Calculate sheet metal area, SMACNA gauge and duct weight for rectangular and round ductwork in galvanized steel, aluminum or stainless — with a seam and waste allowance, insulation area and a BOQ summary by gauge.
Estimating values. Actual gauge depends on pressure class, reinforcement spacing and the project specification; weights use nominal galvanized sheet weights and exclude flanges, supports and sealant.
| Size | Gauge | Length | Area | Weight |
|---|
| Gauge | Thickness | Area | Weight |
|---|
The surface area is the same figure you need for duct insulation or cladding, so the tool reports it separately — without the waste allowance, because insulation has its own waste factor.
For rectangular duct up to 2 in. w.g., the gauge is chosen by the longest side; round duct by its diameter. The auto mode uses this widely used simplified table:
| Gauge | Rectangular — longest side | Round — diameter |
|---|---|---|
| 26 ga | up to 12″ (300 mm) | up to 14″ (350 mm) |
| 24 ga | 13″–30″ (up to 750 mm) | 15″–26″ (up to 650 mm) |
| 22 ga | 31″–54″ (up to 1350 mm) | 27″–36″ (up to 900 mm) |
| 20 ga | 55″–84″ (up to 2100 mm) | 37″–50″ (up to 1250 mm) |
| 18 ga | over 84″ (2100 mm) | over 50″ (1250 mm) |
Medium and high pressure classes, longer reinforcement spacing and some project specifications (for example DW/144 in the UK and Gulf) call for heavier sheet. Select a fixed gauge or a custom thickness when your specification differs.
| Gauge | Nominal thickness | Weight | Weight (metric) |
|---|---|---|---|
| 26 ga | 0.551 mm (0.0217″) | 0.906 lb/ft² | 4.42 kg/m² |
| 24 ga | 0.701 mm (0.0276″) | 1.156 lb/ft² | 5.64 kg/m² |
| 22 ga | 0.853 mm (0.0336″) | 1.406 lb/ft² | 6.86 kg/m² |
| 20 ga | 1.006 mm (0.0396″) | 1.656 lb/ft² | 8.09 kg/m² |
| 18 ga | 1.311 mm (0.0516″) | 2.156 lb/ft² | 10.53 kg/m² |
| 16 ga | 1.613 mm (0.0635″) | 2.656 lb/ft² | 12.97 kg/m² |
20 m of 600 × 400 mm galvanized duct.
Sheet is lost to longitudinal seams (Pittsburgh lock, snap lock), transverse joints, stiffeners and off-cuts. Estimators typically add 10–25% depending on how many fittings the run has and how the shop nests the sheet. Straight runs on a coil line sit at the low end; short runs with many fittings at the high end.
Multiply the duct's surface area by the weight per square metre of its gauge. For rectangular duct the area is 2 × (W + H) × length; for round duct it is π × D × length. Add an allowance for seams, joints and waste.
Under SMACNA low-pressure construction, 26 ga up to 12 in (300 mm) longest side, 24 ga up to 30 in (750 mm), 22 ga up to 54 in (1350 mm), 20 ga up to 84 in (2100 mm) and 18 ga above that. Higher pressure classes may need heavier sheet.
About 1.156 lb/ft², or 5.64 kg/m², including the zinc coating. Its nominal thickness is 0.701 mm.
Almost. Insulation wraps the outer surface, so its area is the duct's net surface area. The tool reports it without the sheet-metal waste allowance, because insulation has its own waste factor.
Typically 10 to 25 percent. Long straight runs need less; runs with many fittings, transitions and short pieces need more. Use your fabricator's historical figure when you have one.
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