Round duct surface Calculator: measuring notes and method
Measure the straight duct surface
Use the outside diameter of the surface being covered. Inside diameter would understate the external circumference.
Allow separately for fittings
Elbows, branches, transitions and flanges are not represented by the entered straight length. For insulation, decide whether you need the duct surface or the outside surface of the insulation.
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 diameter
- 300 mm
- Duct length
- 10 m
- Waste allowance
- 10 %
Sheet area with waste: 10.3673 m²
Imperial worked example
- Duct diameter
- 11.811 in
- Duct length
- 32.8084 ft
- Waste allowance
- 10 %
Sheet area with waste: 111.5922 ft²
Follow the calculation
Lengths in this working are in metres.
- Lateral area = π × 0.3 × 10 = 9.424778 m².
- Allowance multiplier = 1 + 10 ÷ 100 = 1.1.
Reviewed 19 October 2026. How calculations are checked · Report a correction