# Duct Area Calculator

Developed area for rectangular or round duct, plus insulation and steel weight.

- Page: https://jianzhujisuan.com/en/duct-area
- Category: M&E (https://jianzhujisuan.com/en/c/me)
- Chinese version: https://jianzhujisuan.com/zh/duct-area.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Section shape Rectangular duct, Long side 1000 mm, Short side 400 mm, Diameter D 400 mm, Total duct length 30 m, Sheet thickness 0.75 mm, Insulation thickness 30 mm): **Developed area 84.00 m²**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Section shape | select | Rectangular duct | — |  |
| Long side | number | 1000 | mm | Ignored for round duct |
| Short side | number | 400 | mm | Ignored for round duct |
| Diameter D | number | 400 | mm | Only used for round duct |
| Total duct length | number | 30 | m |  |
| Sheet thickness | number | 0.75 | mm | Low pressure: 0.6 up to 630 mm, 0.75 to 1000, 1.0 to 2000 |
| Insulation thickness | number | 30 | mm | 25-40 mm elastomeric or glass wool is typical |

## 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

## Worked example

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.

## Questions

### 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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