There is a difference between surveying a bay or bow window and surveying and fitting a flat UPVC Window.

The Hard Way to Survey a Non Square Bay

A bay pole assembly connects two windows together at the angled part of the bay. To work out the size required to produce a bay layout, see below:

The back span is the internal width of the bay that is measured from inside frame to inside frame. You may have to chop back the plaster on the inside to locate the frame edge. Please note it is surveyed inside and not outside except for the height which is measured from outside e.g. outside brickwork sizes & inside plaster lines.

The projection from the imaginary string line in the drawing below indicates there is 300mm to the inside of the bay. Should you wish to make sure, a string line can be set up for projection measurements.  This is often very helpful on multi-faceted bays.


The internal facet widths are 649mm in the above diagram, 625mm in brackets and 600mm for the centre window. The smaller size would be the window manufacturing size as the UPVC bay pole assembly will then sit in the gap in between the two frames.

The method above allows the calculation of the angle where the two windows meet using trigonometry. We can then use our software to take the sizes and produce a bay plot as above, which we can email to be signed off before we manufacture. Bay windows cannot be produced online yet.

Survey a Bay Window The Easy Way

The above process is all about finding the bay angle so that a deduction can be made to fit the pole in. However, there is an easier way by using an angle finder as shown below. The cost of an angle finder is approx. £30.00 delivered from Screwfix


An angle finder will allow you to measure the angle on the inside of the bay window and the bay facet (window) internal widths to go with the angles. From this, you can then calculate the bay. It is advisable to email South Coast Windows the sizes and window styles as we will send a sign off sheet before any windows are manufactured.

Square Bay Windows

These are the easiest to measure as they do not require an angle finder to measure a square bay. The bay is set at 90 degrees, so all you need to do is measure the inside sizes of each facet. We will then add on the two 90 degree posts. These are 70mm square and increases the external brick size by 140mm (see square post picture). The height is measured from the outside & inside.


How Do Bow Windows Differ from Bay Windows?

A bow window has a very shallow angle and the projection is a lot less than for a bay window. For an existing bow window, measure the internal size brick to brick and not frame to frame. The throw is to the inside of the frame.
Those wishing to convert a flat window into a bow window need to know the external brickwork opening sizes and how many facets you will require as well as the projection you would like. This normally works well at about 1/8th of the width. Please email any queries to

NOTE: When removing an existing bay window, make sure you use Acrow props to fit between the floor and the ceiling as many bays are load bearing.


Bay Jacking Posts
When a bow or bay is load bearing (i.e. it has a bay window above it) to comply with the building regulations it must be fitted with load bearing bay pole jacks. These jacks fit through the external welded sill and can then be adjusted by being wound up or down to compress the bay pole on to a top plate supplied and on to the head of the aperture. These have a load bearing capacity of 2500kg each so a 3 on 3 sided bay will load bear 7500kg.  They come in ones for square bays and non-square bays.