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 · Short side 400 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.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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.
How to work it out
- Compute the developed area: rectangular A = 2 × (long side + short side) × length; circular A = π × D × length. It is perimeter times length, not cross-sectional area.
- Add an allowance of 8-12% for seams and flanges, at the upper end for small ducts, dense flanging and many fittings.
- Compute insulation volume: rectangular V = [(a+2δ)(b+2δ) − a·b] × length; circular V = π÷4 × [(D+2δ)² − D²] × length, where δ is insulation thickness.
- Compute cladding area using the outside perimeter after insulation, not the duct perimeter. This item is often 10% of the total price.
- Steel weight W = A × thickness × 7850 kg. Thickness follows GB 50243-2016: 0.5 mm up to 320 mm long side, 0.6 at 340-450, 0.75 at 480-1000, 1.0 at 1120-2000, 1.2 at 2500-4000.
- Report the three outputs — area, insulation volume and steel weight — because they map onto three different pricing bases: fabrication, insulation and material purchase.
Where it goes wrong
- Using the duct perimeter instead of the insulated outside perimeter. With 30 mm insulation a 1000×400 duct goes from 2800 mm to 3460 mm perimeter, so cladding is under-ordered by 24%.
- Applying 0.75 mm across the board. A 2000 mm duct at 0.75 mm drums and rattles; the code calls for 1.0 mm. Small ducts at 1.0 mm are simply waste.
- Omitting the seam allowance from the developed area. The shop finds itself 10% short, splices on site, and adds leak paths and visible defects.
- Treating flow ÷ velocity as the developed area. That gives the cross-section, a completely different quantity — the two differ by an order of magnitude.
Material and cost
Galvanised ductwork fabricated and installed runs 90-150 CNY/m² including flanges and hangers, with insulation extra; stainless 200-350. Elastomeric insulation at 30 mm is about 25-40 CNY/m² of insulated surface; glass wool is half that but needs cladding. Prices vary widely with thickness, region and volume.
On site
- Duct leakage is a major energy item. Medium-pressure systems need light-leak or leakage testing per the code, and flange gaskets should be closed-cell sponge or rubber — ordinary foam takes a permanent set.
- Insulation thickness is governed by both condensation control and energy, and both must be checked. 30 mm elastomeric is typical for supply ducts; chilled-water pipes need more and a summer condensation check.
- Transitions, tees and elbows must be developed properly rather than treated as sums of rectangles — fittings typically run 15-25% more area than a straight-duct estimate.
- Add reinforcement frames where the long side exceeds 1250 mm and sections are longer than 1.2 m, otherwise the duct hammers in operation and seams can crack.
Basis
- GB 50243-2016 Code for acceptance of construction quality of ventilation and air conditioning works, duct fabrication
- GB 50736-2012 Design code for heating, ventilation and air conditioning of civil buildings, ventilation
- JGJ 141-2004 Technical specification for ventilation ducts
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Duct Area Calculator. 2026-10-25. https://jianzhujisuan.com/en/duct-area
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.