Bow Windows: Sizes, Parts, and What to Check Before You Buy

A bow window is four or more windows of the same size, joined at shallow angles so the run curves out from the wall in an arc. Most have four, five or six units, each turned 10 to 15 degrees from its neighbor, and they stand roughly 7 to 23 inches off the house.

We install bay and bow windows on homes across St. Louis and St. Charles County, and finished jobs are in our photo gallery. Picking between a bow and a bay is a separate question, and the bay vs bow comparison answers that one.

Standard Bow Window Sizes

Three bows drawn from above with 24 inch units: four units stand 8 inches off the wall, five stand 12 inches, six stand 18 inches — Drawn by us to scale, seen from above. Every unit is 24 inches wide and turned 10 degrees from the next. Four units stand 8 inches off the wall, five stand 12 and six stand 18, so adding units makes the bow wider and deeper. Real frames add an inch or two of depth.

Each maker publishes its own table. Andersen’s 2025-26 sizing sheet is public, and it lists its 10 degree casement bow in standard widths about 4 inches apart, up to seven units.

Units	Overall width	How far it sticks out
4	69 1/2 to 143 7/8 in.	7 3/8 to 13 7/8 in.
5	85 7/8 to 164 1/4 in.	10 3/8 to 18 5/8 in.
6	101 5/8 to 164 1/4 in.	14 7/8 to 23 3/16 in.
7	116 5/8 to 164 1/4 in.	19 3/16 to 26 3/8 in.
Height, all four	26 1/8 to 73 7/8 in.

Andersen 400 Series casement bow, 10 degrees, from its 2025-26 sizing sheet.

Stanek’s bow and bay size page shows its four, five and six unit bows at either 10 or 15 degrees. The 15 degree version turns faster, so the same number of units stands further off the wall. The units we install are ProVia, built in three vinyl lines, Aeris, Endure and Aspect, and the bay and bow page has the options.

Is a Bow Deep Enough for a Seat?

Four units stand 7 to 14 inches off the wall. That is a plant ledge. If you plan to set pots on it, ProVia’s product sheet offers a white or beige plastic laminate seat, which it describes as waterproof. Five units give you something you can perch on. Six get close to 2 feet, which most adults can sit on properly.

The seat board also runs back over the wall itself. That adds the thickness of your wall, about 5 inches on most frame houses. St. Louis has a lot of brick houses, and on brick the wall can add 9 inches or more.

When somebody asks me for a window seat, I count units first. If the wall only takes four, a bay usually gives more depth for the same width, and the comparison linked above shows why.

What Holds a Bow Up?

Side view of a bow window: roof kit and flashing at the top, cables with a turnbuckle hanging the unit from the framing, head board, window unit, insulated seat board and a knee brace underneath — Drawn by us from ProVia’s bay and bow product sheet: a side view through the middle unit. Simplified, not to scale.

A bow hangs off the house with no floor under it. Andersen’s 2025-26 sizing sheet says each of its bows comes with two cables, each rated for 500 pounds, and that a load over 1,000 pounds needs extra support. ProVia’s own bay and bow product sheet lists the parts that carry that weight and keep water out.

Head board and seat board. The top and bottom of the box. On ProVia’s units both are oak or birch, and the seat board is 1 1/4 inch furniture grade veneer.
Mullions. The posts between the units. ProVia makes them from TimberStrand laminated lumber and lag bolts them into the head and seat.
Cables. Adjustable cables with a turnbuckle and a 6 inch hook hang the whole unit from the framing above. They are what stops a bow from sagging.
Seat insulation. The standard seat carries R-9, a 1 inch board of rigid foam, and is 3 inches thick overall. The R-21 upgrade uses 3 inches of foam and makes the seat 5 inches thick. Standard foam stops at the wall, and a fully insulated seat is an option. ProVia does not offer an insulated head board.
Panning. PVC channels under the units form a small gutter on the inside, so water that gets past a sash has somewhere to go.
Roof kit. A pre-built hip roof in plywood that overhangs the frame by 2 inches and, as standard, stands about two-thirds as tall as the bow sticks out. It can be covered in copper or painted aluminum, or shingled to match the house. Flashing and shingles are not in the kit; the installer adds them on site.
Knee braces. The brackets under the seat, sold from 6 to 24 inches deep in 2 inch steps. ProVia’s plain 2 by 4 braces only hold the unit until the cables are tightened. The shaped ones are there for looks.

What Goes Wrong With an Old Bow?

Each of the four common failures traces back to a part in that list.

It sags. The cables were never installed, or they have gone slack. The signs are a gap opening along the top of the inside trim, or a sash that binds where it used to swing freely.
The seat is cold. Plenty of older bows have a plain plywood seat with nothing under it. In January you can feel the cold through it.
It leaks at the top. Water nearly always gets in where the little roof meets the siding, not through the glass. Stains on the head board show the leak.
The glass fogs. That is a failed seal between the panes, the same as on any window, and it cannot be cleaned away.

A bow needs a wide wall. You pay for every unit, and more for each one that opens. On an upper floor you can only clean fixed units from a ladder. Your curtains need a bent rod or a ceiling track, because a straight rod will not follow the curve.

Once a year, look at two places: where the roof meets the siding, and the top of the inside trim. Leaks and sags show there first.

