Does this produce a shop-ready flat pattern?

No. It estimates four-panel surface area from centered rectangular interfaces.

Are flanges included?

No. Flange and connection material should be taken off separately.

Can I use this for offset transitions?

The model assumes a centered transition. A deliberately offset transition can require a different developed geometry.

Transition fabrication

Duct Transition Material Calculator

Estimate sheet-metal area for a centered rectangular-to-rectangular duct transition from both interfaces and the transition length.

Transition geometry

Large-end width

Large-end height

Small-end width

Small-end height

Transition length

Fabrication allowance (%)

Dimension unit

The model assumes both rectangular interfaces are centered on the same axis. It estimates four planar panels and should not be treated as a universal shop flat pattern.

Enter both duct interfaces and transition length.

Transition sheet area

Planning developed area

—

—Top/bottom area

—Side area

—Geometric total

—Allowance

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Centered rectangular transition method

The model treats the transition as four planar panels. The top and bottom panels respond to the change in height, while the side panels respond to the change in width.

Top/bottom slant = √(L² + (ΔH ÷ 2)²)
Side slant = √(L² + (ΔW ÷ 2)²)
Top + bottom area = 2 × average width × top/bottom slant
Side area = 2 × average height × side slant

Why each panel is shown separately

Duct transitions are not simply boxes with one dimension changed. Displaying the panel contributions makes it easier to understand which geometric change drives the material area.

Worked example

For 900 × 500 mm reducing to 600 × 400 mm over 800 mm, the width change is 300 mm and the height change is 100 mm. The calculator derives the corresponding slants and sums the four panel estimates.

When a detailed fabrication development is needed

Use an approved shop drawing or CAD development when exact cutting dimensions, seams, corners, offsets, reinforcement or flange details matter. This calculator is for transparent material estimation.

Frequently Asked Questions

Can it calculate an offset transition?

The model is centered. A deliberate offset requires a different geometry treatment.

Does it include flanges?

No.

Is the result exact?

No. It is a four-panel geometric estimate.