A bay and a bow both push out from the wall of the house, and the difference between them is the angle. A bay turns the corner in one step, at 30 or 45 degrees, using three units. A bow turns roughly 10 degrees at a time, using four to six. Every other difference, the wall you need, the price, the way the light falls, comes out of that one number.
We fit bay and bow windows across the St. Louis metro. Nearly every customer arrives having read that one is angled and the other is curved, which is true and does not help anybody decide. What follows is the part that does.
The Short Answer, Side by Side
| Bay | Bow | |
|---|---|---|
| Units | 3 | 4 to 6 |
| Angle | 30 or 45 degrees in one step, or 90 for a box bay | about 10 degrees per unit |
| How far it projects | further, 13 to 19 inches is typical | shallower, for the same width of wall |
| Wall it needs | less | more, and this is the one that rules people out |
| What opens | center pane fixed, the two flankers open | you choose how many of the 4 to 6 |
| Seat inside | deep enough to use | shallower, curved, harder to furnish |
| Cost | lower | higher, more units and more glass |
| Reads as | mid century, colonial, craftsman | Victorian, period, softer |
Why the Angle Decides Everything Else
Both arrive as one assembly. The units are mulled together at the factory, sharing a frame and a mullion at every joint, which is why neither one comes apart into separate windows later. What you are choosing is the whole box, not a row of windows.
Manufacturers build these to fixed angles. Andersen publishes its sizing as 30 and 45 degree bays, a 90 degree box bay, and a 10 degree bow. Those numbers are the whole story.
Think about what a shallow angle costs you. At 10 degrees a unit, four units have only turned 30 degrees between the first and the last. To get any real depth out of that, you need more units, and more units means more width. A bay gets the same depth in one 45 degree step and is done.
So the trade is straightforward. On a narrow wall, a bay projects further. On a wide wall, a bow gives you more glass and a softer line. That is why I ask how much flat wall there is before I ask which one somebody likes the look of. The wall usually settles it.
Which Ones Actually Open?
Whether the units open is the question I get asked most, and the one the comparison articles skip.
A bay, normally two of the three. The center is a fixed picture window, because it is the widest piece of glass and making that sash open would cost a lot for little gain. The two angled flankers are casement or double hung units and both open. Angled outward as they are, they scoop air into the room from along the wall, which is why a bay ventilates better than its glass area suggests.
A bow, however many you pay for. All four to six can be fixed, all can open, or you can mix them. Every operable unit adds hardware and cost, so the usual order has the middle ones fixed and one at each end operable.
Worth knowing before you order: on a bow, opening units at the ends swing into the room’s corners. Check what is under them. A radiator, a dresser or a door swing at the end of the run will stop a casement dead, and that is a problem found on fitting day rather than on paper.
How Much Wall Do You Need?
When I price one of these the first thing I do is put a tape on the flat wall, not on the window. What we usually find on the 1960s ranches around Kirkwood and Webster Groves is roughly 7 feet of wall carrying a 5 foot window, which makes a comfortable 3-window bay and a very tight 4-panel bow. That one measurement moves the quote more than any option on the order form.
Every manufacturer publishes its own chart and they differ, so treat these as the shape of the rule rather than gospel, and ask for the chart covering the unit you are actually quoted.
Broadly, a bay starts working at a little over three and a half feet of opening. A bow wants five feet before it looks like anything, and it is happier with eight or more. Below that the curve is so slight that you have paid for four units and got a flat window with extra seams in it.
A tape will not tell you what is inside that wall, and neither will we until the old unit is out. Any homeowner can measure the flat wall themselves in two minutes, though, and it is the single number worth having before you call anybody.
Two things people forget when they measure the wall. The rough opening needs framing either side, so the flat wall has to be wider than the window itself, and the sill has to carry the load of a box that now sticks out past it. And if the new opening is wider than the old one, somebody has to put a bigger header over it, which is structural work and a different conversation from swapping a window. Our bay window replacement guide covers how these are carried and what is behind the seat, and it is worth ten minutes before you commit either way.
Are Bay Windows Outdated?
People ask this and nobody answers it straight, so here it is. No, but the 1990s version is.
