (June–September) develops white rust on outer wraps within 1–2 weeks. Store coils vertically (eye to sky), cover with breathable tarpaulin, and ensure ventilation. Discard any coil showing white rust on the first 2–3 wraps — contamination damages the plasma cutter guide and TDC machine rollers.
At fire damper locations in duct systems, SMACNA and NBC 2016 require the duct section within the fire-rated wall/slab to be fabricated in minimum 1.6mm (16G) GI sheet — regardless of the system duct gauge. Failure to uprate the gauge at fire damper pockets is a common site quality deficiency caught during fire system commissioning and audit.
Tools and references to support your HVAC sheet metal design and procurement in India
Common questions from HVAC contractors, MEP engineers and procurement teams in India