Rectangular Duct Transition Calculator

Concepts behind this calculation

Calculate minimum axial length or check wall angles for centered and offset rectangular transitions. Offset modes evaluate all four walls and show signed center offsets in plan and elevation.

Transition length follows the lateral distance each wall must move between two rectangular openings. A centered transition divides each width or height change equally between opposite walls. With signed center offsets, one wall may remain flush while the opposite wall takes the full change, or both walls may slope in the same direction.

The selected criterion is a per-wall angle measured from the original duct axis. It differs from a two-wall included angle and from the angle of the shifted centerline. The largest absolute wall displacement controls the minimum axial length. Geometry alone does not determine pressure loss or a fabrication pattern.

How to use this calculator

1. Select Transition calculation and enter Start inside width, Start inside height, End inside width, and End inside height.
2. For offset modes, establish Geometry and per-wall limit reference and End planes and angle basis, then enter End-center horizontal offset and End-center vertical offset.
3. Enter Selected maximum wall angle from duct axis and, for a length check, Available axial length.
4. Review all four wall displacements and angle margins.

Calculation method and applicability

Offset wall displacements: left/right = X ∓ (W₂−W₁)/2; bottom/top = Y ∓ (H₂−H₁)/2. Lmin = max absolute wall displacement / tan(selected wall limit); each signed wall angle = atan(displacement/L). Centered modes use X=Y=0.

X and Y = signed end-center offsets (in); W/H = full clear width/height (in); L = axial separation (in). The selected wall limit and reported wall angles are degrees; trigonometric evaluation uses the corresponding angle conversion.

Input definitions and units

Transition calculation

Centered modes split each size change equally. Offset modes use signed end-center positions and evaluate all four walls. The selected limit is a per-wall angle from the original axis, not an included angle or universal code limit. Choices: Minimum length from selected wall angle; Check wall angles for an available length; Offset rectangular ends — minimum axial length; Offset rectangular ends — check four wall angles.

Start inside width (in)

Measure clear inside dimensions; use the same units for all four end dimensions and length.

Start inside height (in)

Use the rectangular opening before the transition.

End inside width (in)

Use the rectangular opening after the transition.

End inside height (in)

Clear inside height at the parallel end plane. The separate signed offsets position its center in offset modes.

Selected maximum wall angle from duct axis (°)

Enter the angle specified for this job, measured along one sloping wall from the duct axis. No universal default is assumed.

Available axial length (in)

Axial separation of the parallel end planes, measured along the original duct axis; this is not the sloping center-to-center distance. Applies to Transition calculation: Check wall angles for an available length or Offset rectangular ends — check four wall angles.

Geometry and per-wall limit reference

Identify the actual drawing, fixed opening-view axes, end-center offsets and project per-wall angle criterion. Confirm the criterion is not a two-wall included angle or an angle from the shifted centerline. A note does not authenticate the drawing. Applies to Transition calculation: Offset rectangular ends — minimum axial length or Offset rectangular ends — check four wall angles.

End planes and angle basis

Confirm parallel rectangular end planes, no twist/rotation between openings, four straight planar walls and a per-wall limit measured from the original axial direction. Multi-piece offsets require a separate layout. Choices: Not established; Parallel untwisted ends and actual per-wall criterion established. Applies to Transition calculation: Offset rectangular ends — minimum axial length or Offset rectangular ends — check four wall angles.

End-center horizontal offset (in; positive right)

Relative to the start center in one fixed opening view looking from start toward end. Positive moves the end center right; negative moves left. This is not an edge-to-edge offset. Applies to Transition calculation: Offset rectangular ends — minimum axial length or Offset rectangular ends — check four wall angles.

End-center vertical offset (in; positive up)

Same fixed view and start-center origin. Positive moves the end center up; negative moves down. Edge alignment follows from the actual widths/heights and center offset. Applies to Transition calculation: Offset rectangular ends — minimum axial length or Offset rectangular ends — check four wall angles.

How to interpret the result

Identify the controlling wall and compare each numerical angle with the project criterion. Use the schematic for orientation; its independent scales do not replace the dimensions.

Worked examples

Assumptions and limits

• Centered modes retain their original two-plane geometry. Offset modes require parallel rectangular end planes with fixed untwisted axes and four straight planar walls. Twisted/rotated ends, nonparallel planes, round/oval transitions and mitered multi-piece offsets need different geometry.
• All end dimensions, signed center offsets and axial separation use inches. Positive X is right and positive Y up in one fixed opening view looking from start toward end; offsets are center-to-center, not edge-to-edge.
• The selected maximum is a project per-wall projected tilt from the original axis. Included wall angles and center-to-center tilt are separate reported quantities. Parallel offset walls can have zero included angle while exceeding the selected per-wall limit.
• The 2022 NASA UFGS guide specification illustrates editable side-angle criteria; it is not a nationwide adopted requirement. ASHRAE fitting examples can use included angles. No source or project angle is automatically imposed here.
• This is geometry only, not a pressure-loss, noise, structural, full installed-envelope, fabrication flat-pattern, seam allowance, panel-cutting or code-compliance calculation. The diagram uses independent axial/transverse scales; numerical dimensions govern.
• Offset minimum mode permits same-size translated ends, but an unchanged aligned section has no taper minimum and is withheld. Positive available-length checks may assess that straight aligned case. Centered minimum mode retains its existing requirement for a size change.

Sources

• MachineCalcs: Duct Transition CalculatorDirect peer calculator exposes end dimensions, transition length and a selected maximum taper angle; its 400 × 250 mm to 300 × 200 mm, 15° example rounds to 187 mm minimum length.
• Planichapa: Rectangular Transition Flat Pattern CalculatorFabrication geometry reference for centered rectangular transitions; this calculator does not reproduce its sheet-metal allowances or flat pattern.
• ASHRAE Handbook, Fundamentals 2025, Chapter 21: Duct DesignDuct-design and fitting context only. It does not establish a universal transition angle; use the project-specified geometry limit.
• NASA UFGS 23 31 13.00 40, Metal Ducts, August 2022Physical/printed pages 20–21, Section 3.3.1.7 uses editable transition-side angles measured from the straight run. This is guide-specification context, not a universal code limit or a complete eccentric-transition formula.
• Ruthen Systems: Hopper and Rectangular Transition CalculatorActual parallel axis-aligned end geometry and signed X/Y center-offset controls observed. Fabrication-panel calculations and angles use their own conventions; this tool does not export their flat patterns or allowances.