Insights · Reference
Duct drawings talk in gauge; machines are rated in millimetres. This reference converts sheet metal gauge to millimetres and inches for galvanised duct steel, then shows which SBKJ duct machine forms each gauge — with the machine thickness ranges taken verbatim from the SBKJ Product Catalog 2026.
Sheet metal gauge is an older imperial system: the higher the number, the thinner the metal. It is not linear, and the thickness a gauge number represents differs between galvanised steel, stainless and aluminium. The table below is the Galvanized Steel Sheet Gauge standard — the one that applies to the galvanised coil most HVAC duct is built from — with nominal inch and millimetre equivalents and where each gauge typically lands in duct work.
| Gauge (GSG) | Nominal inch | Nominal mm | Typical duct use |
|---|---|---|---|
| 30 | 0.0157″ | 0.40 mm | Light flexible-duct collars, lightest fittings |
| 28 | 0.0187″ | 0.48 mm | Light round duct and fittings |
| 26 | 0.0217″ | 0.55 mm | Small low-pressure duct — the common light gauge |
| 24 | 0.0276″ | 0.70 mm | General low-pressure rectangular and round duct |
| 22 | 0.0336″ | 0.85 mm | Mid-size duct, higher pressure |
| 20 | 0.0396″ | 1.01 mm | Large duct, medium/high pressure |
| 18 | 0.0516″ | 1.31 mm | Large/high-pressure duct, plenums, heavy fittings |
| 16 | 0.0635″ | 1.61 mm | Industrial duct, dust/fume, heavy plenum |
| 14 | 0.0785″ | 1.99 mm | Heavy industrial, abrasion-handling duct |
| 12 | 0.1084″ | 2.75 mm | Plate work — cut, not roll-formed as duct |
Gauge values follow the Galvanized Steel Sheet Gauge standard and are nominal coated thicknesses — confirm against your material certificate. Stainless steel and aluminium use different gauge-to-thickness scales; when in doubt, specify in millimetres.
The next table is the other half of the answer: the thickness range each SBKJ duct-forming machine actually handles, in galvanised, stainless and aluminium, taken verbatim from the Product Catalog 2026. Read it against the chart above to match a gauge to a machine.
| Machine | Model | Galvanised | Stainless | Aluminium |
|---|---|---|---|---|
| Spiral tubeformer (round duct) | SBTF-1500 | 0.4–1.2 mm | 0.4–0.8 mm | — |
| SBTF-1500C | 0.4–1.2 mm | 0.4–1.2 mm | — | |
| SBTF-1602 | 0.4–2.0 mm | 0.4–1.2 mm | 0.4–3.0 mm | |
| SBTF-2020 | 0.4–2.0 mm | 0.4–1.2 mm | 0.4–3.0 mm | |
| Auto duct line (rectangular) | SBAL-V | 0.5–1.5 mm | 0.5–1.5 mm | — |
| SBAL-III | 0.5–1.2 mm | 0.5–1.2 mm | — | |
| SBAL-II | 0.5–1.2 mm | 0.5–1.2 mm | — |
Source: SBKJ Product Catalog 2026, manufacturer nameplate specifications. Spiral tubeformers form round duct Φ80–Φ2500 mm; auto duct lines form rectangular TDF duct up to 1500 mm wide. Full per-model figures: spiral comparison and auto line comparison.
30–20 gauge (0.40–1.0 mm). The everyday HVAC range. Every SBKJ spiral tubeformer and every auto duct line forms it — they all run down to 0.4–0.5 mm. If your work is standard low-pressure galvanised duct, any model in the range covers it.
18 gauge (≈1.3 mm). Above 1.2 mm you narrow the field. In round duct that means the SBTF-1602 or SBTF-2020 (galvanised to 2.0 mm); in rectangular, the SBAL-V (to 1.5 mm). The lighter SBTF-1500/1500C and SBAL-III/II top out at 1.2 mm.
16–14 gauge (≈1.6–2.0 mm). Heavy-gauge galvanised round duct is the SBTF-1602 / SBTF-2020 territory (0.4–2.0 mm). For seam-welding that gauge, the SBFN-100 seam welder reaches 2.0 mm and the medium-frequency welder reaches 3.0 mm in stainless or carbon steel.
12 gauge and heavier (2.75 mm+). This is plate, not roll-formed duct. You cut it rather than form it: shearing handles 2–4 mm, plasma and laser to 8 mm. Aluminium is the exception — the SBTF-1602/2020 roll aluminium to 3.0 mm.
Gauge selection is set by duct size and pressure class, not preference. As a working rule of thumb for low-pressure rectangular duct:
Treat this as orientation, not specification. Always confirm the required gauge against SMACNA HVAC Duct Construction Standards or AS/NZS 4254 for your exact size, pressure class and reinforcement — the standard, not the rule of thumb, governs the job.
Tell us your gauge and duct type for a machine recommendation →
24 gauge galvanised steel is a nominal 0.70 mm (0.0276″). It is one of the most common HVAC duct gauges, and every SBKJ spiral tubeformer and auto duct line forms it — they all run down to 0.4–0.5 mm.
In galvanised steel, roughly 30 to 14 gauge (0.4–2.0 mm). SBTF-1602/2020 form galvanised 0.4–2.0 mm and aluminium 0.4–3.0 mm; SBTF-1500/1500C form 0.4–1.2 mm; SBAL-V forms 0.5–1.5 mm; SBAL-III/II form 0.5–1.2 mm. For plate over 2 mm, shearing, plasma and laser cut up to 8 mm.
Gauge is an older imperial scale where a higher number means thinner metal (26 ga is thinner than 20 ga). Millimetres measure thickness directly. The two are not linear, and the gauge-to-mm value differs by material, so convert with the correct standard. SBKJ catalogue figures are in millimetres.
Most low-pressure duct is 26 to 20 gauge (0.55–1.0 mm), increasing with duct size and pressure. Small low-pressure duct uses 26 gauge; large or higher-pressure duct uses 22–18 gauge. Confirm against SMACNA or AS/NZS 4254 for your pressure class.
Tell us the gauge and duct type you build and an SBKJ engineer will tell you which machine forms it — within 12 hours, and not a salesperson.
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