Round-to-Rectangular Duct Transition Calculator
Estimate material area for a centered transition between a circular duct and a rectangular duct, with the two interface perimeters kept visible.
Interface and transition inputs
Round-to-rectangular transition result
How the mixed-shape estimate works
The round interface perimeter is πD. The rectangular interface perimeter is 2(W + H). The calculator uses the average of these perimeters and a representative slant based on transition length and the difference in cross-sectional area.
Rectangular perimeter = 2(W + H)
Area difference = |round area − rectangular area|
Representative radial change = √(|area difference| ÷ π)
Representative slant = √(L² + radial change²)
Approx. surface area = average perimeter × representative slant
Why this is intentionally approximate
Round-to-rectangular transitions can be broken into multiple developed panels or gores. Different segmentation and fabrication methods can produce different flat patterns even when the interface dimensions are identical.
Worked example
For a 500 mm round end, 600 × 400 mm rectangular end and 700 mm transition length, the calculator first compares the two interface perimeters, then develops a representative slant to produce an approximate surface area before the fabrication allowance is applied.
When to use a detailed fabrication method
Move to an approved drawing or CAD development when the fitting must be fabricated to exact cut dimensions, when the transition is offset, or when seams, corners, reinforcement and flanges materially affect the final material takeoff.
Frequently Asked Questions
Can the rectangular end be larger than the round end?
Yes. The approximate model works with either direction of cross-section change.
Does this calculate a flat cutting pattern?
No.
Are flanges included?
No.