Can I lay out a plenum box here?

The four sides, yes: the Rectangular Transition with the same rectangle at both ends draws them as one strip with the folds. The top, the takeoff holes and the seams are laid out by hand.

How do I lay out a plenum transition?

Each face is a flat trapezoid: its two widths, bottom and top, and its height on the face, √(h² + step²), the step being half the difference across that face. The Rectangular Transition page draws the four faces in one strip and gives the fold angle.

How do I make an offset plenum transition?

Offset one way, on the Discharge Chute page with the tilt at 0: the larger rectangle is the base, the smaller the outlet, and the offset along X is (X − N) / 2 for flat on one side. Offset both ways is not covered.

Does the site lay out duct board plenums?

No. Duct board is grooved and folded with its own allowances; the patterns here are for sheet metal.

What gauge should a plenum be?

The one the job spec or SMACNA's tables give. The page takes the gauge you pick from the list and works the pattern on its mean line.

Is the plenum layout free?

The figures and the drawing need no account. The full-size template takes a free account, which also exports 1 part a month in DXF; unlimited DXF export is the paid full access.

Plenum box and plenum transition layout: the sides, the offset and the round outlet

Updated on 28 October 2026

A plenum is the box on the supply or return of a furnace or air handler, and a plenum transition takes its opening to another size: the old plenum, the trunk line or a round duct. The site has no plenum page as such: the Rectangular Transition lays out the sides of the box and the transition between two rectangles, the Discharge Chute the transition offset to one side, and the Square to Round the round outlet. All three work in inches: set the page to inches and every result comes in fractions to the nearest 1/16 in, the thickness field takes a gauge from a list, and a free account adds the full-size template on Letter sheets and the DXF.

This page says what each of them draws for a plenum, with worked figures, and what is left to do by hand: the top, the takeoffs, the connections and the seams.

What the site draws, and what it does not

The piece	Page	Covered
Plenum box, the four sides	Rectangular Transition, with the same size at both ends	Yes: one strip with three folds
Plenum transition, centred: reducer, expander, or wider one way and narrower the other	Rectangular Transition	Yes
Plenum transition offset one way, flat on one side	Discharge Chute, with the tilt at 0	Yes, when the smaller opening lies inside the larger one seen from above
Plenum transition offset both ways, flat on two sides	None	No
Plenum to a round duct	Square to Round; off centre, Eccentric Rectangle to Round	Yes; see the duct transition guide
The top or end cap, the takeoff holes, starting collars	None	No
S and drive, TDC or TDF connectors, Pittsburgh and other seam allowances	None	No: every pattern is net
Duct board plenums	None	No: board is grooved and folded with its own allowances, not laid out on the mean line of a sheet

The plenum box

The four sides of a straight plenum box open into one strip: the perimeter by the height, folded at three corners and closed at the fourth. Give the Rectangular Transition the same rectangle at both ends and it draws that strip with its fold lines, worked on the mean line: each side comes off one thickness, so the box closes at the outside size you typed.

A 20 × 18 in box (508 × 457 mm), 24 in tall (610 mm), in 24 gauge: the strip is 75-7/8 × 24 in (1928 × 610 mm), 2 × (20 + 18) less four thicknesses, with the folds at 90°. Cut it on the middle fold and it makes two equal L's, for a box with two corner seams instead of one.

What the strip is not: the top, the hole for each takeoff, the connection to the unit and the seam allowances. The top is a flat panel laid out by hand, and the takeoff holes are cut where the runs leave, for a starting collar or a rectangular takeoff.

The plenum transition, centred

Every face of a transition between two rectangles is a flat trapezoid: its bottom edge is parallel to its top edge, so the face is its own pattern and nothing is approximated. Each face lays out with a tape and a square, from three figures.

Height of a face, measured on the face: s = √( h² + step² ) h is the length of the transition, square to both ends. The step is how far the face moves in or out, half the difference across it: (Y − Y2) / 2 for the faces of width X, (X − X2) / 2 for the faces of width Y.

Corner line: L = √( h² + ((X − X2) / 2)² + ((Y − Y2) / 2)² ) The fold between two faces, and the check: both slanted edges of every face you lay out must measure L.

A 24 × 20 in plenum (610 × 508 mm) to a 20 × 18 in unit opening (508 × 457 mm), 8 in long (203 mm), in 24 gauge:

Face	Bottom	Top	Height on the face
The two 24 in faces	24 in	20 in, stepped in 2 in at each end	8-1/16 in (205 mm)
The two 20 in faces	20 in	18 in, stepped in 1 in at each end	8-1/4 in (209 mm)

The corner lines come out 8-5/16 in (211 mm), and the page gives the fold between faces as 91.7°. In one piece, the way the page draws it, the strip needs a 69-3/4 × 43-11/16 in blank (1772 × 1110 mm) and 50 Letter sheets at full size: cut it on the folds and nest the four faces, or mark them from the table, and most of that offcut goes back on the rack.

