What size boot does a 4 × 10 register take?

One with a 4 × 10 in opening: registers are sold by the size of the opening they drop into, not by their face. The round end is the duct that feeds the register, which the duct design sets from the airflow the room needs.

Can the round duct be wider than the register's narrow side?

Yes. A 6 in duct on a 4 × 10 register is a normal boot, on the Angular Boot page with the duct from the side and on the Square to Round page with the duct from below.

Why does the boot pattern have creases?

On the two sides that run along the duct, no point of the round end has its tangent parallel to the side, so the flat face and the curved part meet at an angle instead of smoothly. The crease is part of the boot, and the note on the result says to mark it.

Where do I measure the depth and the offset from?

The depth, from the register face, the plane of the rectangle, to the centreline of the duct. The offset, along the duct, from the middle of the register opening to the centre of the round end.

Does it do ceiling boots?

Yes: a ceiling boot is a floor boot upside down. Duct from the side, the Angular Boot; duct from above, the Square to Round.

Is the boot 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.

Register boot layout: from a floor, wall or ceiling register to a round duct, in inches

Updated on 28 October 2026

A register boot takes a round duct to the rectangular opening of a floor, wall or ceiling register. The Angular Boot page lays out the boot whose duct comes in from the side, running along under the floor: 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.

The boot whose duct comes straight up from below is a square to round, on another page. Both are here: what each lays out, what to measure, a worked example and what the pattern leaves for you to add.

What the Angular Boot lays out

A transition from a rectangle, the register opening, to a round duct that runs parallel to it: the duct lies at a depth H below the register face, and the boot ends on a cut across the duct. A square cut gives an upright round end, the plain end boot, where the duct or a collar connects. An angled cut gives a sloping end, which is an ellipse rather than a circle; the duct then has to be cut at the same angle, and that cut is the Truncated Cylinder page, with the same diameter and angle.

Like the other transitions here, the boot is four flat triangles, one on each side of the rectangle, and four fans at the corners. On the two sides that run along the duct the flat triangle cannot meet the round end smoothly, so the boot takes a crease on each of them; the note on the result says where to mark it. The creases are part of the boot, not a fault in the pattern.

What to measure

Field	On a register boot
X, the side along the duct	The register size in the direction the duct runs: 10 for a 4 × 10 register with the duct coming in at one end.
Y, the side across the duct	The other register size: 4 in the same case.
D, the round duct	The duct size: 6 for 6 in duct. It can be wider than Y; a 6 in duct on a 4 in wide register is normal.
H, to the duct centreline	From the register face, the plane of the rectangle, to the centreline of the duct. It has to be more than half of D, so that the whole duct is below the face.
V, the offset	Along the duct, from the middle of the register opening to the centre of the round end. Half of X puts the round end right under the end of the opening; more puts it past the end.
ANG, the cut angle	0 for a square cut and an upright round end; more slopes the end.
DV, divisions	How many points the round end is divided into, a multiple of 4. 16 is the page's own.
Thickness	The gauge, from the list.

A register is sold by the size of the duct opening it drops into, not by its face: a 4 × 10 register fits a 4 × 10 opening, and its face is bigger. That opening is the rectangle of the boot. Type decimals: 5.5 for 5-1/2 in.

Worked example: a 4 × 10 floor register on 6 in duct

A 4 × 10 in floor register (102 × 254 mm), fed by a 6 in duct (152 mm) that runs under the floor and comes in at one end. The duct centreline is 5-1/2 in below the floor (140 mm), and the boot ends 6-1/2 in from the middle of the register (165 mm), 1-1/2 in past the end of the opening. Square cut, 26 gauge.

Field	Value
X and Y	10 and 4
D	6
H	5.5
V	6.5
ANG	0
DV	16
Thickness	26 gauge, 0.0217 in (0.55 mm)
Plate	18 × 14-11/16 in (457 × 373 mm)
Full-size template	6 Letter sheets

The result carries the notes for this part: where to mark the two creases, and that the angle between faces changes from crease to crease, so the boot is formed to fit rather than folded to one angle.

Angular Boot, Rectangle to Round flat pattern: drawing of the template with the worked example dimensions — The pattern the Angular Boot page draws for its own example: an 11.8 × 5.9 in rectangle (300 × 150 mm), a 9.8 in duct (250 mm) cut at 30°, its centreline 7.9 in from the rectangle and the round end 7.9 in from its middle (200 mm each), in 3 mm plate. The cut is in purple and the triangulation in orange.

Straight boots, ceiling boots and the rest

Straight boot, the duct coming straight up from below, centred: the Square to Round page, with the register size as X and Y, the duct as D and the depth of the boot as H. It takes a duct wider than the register's narrow side: a 4 × 10 register on 6 in duct, 6 in deep, in 26 gauge, is a 19-3/8 × 17-3/4 in plate (492 × 450 mm).
Straight boot with the duct off centre: the Eccentric Rectangle to Round page, but only when the round fits inside the register opening seen from above, so not for a 6 in duct under a 4 in wide register.
Ceiling boot with the duct coming in from the side: the same Angular Boot, upside down. With the duct coming down from above: the Square to Round, upside down.
Box boot, a plain box with a collar in its end or its bottom: not a transition. Its sides are flat panels and its collar a plain band, and no page here lays out the box with its hole.
Stack head, from rectangular wall stack to the register: not covered.

What the pattern does not include

The tabs on the rectangular end that bend over the edge of the floor or ceiling opening to hold the boot. Add them along the sides of the rectangle.
The crimp or bead on the round end, and any collar. A straight collar is a plain band, on the Cylinder page.
Seam allowances. The pattern is net, with the seam edges butted: a Pittsburgh, a snap lock or a lap for screws goes on along the seam edges, from the chart of your lock former.

For plasma or laser cutting, the free account exports 1 part a month in DXF; unlimited DXF export is the paid full access. A boot of the example's size prints on six Letter sheets, so for a few boots the paper template is the quicker route, and for a run of them the DXF.

Frequently asked questions

What size boot does a 4 × 10 register take?: One with a 4 × 10 in opening: registers are sold by the size of the opening they drop into, not by their face. The round end is the duct that feeds the register, which the duct design sets from the airflow the room needs.
Can the round duct be wider than the register's narrow side?: Yes. A 6 in duct on a 4 × 10 register is a normal boot, on the Angular Boot page with the duct from the side and on the Square to Round page with the duct from below.
Why does the boot pattern have creases?: On the two sides that run along the duct, no point of the round end has its tangent parallel to the side, so the flat face and the curved part meet at an angle instead of smoothly. The crease is part of the boot, and the note on the result says to mark it.
Where do I measure the depth and the offset from?: The depth, from the register face, the plane of the rectangle, to the centreline of the duct. The offset, along the duct, from the middle of the register opening to the centre of the round end.
Does it do ceiling boots?: Yes: a ceiling boot is a floor boot upside down. Duct from the side, the Angular Boot; duct from above, the Square to Round.
Is the boot 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.