How to measure a bay window

Which kind of bay have you got?

All four are measured the same way. Our bay designer takes 2 to 7 frames, with corners from 45° to 179°, to one decimal place, so the corners don't have to match. Each frame can be any of our window styles that can be made at your bay's height, and the designer only offers those.

A corner window is two frames meeting at one corner post on the corner of the building, usually at 90°. In the designer it's a 2-frame bay, and its backspan is the diagonal between the two frame ends. Both its frames are end faces, so measure each one from the end of the old frame to the centre of the corner post, or measure from the plaster line.

What you'll need

• A steel tape measure, and someone to hold the other end on the longer faces.
• A sliding bevel and a protractor, or a digital angle finder, for the corners. No angle finder? You can find the angles with the tape instead: see finding the angles with only a tape.
• A spirit level, to check the head and the base are level from end to end.
• A pencil and paper. Sketch the bay from above, number the faces and the corners as the designer numbers its frames and corners, with face 1 and corner 1 on your right as you look out, and write each measurement on the sketch as you go: every height reading, each face's internal width, each corner's angle, the backspan and the projection, and, for the safety glass, the height inside from the floor to the bottom of the old window frame. Note which side you want each window to open on too. The designer lets you choose for each frame, seen from outside, the way we draw and make windows, so its left and right are the other way round from what you see in the room.

1. The overall height

The overall height is the external size of the frames, with the new uPVC cill included. Measure it outside, from the bottom of the old cill, where it sits on the brickwork, up to the underside of the head (the lintel or beam the old frames sit under), at each end and at every corner. Use the smallest and take 10mm off: that's the overall height of the whole bay. Don't add anything for the new cill: it goes where the old one was, inside this height. The old cill always comes out, so never measure from the top of it. The 10mm is for fitting clearance.

When you choose a cill, the designer takes its 30mm face off the height you enter and makes the frames from the rest. So a 1,200mm bay with a cill has 1,170mm frames, and its bay poles are 1,170mm long too.

Which cill? The 150mm cill, the same as our windows, is chosen for you. The 150mm is how deep it is, front to back: every cill is 30mm tall, and that's inside your overall height. Choose the 180mm or 225mm cill if you need one deeper to reach past the brickwork below, so rain drips clear of the wall. Only choose the 85mm stub cill or No cill if your fitter asks for it. Not sure? Ask us before you order.

Small differences between the readings are normal: using the smallest and taking 10mm off already allows for them. If the readings differ by more than 10mm, the head or the base isn't level. Note it on your sketch and talk to your fitter before you order.

Low bays: safety glass

Low bay? Measure inside, from the floor to the bottom of the old window frame. If that's under 800mm, the glass will be within 800mm of the floor, so it must be toughened safety glass: choose toughened, or ask us if you're close. Our toughened safety glass guide explains where it's needed.

2. The internal width of each face

Every width is an internal width: measure it inside the room, along the inside of the bay, never outside.

We make each frame from the internal width you enter, so this is the measurement to get right. We number the frames left to right as seen from outside, the way we draw and make them. Standing inside looking out, frame 1 is the one on your right. The designer lists the frames in that order, frame 1 first.

• Between two corners: measure from the centre of one corner pole or post to the centre of the next.
• The two end faces have a pole at one end only. At the wall end, dig the plaster back on the inside until you can see the end of the existing frame, and measure from there to the centre of the pole. If you can't, measure from the plaster line and use that figure as it is.
• Measure every face at the top, middle and bottom, and use the smallest.

3. The angle at each corner

Measure the angle inside the room, between the two faces that meet at the corner. A usual splayed bay is 135°, a box bay 90°, and a bow wider still. Measure every corner, even if the bay looks symmetrical, and enter what you measure.

• Sliding bevel: open it in the corner with the stock against one face and the blade against the other, lock it, then lay it on a protractor with the stock along the base line and read the angle.
• Digital angle finder: open the arms into the corner and read the angle. Some angle finders read the outside of the corner. On a bay, if yours shows a small angle such as 45° or 30°, take it away from 180°: 45° means 135° inside, and 30° means 150°.
• Measure at the bottom, on the window board inside, and near the top if you can. If the two readings differ, talk to your fitter before you order.

In the designer, leave Same angle at every corner switched on if all your corners match. If they don't, switch it off and enter each one. We number the corners the same way: corner 1 is between frames 1 and 2, the corner furthest to your right as you stand inside looking out.

No angle finder? You can find the angles with a tape instead, as the next section shows.

4. Check the backspan

As you type your widths and angles, the bay designer works out the internal backspan and the projection, so you can check your figures against the room before you order.

Check it: measure straight across the inside of the bay, between the same two points you measured the end faces from, and out to its furthest point for the projection. Yours should match ours within about 5mm. If you measured the end faces from the plaster line, tick "I measured the end faces from the plaster line" under the backspan in the designer: it then shows the backspan and projection to expect plaster line to plaster line, with your widths left as they are. If they don't match, check your widths and angles before you order.

Bay poles and cill welds: what they are and what they cost

Each frame is priced exactly as the same window on its own, at the internal width you enter and the bay's overall height, with the options you choose for the bay. Two things go on top.

• Bay poles: a bay pole joins two frames at a corner, so there's one at every corner: a three-frame bay has two. Each pole is as long as the frame height: the overall height less the 30mm cill face when you choose a cill, so a 1,200mm bay with a cill has 1,170mm poles, £70.20 each. With no cill, they're the full overall height. Poles cost £60 per metre, charged by their exact length.
• Cill welds: a cill weld joins the cills of two frames where they meet at a corner. When you choose a cill, there's one at every corner, at £25 each. With no cill, there are no welds.

Both include VAT, like every price on our site. The pole depends on the angle: a 90° corner takes a 90° pole, a corner from 134° to 136° takes a 135° pole, and any other angle takes a variable pole. The designer picks the right one for each corner.

We deduct for the poles when we make the frames. That's why you enter each width exactly as you measured it and never take anything off yourself, not even the 10mm you'd take off a flat window.

Worked examples

Delivery is extra: priced by straight-line distance from our factory (M28 2LY) to your postcode district, from £30, to mainland Britain addresses within 300 miles, not Northern Ireland, the Scottish Highlands or islands reached by ferry. Or collect free from the factory.

Before the old bay comes out

In some bays the roof, or the wall above, rests on the old frames. If yours might, get a builder or structural engineer to look at it before you order. Have them support it before the old frames come out, and ask them how it should be carried once the new bay is in.

We supply only: we make the bay to your sizes and deliver it, and you or your fitter fit it. Our hiring a fitter guide covers what to ask before you book one.

Common mistakes

• Taking something off the widths for the poles. We do that when we make the frames.
• Taking 10mm off the widths, as you would for a flat window.
• Measuring the widths outside the bay instead of the internal widths, inside the room.
• Numbering the faces or the corners from your left as you stand in the room. Face 1 and corner 1 are on your right as you look out.
• Guessing the angles because the bay looks like a standard one.
• Using the largest of the three readings, or averaging them.
• Measuring the height inside the room, or from the top of the old cill.
• Forgetting the 10mm off the height, or adding the cill on top.
• Measuring the backspan from wall to wall across the room, instead of between the two points you measured the end faces from.
• Skipping the backspan check.
• Changing your end face widths to check the backspan. Leave them as you'll order them, and if you measured from the plaster line, tick "I measured the end faces from the plaster line" under the backspan.