The Rectangular Transition does not care which end you call the bottom: a reducer turned over is an expander. It also takes one end wider and the other narrower, 24 × 16 to 20 × 20 say, which is routine on a plenum. The Regular Pyramid Frustum draws the same fitting when both ends are square, but it takes the sizes across the corners; the Rectangular Transition takes the sides as you measure them.

Rectangular Transition flat pattern: drawing of the template with the worked example dimensions — The strip the Rectangular Transition page draws for its own example, a 31.5 × 23.6 in opening (800 × 600 mm) to 15.7 × 11.8 in (400 × 300 mm), 19.7 in apart (500 mm), in 3 mm plate: four flat trapezoids, the cut in purple, the three folds in orange and the seam at a corner in green.

Offset to one side: the Discharge Chute at 0°

The Discharge Chute is a hopper whose outlet can tilt and sit off centre. With the tilt at 0 it is a transition between two parallel rectangles, offset along one of them: the plenum transition that is flat on one side, against the back of the unit or a wall.

Field	On a plenum transition
X and Y, the base	The larger rectangle, with X in the direction of the offset.
N and Z, the outlet	The smaller rectangle: N along X, Z along Y. Z cannot be more than Y.
V, the offset	Along X, centre to centre. Flat on one side: V = (X − N) / 2.
ANG	0
H	The length of the transition, square to both ends.

The chute calls the larger end the base whichever way up the fitting goes in: a plenum widening from the unit is the same fitting, turned over. The same 24 × 20 to 20 × 18, 8 in long, flat at one 20 in end, is X 24, Y 20, N 20, Z 18, V 2. The flat end is a trapezoid 20 in to 18 in and 8 in tall; the opposite end steps in 4 in and measures 8-15/16 in on the face (227 mm); the two 24 in faces keep their 8-1/16 in, square at the flat end. In one piece the blank is 66-1/4 × 41-1/16 in (1682 × 1043 mm).

The offset runs along X only, and the outlet stays centred along Y: a transition flat on two sides, offset both ways, is not on any page.

Plenum to a round duct

A plenum that ends in a round duct is a square to round: centred on the Square to Round page, off centre on the Eccentric Rectangle to Round. A 24 × 20 in plenum to a 16 in round (406 mm), 12 in long, in 24 gauge, is a 59-5/8 × 32-1/4 in plate (1514 × 819 mm). Offsets, flat sides and what that pattern leaves out are in the duct transition guide.

Gauge, folds, seams and connections

Gauge. The page takes the one you pick from the list and works on its mean line; which gauge a plenum takes comes from the job spec or SMACNA's tables. On 24 gauge the thickness takes about a tenth of an inch off the box above, round its perimeter.
Folds. The patterns are worked to a sharp corner on the mean line; the note on the result gives the deduction if your brake leaves an inside radius.
Seams and connections. None are added: a Pittsburgh, a snap lock, a standing seam, S and drive edges or TDC and TDF connectors go on along the edges, from your machine's chart.
Lined plenums. The metal is the clear inside size plus twice the liner: a 20 × 18 clear plenum with 1 in liner is 22 × 20 in metal.

For plasma or laser cutting, the free account exports 1 part a month in DXF; unlimited DXF export is the paid full access.

Frequently asked questions

Can I lay out a plenum box here?: The four sides, yes: the Rectangular Transition with the same rectangle at both ends draws them as one strip with the folds. The top, the takeoff holes and the seams are laid out by hand.
How do I lay out a plenum transition?: Each face is a flat trapezoid: its two widths, bottom and top, and its height on the face, √(h² + step²), the step being half the difference across that face. The Rectangular Transition page draws the four faces in one strip and gives the fold angle.
How do I make an offset plenum transition?: Offset one way, on the Discharge Chute page with the tilt at 0: the larger rectangle is the base, the smaller the outlet, and the offset along X is (X − N) / 2 for flat on one side. Offset both ways is not covered.
Does the site lay out duct board plenums?: No. Duct board is grooved and folded with its own allowances; the patterns here are for sheet metal.
What gauge should a plenum be?: The one the job spec or SMACNA's tables give. The page takes the gauge you pick from the list and works the pattern on its mean line.
Is the plenum layout free?: The figures and the drawing need no account. The full-size template takes a free account, which also exports 1 part a month in DXF; unlimited DXF export is the paid full access.