This guide covers everything you need to know about Galvanized Steel Gauge Chart. Key takeaways: detailed specifications, cost comparisons, pros and cons, maintenance requirements, and expert recommendations from Preda Machine — a leading HVAC duct machinery manufacturer since 2008, serving 60+ countries.
In This Guide
Galvanized Steel Gauge to MM Conversion Chart
| Gauge | Thickness (mm) | Thickness (inch) | Weight (kg/m²) | Common HVAC Use |
|---|---|---|---|---|
| 28 GA | 0.40mm | 0.0157" | 3.25 kg/m² | Small residential duct, liners |
| 26 GA | 0.50mm | 0.0197" | 4.06 kg/m² | Residential, small commercial |
| 24 GA | 0.60mm | 0.0236" | 4.87 kg/m² | Standard commercial duct |
| 22 GA | 0.80mm | 0.0315" | 6.49 kg/m² | Medium commercial, fittings |
| 20 GA | 1.00mm | 0.0394" | 8.11 kg/m² | Large duct, high pressure |
| 18 GA | 1.20mm | 0.0472" | 9.73 kg/m² | Heavy-duty, industrial |
| 16 GA | 1.50mm | 0.0591" | 12.17 kg/m² | Very large duct, structural |
| 14 GA | 2.00mm | 0.0787" | 16.22 kg/m² | Special applications |
Industry Standard
24 gauge (0.60mm) galvanized steel is the most common thickness for commercial HVAC ductwork, specified by SMACNA for ducts up to 48" wide at pressures up to 2" WG. 26 gauge is used for smaller ducts and residential applications.
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SMACNA Duct Construction Standards
SMACNA's "HVAC Duct Construction Standards" specifies minimum galvanized steel gauge based on duct dimension and operating pressure:
| Duct Width | ½" WG (Low) | 1" WG (Medium) | 2" WG (High) | 3" WG (Very High) |
|---|---|---|---|---|
| Up to 12" (300mm) | 28 GA | 26 GA | 24 GA | 22 GA |
| 13-24" (300-600mm) | 26 GA | 24 GA | 24 GA | 22 GA |
| 25-36" (600-900mm) | 24 GA | 24 GA | 22 GA | 20 GA |
| 37-48" (900-1200mm) | 24 GA | 22 GA | 22 GA | 20 GA |
| 49-72" (1200-1800mm) | 22 GA | 22 GA | 20 GA | 18 GA |
| 73-96" (1800-2400mm) | 20 GA | 20 GA | 18 GA | 16 GA |
Note: These are minimum gauges. Additional reinforcement (TFD flanges, standing seams, cross breaks) may be required for larger ducts at higher pressures. Always consult the latest SMACNA manual for project-specific requirements.
Weight per Standard Sheet
| Gauge | 4×8 ft Sheet (kg) | 4×10 ft Sheet (kg) | 1×2m Sheet (kg) |
|---|---|---|---|
| 28 GA (0.40mm) | 9.6 kg | 12.0 kg | 6.5 kg |
| 26 GA (0.50mm) | 12.0 kg | 15.0 kg | 8.1 kg |
| 24 GA (0.60mm) | 14.4 kg | 18.0 kg | 9.7 kg |
| 22 GA (0.80mm) | 19.2 kg | 24.0 kg | 13.0 kg |
| 20 GA (1.00mm) | 24.0 kg | 30.0 kg | 16.2 kg |
| 18 GA (1.20mm) | 28.8 kg | 36.0 kg | 19.5 kg |
| 16 GA (1.50mm) | 36.0 kg | 45.0 kg | 24.3 kg |
Weight calculation: Weight (kg) = Area (m²) × Thickness (mm) × 7.85 (density of steel). Add approximately 5% for galvanized coating weight.
Cost Comparison
| Gauge | Cost per 4×8 Sheet (USD) | Cost per m² (USD) | Relative Cost |
|---|---|---|---|
| 28 GA | $25-$35 | $8.5-$12 | 1.0x |
| 26 GA | $30-$42 | $10-$14 | 1.2x |
| 24 GA | $35-$50 | $12-$17 | 1.4x |
| 22 GA | $45-$65 | $15-$22 | 1.8x |
| 20 GA | $55-$80 | $18-$27 | 2.2x |
| 18 GA | $70-$100 | $24-$34 | 2.8x |
Prices are approximate factory-direct rates for G90 galvanized steel in 2026. Actual prices vary with steel market conditions, quantity, and supplier. Coil stock (for auto duct lines) is typically 10-15% cheaper per kg than sheet stock.