Are Bows Dated? What a Modern One Looks Like

A bow in a living room with white casement units, no grids, and potted plants along the seat board

Bows are still made, and the curve itself has not gone out of style. What ages a bow is small panes split up by grids, with a rail across every opening.

A current bow uses casement units with one clear pane each, like the ones above. The roof is copper or painted aluminum. ProVia’s bay and bow product sheet lists exterior cladding in Coal Black and Classic Bronze as well as White, Beige and Sandstone, and 28 colors for the aluminum roof. The copper roof is 16 ounce sheet, left bare to weather green or given an Incralac protective coat that keeps it shiny.

If you like the curve but not the period feel, I would change the units to casements first. With no meeting rail across the middle, the arc looks clean rather than busy.

How to Measure the Bow You Have

You can measure an existing bow yourself in about ten minutes. It will not give you an order size, but it makes any quote easier to check.

Count the units, and note which ones open.
Measure the width in a straight line along the wall, from the outside edge of one end frame to the outside edge of the other. Do not run the tape around the curve.
Measure the height of one unit, from the top of the frame to the bottom.
Measure the depth, from the siding straight out to the face of the middle unit.
Take a photo of the roof and one of the underside, so whoever looks at it can see the braces and the roof covering.

Makers size these by the frame, not the glass, which is why a tape across the panes tells you nothing useful. Andersen’s 2025-26 sizing sheet gives its widths from outside frame to outside frame, and its depth from the sheathing under the siding. Andersen’s bay and bow article adds that its unit height leaves out the head and seat boards.

What a tape cannot show you is what is behind your trim. Are the support cables still there, and has the seat board rotted? How was the header framed? Those answers come out with the old unit, and the bay window replacement guide walks through how that day goes.

What Moves the Price

There is no honest single price for a bow. Two of the same width can differ by the cost of the roof alone.

Unit count. Every unit is another window.
How many open. An opening unit costs more than a fixed one.
The seat. R-21 insulation costs more than the standard R-9.
The glass. Double or triple pane, a low-E coating, argon fill, and tinted or privacy glass all change the number.
The roof. Shingles to match the house, painted aluminum, or copper. Andersen’s sizing sheet says its bows ship as the basic unit only, with the roof from another maker.
The inside finish. Unfinished, stained, painted or laminate.
The opening. A straight swap into the same hole is the simple job. A wider hole means new framing and a new header.

Opening your wall also adds time on site compared with a straight swap. The bay vs bow comparison weighs the two on cost.

When a Bow Is the Wrong Choice

Is a bow worth it? A bow is not for every wall: the narrowest four-unit bow on Andersen’s sizing sheet is 69 1/2 inches wide, and these are the walls where I would pick something else.

Less than about 6 feet of wall. Four units is the usual minimum, and on a narrow wall they leave you a shallow ledge and a lot of seams.
No room outside. Next to a walkway or the edge of a driveway, 18 inches of glass sticking out is a hazard.
Over a kitchen sink. A garden window is the better choice there, with a glass top for plants.
A bump-out with a floor under it. That is building work, not a window order, and we stick to windows and doors.

Common Questions

A bow window is a set of four or more windows of equal size, joined at shallow angles so they curve out from the house in an arc. The angle is usually 10 to 15 degrees between units. Most have four, five or six units and stand 7 to 23 inches off the wall.

Some units do and some do not, and you choose which when you order. Each unit can be fixed, casement or double hung. An opening unit costs more than a fixed one, so a lot of people open two, one near each end, and keep the rest fixed.

On Andersen’s 10 degree casement bow, four units run 69 1/2 to 143 7/8 inches wide. Five run 85 7/8 to 164 1/4 inches, and six run 101 5/8 to 164 1/4 inches. Heights run 26 1/8 to 73 7/8 inches. Andersen’s sizing sheet also lists custom sizes in 1/8 inch steps when a standard size does not fit the opening.

Four or more, all the same size. Four, five and six are the usual counts, and Andersen’s 2025-26 sizing sheet goes up to seven. Three units set at a steeper angle is normally called a bay.

With six units, usually yes. They stand 15 to 23 inches off the wall, and the seat board runs back over the wall for about 5 more. Four units give 7 to 14 inches, which is a ledge for plants rather than a seat.

Cables. Adjustable cables with turnbuckles hang the unit from the framing above, hidden under the little roof. The braces you see under many bows are often decorative; ProVia’s plain 2 by 4 braces only hold the unit until the cables are tightened.

A bow needs a wide wall, costs more with every unit, and on a narrow wall gives only a shallow seat. Old ones tend to sag when the cables fail, leak where the roof meets the siding, and lose heat through a seat with no insulation under it.

Often, yes, if the wall is wide enough and the framing above can carry the weight. The opening has to be checked, and sometimes widened with a new header. Measure your flat wall first. Under about 6 feet, a bow will be tight.

Getting Yours Looked At

ProVia’s own dealer locator lists us as a Platinum dealer, so you can check that on their site. Our workmanship carries a 10 year guarantee. Since 2014 we have installed windows and doors on St. Louis area homes, and we do residential work only.

Call us at 314-277-5952, or ask for an estimate online, and I or one of our team will come out to measure your wall and look under your seat. You can read who we are before you call.