Rectangular duct surface Calculator: measuring notes and method
Measure the outside dimensions
Use outside width and height for sheet or wrap area. Inside clear dimensions describe the air passage rather than the surface being covered.
Separate fabrication details
Prepare a separate material schedule for fittings and fabrication details. A percentage applied to straight duct area does not identify each seam or transition.
Formula
Round surface = π × diameter × length. Rectangular surface = 2 × (width + height) × length. Apply waste.
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Duct width
- 0.4 m
- Duct height
- 0.25 m
- Duct length
- 10 m
- Waste allowance
- 10 %
Sheet area with waste: 14.3 m²
Imperial worked example
- Duct width
- 1.3123 ft
- Duct height
- 0.8202 ft
- Duct length
- 32.8084 ft
- Waste allowance
- 10 %
Sheet area with waste: 153.9239 ft²
Follow the calculation
Lengths in this working are in metres.
- Lateral area = 2 × (0.4 + 0.25) × 10 = 13 m².
- Allowance multiplier = 1 + 10 ÷ 100 = 1.1.
Reviewed 19 October 2026. How calculations are checked · Report a correction
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