How to Choose the Right Gauge
- Check project specifications — The mechanical engineer will specify the required gauge based on SMACNA standards and project pressure class.
- Consider duct size — Larger ducts require thicker gauges to prevent flexing and oil-canning.
- Factor in pressure — Higher static pressure systems need thicker gauges and more reinforcement.
- Balance cost vs durability — Using a thicker gauge than required increases material cost by 20-40% but may reduce reinforcement needs.
- Check equipment capacity — Ensure your auto duct line or lockformer can handle the specified thickness.
Common Gauge Selection Mistakes
- Using 26 GA for everything — 26 GA is too thin for ducts over 24" wide at medium pressure. The duct will flex and make noise.
- Ignoring fittings — Fittings (elbows, tees, reducers) should be one gauge heavier than straight duct due to higher stress concentrations.
- Confusing gauge with thickness — Gauge numbers are inverse: higher gauge = thinner material. 24 GA = 0.60mm, not 2.4mm.
- Underestimating transportation — Thinner gauges dent more easily during shipping and handling. Use 24 GA minimum for pre-fabricated duct shipped to site.
Frequently Asked Questions
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Zinc Coating Weights and What the Designation Means
Gauge tells you the thickness; the coating designation tells you how long it will last. Two sheets of identical gauge can differ in service life by a factor of three purely on coating weight.
| Designation | Coating mass (both sides) | Approx. thickness per side | Typical service life, indoor dry | Typical use |
|---|---|---|---|---|
| Z100 | 100 g/m² | 7 µm | 10 – 20 years | Indoor, conditioned air |
| Z180 | 180 g/m² | 13 µm | 20 – 35 years | Standard commercial duct |
| Z275 | 275 g/m² | 20 µm | 35 – 50 years | Exposed, humid, kitchen extract |
| Z350 | 350 g/m² | 25 µm | 50+ years | Coastal, industrial, external |
| AZ150 (aluzinc) | 150 g/m² | 20 µm | 40 – 60 years | High-temperature and external |
Cut Edges Are the Weak Point
Zinc protects steel sacrificially, so a sheared edge is protected for a few millimetres either side of the cut. But a welded or plasma-cut edge loses the coating entirely. On kitchen extract and external duct, specify a heavier coating or plan to touch up cut edges — this is where corrosion always starts.
Calculating Sheet and Duct Weight
Weight matters for hanging design, for transport, and for quoting. Use the base steel thickness, then add roughly 3–5% for the zinc layer.
Weight (kg/m²) ≈ thickness (mm) × 7.85
| Gauge | Thickness (mm) | Sheet weight (kg/m²) | Standard 1220 × 2440 sheet | 1 m of 400 mm round duct |
|---|---|---|---|---|
| 26 ga | 0.55 | 4.3 | 12.8 kg | 5.4 kg |
| 24 ga | 0.70 | 5.5 | 16.4 kg | 6.9 kg |
| 22 ga | 0.85 | 6.7 | 19.9 kg | 8.4 kg |
| 20 ga | 1.00 | 7.9 | 23.4 kg | 9.9 kg |
| 18 ga | 1.25 | 9.8 | 29.2 kg | 12.3 kg |
| 16 ga | 1.60 | 12.6 | 37.4 kg | 15.8 kg |
For a duct run, multiply the per-metre figure by length and add 15–25% for flanges, hangers and fittings. A 60 m run of 400 mm duct at 22 ga is roughly 500 kg as installed — a number worth knowing before you spec the hangers.
Gauge and Its Effect on Welding and Forming
| Gauge | Pittsburgh lock | TDF flange | Spot weld | Plasma cut quality |
|---|---|---|---|---|
| 26 ga (0.55 mm) | Excellent | Acceptable, needs care | Burn-through risk | Excellent, fast |
| 24 ga (0.70 mm) | Excellent | Good | Good | Excellent |
| 22 ga (0.85 mm) | Good | Good | Good | Good |
| 20 ga (1.00 mm) | Good, check machine limit | Good | Good | Good |
| 18 ga (1.25 mm) | Machine dependent | Needs heavier former | Good | Slower, more dross |
| 16 ga (1.60 mm) | Beyond most lockformers | Beyond standard formers | Good | Slow |
Above 1.2 mm the economics shift: forming forces rise steeply and the lock becomes impractical. That is the point at which shops move to welded construction and a plasma or laser cut rather than a lockformer.
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