The angle answer
Most UK bays are one of three shapes. A square or box bay has corners at 90°. A splay bay, the most common kind, has angled side sections that usually meet the front at about 135°. A curved bay follows a continuous arc with no true corners. The angle matters because shutter frames meet at each corner, and on a splay bay the corners need to match within about 2° for the panels to fold flat and close neatly.
Why an angle, not just a width
A flat window only needs a width and a height. A bay is several windows joined at corners, and every corner is a joint the shutter frame has to meet. If the frame pieces are cut to the wrong angle, a gap opens up on one side of the corner or the frames push against each other, and the panels either side stop sitting square.
That is why bay shutters are built around the bay's exact angles, with mitred corner posts, rather than as separate shutters pushed together. Knowing your angle before anyone quotes also helps you understand why two bays of similar width can end up at different prices.
The three shapes, and how to spot yours
Square bays are common on many 1930s semis. Splay bays are the classic Victorian and Edwardian front. Curved bays are rarer and usually need each flat pane treated as its own narrow section.
| Shape | Corner angle | How to recognise it |
|---|---|---|
| Square or box bay | 90° | Sides come straight out from the wall, like three sides of a box |
| Splay bay | Usually about 135° | Sides angle outwards, the front is wider than the back |
| Curved or bow bay | No true corner | Glass follows a smooth curve, often many narrow panes |
Reading the angle yourself
You do not need special kit. A sliding bevel from a toolbox, or a free angle finder app on a phone laid flat against the reveal, will give you a reading. Measure the inside angle where each side section meets the front section, at sill height.
- 1Clear the sill so the tool can sit flat against both faces of the corner.
- 2Read the left-hand corner, then the right-hand corner.
- 3Repeat each reading near the top of the window.
- 4Compare: on a well-built splay bay the two corners agree within about 2°.
When the corners do not match
Older houses settle, and bays were often built by eye. If the two corners differ by more than a couple of degrees, or the top and bottom readings disagree, the bay is out of square. That is normal and fixable, but it means the frame has to be scribed to the real shape on site rather than made to a textbook angle.
The important thing is that your fitter knows before the shutters are made. Finding it on installation day is how bay projects go wrong. We take our own readings on every survey regardless of what the customer has measured.
How the angle feeds into price
Bay shutters are priced per section, and most bays have 3 to 5. The angle itself does not add much cost, but an out-of-square bay that needs scribing, or a curved bay broken into many narrow sections, takes more making and fitting time. Written quote after a free survey.
For the full measuring routine, including widths, heights and the 70mm recess depth needed for an inside fit, see our step by step guide to measuring a bay window for shutters.
Bay angle questions
What angle is a standard bay window?
Splay bays, the most common UK type, usually meet at about 135°. Square or box bays meet at 90°. Curved bays have no true corner.
How accurate does my angle reading need to be?
On a splay bay, the corners should agree within about 2°. If they differ by more, the bay is out of square and the frame will need scribing on site.
Does a 90° bay cost less than a splay bay?
The angle alone makes little difference. Bay shutters are priced per section, usually 3 to 5, and the size and material matter more.
Can you fit shutters to a curved bay?
Yes. Each flat pane is treated as its own section, which usually means more, narrower panels.