What dates a bay is not the shape. It is the trim: heavy white grids on every pane, a shingled roof that does not match the house, and oak veneer on the seat board. Take those away and the same bay reads as current. The ones that look tired around here are almost always original to the house and have been sitting there for thirty years with the sealed units fogged.
A bow has the opposite problem. The curve reads as period, so it sits well on a Victorian or a older brick house and can look bolted on to a plain 1970s ranch.
The Other Windows That Stick Out
One thing both share, and it is the part that fails first. Each carries its own small roof, a hip roof on most bays and a shed roof on many bows, flashed into the siding above. That flashing is where the water gets in, and it is covered properly in the guide linked above rather than repeated here.
Bay and bow are two of five. If you are trying to put a name to something you already have, it is probably in this list.
- Bay. Three units, one step out at 30 or 45 degrees.
- Bow. Four to six equal units in a shallow arc, about 10 degrees apiece. Sizes and parts are in our bow windows guide.
- Box bay. A bay at 90 degrees, so the sides are square to the wall and the plan is a rectangle rather than a trapezoid. Deepest seat of the lot.
- Garden window. A small glass box over a kitchen sink, with glass on the top as well as the sides. It is a greenhouse for herbs, not a seat.
- Oriel. Any of the above on an upper floor with nothing underneath it, carried on brackets or corbels off the wall. Common on older St. Louis brick.
People also say bump out, which is not a window type at all. It usually means a box bay, and occasionally it means a piece of the room that was pushed out with a floor under it, which is building work rather than a window order.
Which One Should You Choose?
Take the questions in this order and it usually answers itself.
- How much flat wall is there? Under five feet, the decision is made for you. It is a bay.
- Do you want to sit in it? A bay, and preferably a box bay. A bow’s seat is curved and shallow, and furniture does not like it.
- How much does it need to open? A bay gives you two operable units as standard. A bow gives you as many as you want to pay for.
- What is the house? Victorian, older brick, anything with curves already: a bow. Mid century, colonial, ranch: a bay.
- What is the budget? For the same width of wall the bow costs more, every time. More units, more glass, more hardware.
If the first four point different ways, the wall wins. I have never seen anybody regret a bay that fitted, and I have seen people regret a bow squeezed onto a wall that was too narrow for it.
When This Is Not Our Job
- Repairing the one you have. A failed seal on a single pane, a stuck crank, a sagging seat board. That is glass and carpentry work. Nobody needs a window company for it, and we would be the wrong call. New units are what we supply and fit; we do not rebuild old boxes.
- Building a new opening where there is no window. Cutting a fresh hole in a wall and heading it is a builder’s job with a window at the end of it.
- Anything commercial. These units are engineered for homes, and residential work across the metro is the whole of what we take on.
Common Questions
What a Real Quote Has to Say
Three things belong on any quote for one of these. How many units, and how many of them open. Whether the price includes the roof and the seat board or only the window. And whether the opening is being widened. Ask for the frame material too, since vinyl, fiberglass and clad wood price differently. Then ask for the U-factor, the SHGC, and whether the sealed units carry a low-e coating and an argon fill. A quote that skips those is a number, not a price. Set two of them side by side and you are comparing guesses, because the difference is nearly always hiding in one of the three.
People ask whether either one is worth it. On the right wall, yes: it is the one window change that alters a room rather than just refreshing it. On the wrong wall it is an expensive way to get a flat window with seams. As for how long, a straight swap into a sound opening is usually a day, and widening the opening turns it into two or three.
The honest limit on all of this: everything above tells you which shape suits your wall and what to ask. What it costs at your house is a different question, and you cannot get that number off a screen. That number lives behind the siding, and nobody sees it until the old box is off the wall. Finished work is in our photo gallery.
The units are ProVia. Their own dealer locator badges us Platinum and lists the showroom, and that page is theirs, not ours, so it is the one to check. The bay and bow installation page sets out what we fit, and there is a 10 year guarantee on our workmanship. Ask for a visit and somebody will put a tape on that wall and tell you which of the two it will carry. The number is 314-277-5952.
