Duct Area Calculator
Developed area for rectangular or round duct, plus insulation and steel weight.
Quick answer
With the default inputs: Section shape Rectangular duct · Long side 1000 mm · Short side 400 mm etc.Developed area 84.00 m²
Edit the inputs below to recalculate.Inputs
Result
Developed area
84.00m²
| Area per metre | 2.800m²/m |
| Total length | 30.0m |
| Insulation volume | 2.628m³ |
| Cladding area | 91.20m² |
| Galvanised steel weight | 494.6kg |
| Sheet weight per m² | 5.89kg/m² |
| With 10% waste | 92.40m² |
Measurement rules: rectangular duct area = 2 × (long + short) × length, round = π × D × length; flanges, stiffeners, access doors and test holes are not deducted, and seam allowances are not added. Insulation volume is the net volume enclosed by the insulated outline, (a+2δ)(b+2δ) − a·b, not area times thickness — the simple product misses the four corners.
Formula
| Rectangular area | A = 2 × (long + short) × length |
| Round area | A = π × D × length |
| Rectangular insulation | V = [(a + 2δ)(b + 2δ) − a·b] × length |
| Round insulation | V = π ÷ 4 × [(D + 2δ)² − D²] × length |
| Steel weight | W = A × thickness × 7850 |
30 m of 1000×400 rectangular duct at 0.75 mm with 30 mm insulation: 84 m² area, 2.628 m³ insulation, 91.2 m² cladding, 494.6 kg of galvanised sheet.
FAQ
Why are flanges and access doors not deducted?
The measurement rule says so explicitly: duct area is taken without deducting access doors, test holes, supply or return grilles, flanges, stiffeners or hanger brackets, and seam overlap is not added either. These items are small relative to the duct body and the effort of deducting each exceeds the error. So pricing is simply area times rate.
Why not just multiply area by thickness?
Insulation wraps the outside, so the four corners are covered twice. For 1000×400 duct with 30 mm: the naive method gives 2.8 × 0.03 × 30 = 2.52 m³ while the correct one gives [(1.06×0.46) − 0.4] × 30 = 2.628 m³, a 4% shortfall. The smaller the section and the thicker the insulation, the bigger the gap — and elastomeric insulation is bought by volume.
How is duct sheet thickness chosen?
By system pressure class and the long side. Low pressure (up to 500 Pa): 0.5 mm up to 320, 0.6 to 630, 0.75 to 1000, 1.0 to 2000, 1.2 above. Medium (1500 Pa) and high (2500 Pa) classes go one step thicker and need stiffening frames. Undersized sheet pants audibly in service.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.